Airfoiled stab

I am currently designing a new stunter and would like to put an airfoiled stab on this one. Does anyone have any ideas on what thickness this should be and or the shape of the airfoil. Any ideas would be appreciated. I will be using my trusty ST 60.

My last 4 planes have had airfoiled stabs. Mini-wings really. D-Tube construction.

Thickness depends on the planes size. the airfoil I use is essentially and 18% thickness with a sharp LE.

Randy, that sharp LE is, accoding to Bene Rodrigues, undesirable for windy conditions.

I'm just "selling the fish" as I was told...

In Christ,

Ion,

Not in my experience.

1/2" - 3/4" would be the "usual" tchickness for a .60 ship.

About the shape, Bené Rodrigues told me that the shape of the Vector .40 stab/elevator is good for calm weather, and not good for windy days.

His first "Cheyennes" had the Vector shape, but now he uses somwthing like a NACA 10 or 15%.

In Christ,

I noticed that Jim Casale used a straight triangle shape for both the stab and elevators. Any thoughts on that.

>I am currently designing a new stunter and would like to put
>an airfoiled stab on this one. Does anyone have any ideas on
>what thickness this should be and or the shape of the
>airfoil. Any ideas would be appreciated. I will be using my
>trusty ST 60.

Based on pretty extensive experimentation with me and my buddies I would recommend a more-or-less arc airfoil with about 5-6% thickness, and a LE radius of around 1/16-3/32". Look at any of the "Gemini-of-the-month" models from FM in the late 70's and 80's. They used the CSC stab/elevator cores, and I think this was definitely right.

I prefer a lower aspect ratio than the older models. I recommend 4.25-4.5:1, partly for aerodynamics, but mostly for structural and rigidity reasons.

Hey Brett, what were the conclusions that you all had with the positive incidence on the stab?

Anyone: What are the advantages of an airfoiled stab over the more common "flat" one?

LAST EDITED ON Sep-30-03 AT 03:49 PM (CDT)
 
>Anyone: What are the advantages of an airfoiled stab over
>the more common "flat" one?
>

AIrfoiled was always the standard, since "Stunting can be smooth" days. The ultimate in this idea were probably the Imitation and it''s full-fuse bretheren from the early 80s.

Flat came into vogue in the early 80's when a number of people noted the good tracking and very reliable corner exits of Gid Adkisson's "Laser". The "Laser" had a very thin (3/16-1/4") flat section with a radiused LE, a very low aspect ratio (something like 3.5:1) and lots of functional bracing wires to keep it straight. It was also dead easy to align. Several airplanes, notably the Impact, used a somewhat thicker, somewhat higher aspect ratio tail in an attempt to reproduce the laser's corner exit. It had to be thicker because nobody wanted to use the bracing wires.

Of course the Impact is probably the most successful desgn in stunt history, so it was somewhat influential (at least among those of us who paid attention). So there were a lot of airplanes built with 1/2" thick flat stabs of moderately low aspect ratio - the Imitation XL, first Infinity, the "Final Edition", and all but the first "Star Gazer". The original Trivial Pursuits (the cream/maroon one of Ted's and David's first "Star Gazer" (yellow/green) built from the same batch of parts) both had airfoiled tails.

One thing we noticed was the while the airfoiled tail models had other issues (Ted's being 68 oz, and both of them with too much built-in rudder offset), they both turned much more reliably than any of the flat-tail airplanes. The worst of the bunch, by far, was Ted's "Great Expectations" that flew to a highly-placed finish at the Shanghai W/C and won the 95 NATs. But David's contemporary airplane (the faded blue/white Star Gazer) was only a little bit better, and the other two (blue/yellow with soft mount (big mistake) and the maroon/yellow "Kiev" airplane) were no great shakes, either.

We had been having good success with the use of turbulators on the wings. Ted's original Trivial Pursuit tended to stall when flown in the rain, and lengths of flying lines taped to the top and bottom of the wing at about the high point of the airfoil fixed it conclusively. Same with Jim "Uncle Jimby" Aron's appalling "Frankenstunt", and the pretty, but incredibly overweight Boogaloo 3 (615 squares/71 oz). This led David to started experimenting with turbulators on the tail. It's not clear what helped what, but after some fiddling, it was discovered that the airplanes with inconsistent turns seemed to get a lot better when you taped wires right along the point of the leading edge, and right at the point the radiused leading edge met the flat part of the stab.

That's not to say that all the flat-stab airplanes turned inconsistently. The Imitation XL had no issues with tracking or inconsistent corners (although there is a different but related story there...) as a result of the flat airfoil, the first Inifinty probably had only a minor problem, although it didn't last long enough to really tell for sure, and obviously most of the Impacts were fine as they were. Adding wires and other interesting gadgets to the tail really didn't seem to help - but then again, there wasn't anything to fix.

Nontheless, it certainly appeared that the 1/2" thick flat stabs were more prone to problems than the others, and that the issue had something to do with the airfoil on the stab. If it wasn't the airfoil, why did turbulators help? David's soft-mount airplane also had a removable stabilizer after Paul made an offhand comment on time that he had never had a tracking problem since he began making take-apart airplanes and could shim them. This made it really easy to experiment with the tail incidence, and also the airfoil. These experiments were documented in the "DE-TAils article. The net result was that slight positive incidence and an airfoiled tail made that airplane turn the most consistent corners. The experimental, Monokoted, tail made for that article was used to win at least 2 NATs.

The incidence was hardly a new idea, but I think it was pretty new to David and I, at least in terms of tracking and corner exit. Of course, everybody knew that you could adjust the inside/outside turn difference by altering the flap/elevator neutral. Adding down elevator at neutral flap makes it turn quicker outside, and up elevator at neutral flap makes it turn quicker inside. What got me to thinking about it was an experience I had at the Northwest regionals one year. I had, as is typical, not picked up a handle since the previous October. I got the airplane down, put it togther at the contest, and started trying to fly it. I almost couldn't fly it level, and darn near crashed. I got to looking at it, and for some reason, the elevator had gotten a bit of "up" at neutral flap - like maybe 1/32" at the trailing edge. I couldn't see how that made all that much difference, but what the heck, that why I use ball links. Took off the hatch, turned the pushrod in two turns, and then went and flew it. Completely stable, turned and exited flat as a pancake, completely different airplane. This is what led me to (unbeknownst to my buddies) *build in* about 1/64" of positive incidence in the first Infinity. While it never got around corners with a lot of finesse (because the tail moment was too long at 21"), it certainly could fly level well.

Cut to about 5 years later. David decided to do his tail airfoil test. Once he had done all the different airfoils, I suggested that he also to some experiments with incidence, and finally owned up to the intentional "misalignment" I had been using. The results were in the article, but it turned out that the slight positive incidence (David went all the way to .6 degrees - close to 3x the amount I had been using) was much more significant than the airfoil shape. Even the old flat stab that didn't track all that well at 0-0 worked pretty good at +.3. The airfoiled stab (shaped like the one described in the previous post) was still somewhat better.

As a result of all this, Ted, when refinishing the Great Expectation to become the "Pond Scum", built a sharp LE onto the otherwise flat stab. David left the airfoiled stab at .6 degrees positive, and I built the second Infinity with a very similar airfoil at the same .25 degrees I had used before, and also radiused the edges of the hinge lines like Paul had been doing for years. We were all very happy with the results, and in applying the same principles to other's airplanes, it seemed to be reasonably repeatable.

I have only speculation on the reason that the incidence seems to help. The flat stab "issue" seems a lot more understandable. I speculate that, as built, they tend to have a very steep pressure gradient that abruptly falls off once the stab gets flat. This has got to make it pretty tough for the air to get "around the corner" and flow smoothly across the flat surface. I think the net result is that it has a much greater tendency to hunt a little as the flow attaches and detaches to the top and bottom of the tail. The turbulators would help because they would help "trip" the air around the corner so it stays attached more securely. An airfoiled stab of course had a much more gently pressure gradient that would be much less prone to detaching. Really thin falt stabs, like on the 'Laser", probably self-turbulate and thus avoid the problem with thick and flat.

Has anyone experimented with actual airfoil sections on these 'flat plate' stabs.

Gottingen Flat Plate for example, is flat from 10% chord to 70% and so, provides a formal shape to aim for.

It's difficult to reproduce reliable results using the 'If it looks right, it probably is' approach.

I went flying Sunday with a local guy whose stabilizer is flat with approximately a semicircle for a leading edge. His engine was spewing after-run oil, much of which hit the tail. I noticed that after he flew, the oil had pooled along a spanwise line just aft of the radiused part of the stabilizer LE. I guess that the flow separates at the break from round to straight, as you mentioned. I flew the model. It flew extremely well. So the flow is doing something one would expect to be bad, but it seems to have no ill effect.

>I went flying Sunday with a local guy whose stabilizer is
>flat with approximately a semicircle for a leading edge.
>His engine was spewing after-run oil, much of which hit the
>tail. I noticed that after he flew, the oil had pooled
>along a spanwise line just aft of the radiused part of the
>stabilizer LE. I guess that the flow separates at the break
>from round to straight, as you mentioned. I flew the model.
> It flew extremely well. So the flow is doing something one
>would expect to be bad, but it seems to have no ill effect.

Hey, empirical evidence! Did this guy happen to have his CG relatively far forward, meaning there was a fair amount of tail load (that would suggest a relatively high AoA, and potentially overcoming tendency to have "cyclic separation").

I really to think that airfoiling the stab makes the response more linear/less discontinuous - even though both PW and I have gotten away with it with no apparent ill effects.

Maybe so. It was a dadgum four-stroke. Those people tend to operate with pretty-far-forward CGs. So having a four-stroke gives a person the license to have a forward CG, which masks crudeness in his airplane.

>Maybe so. It was a dadgum four-stroke. Those people tend
>to operate with pretty-far-forward CGs. So having a
>four-stroke gives a person the license to have a forward CG,
>which masks crudeness in his airplane.

Uh, yeah.

Are you guys coming down for Golden State?

I'd like to go. Clovis is a great contest. My wife has to be back here Monday morning, though.

Howard,

Now you've done it. You've have subtlely disparaged Dirty Dan's new Impact and he didn't even read it. Where's the fun in that?

You're going to have to be more direct!

I was flying my 'Easy' today and was on kind of a schedule, so I did not wipe it down between flights. The stab is a 1/4 in thick flat plate with semicircular leading edge. There was a line of oil right along the transition to the flat plate, and oil behind, as Howard described. So now we have two observations of the same phenomenon.

In building the Lancair I read threads about how competition glider pilots add turbulator tape to the wings to improve lift and speed, etc. They smear oil on the wings lengthwise on the top and bottom and go fly. As you have observed where the oil appears after a flight there is a definite separation line. It is at that point where they add the turbulator tape to "re-attach" the air flow and thus reduce drag; conversely inprove performance.

Many Lancair fellas have tried to repeat the same experiment in hopes of increasing airspeed. Apparantly the speed improvement is negligible for the effort expended and CG differences in real aircraft change which changes AOA. etc.

Turbulator tape is a teflon tape that is zig-zagged like it was cut with pinking shears.

Bob Smiley

A Lancair 36 builder/pilot trying to be a mdoeler

>. . .and also radiused the edges of the hinge lines like
>Paul had been doing for years. We were all very happy with
>the results, and in applying the same principles to other's
>airplanes, it seemed to be reasonably repeatable. . .

Hi Brett,
I've always tried to keep my trailing edges thin (1/16 inch) and sharp. Likewise, my hinge lines are "sharp"- wing and stab are square, flap and elevator have crisp 45° chamfers. Are radiused TE's and hinge lines better? Why?
Thanks!
Derek Moran

> Likewise, my hinge lines are "sharp"- wing and
>stab are square, flap and elevator have crisp 45° chamfers.
>Are radiused TE's and hinge lines better? Why?

I don't know about the TE - I never really payed any attention to the TE, except that sharp ones tend to get chipped easily.

My idea was to copy Paul. I imagine the reason Paul did it was to make sure that nothing about the flow over the stab changed abruptly as you deflected it. Sharp edges give the impression that the flow around them will change a lot with just a little change of external conditions. But I don't know for sure.

I imagine the reason Paul did
>it was to make sure that nothing about the flow over the
>stab changed abruptly as you deflected it. Sharp edges give
>the impression that the flow around them will change a lot
>with just a little change of external conditions. But I
>don't know for sure.
>
> Brett

That was the exact reason it was done.

>Hey Brett, what were the conclusions that you all had with
>the positive incidence on the stab?

I think all the airplanes built recently around here have had between .25-.5 degrees of positive incidence built in. David's experiment was for his model only, but all three of my airplanes built since 1988 have had it, and I think Ted's new model does (or will), too. It seems to make no detectable difference in inside/outside turn, but it does seem to help the tracking quite a bit. If nothing else, it pretty much gurantees that the stab is not at a *negative* incidence, which seems to be the kiss of death for tracking.

It seems to be pretty universally beneficial on airplanes with the stab higher than the wing. I don't know about (but I doubt) the benefits on an "in-line" model.

And just to make sure we are all talking about the same thing - positive means LE higher than TE. This would be expected to create a slight *nose-down* moment.

If you want to experiment, drooping the elevator slightly on a 0-0 airplane is nearly equivalent (which is how I got the idea in the first place).

I think that it's definitely a more generally beneficial solution than downthrust.

I've seen how horribly a ship with negative stab incidence behaves, and also the benefit of positive incidence or (in my case) its equivalent elevator trailing edge droop (about 1/8").

Initially I was trying to achieve identical inside and outside turns, and I attributed the need for the elevator droop to "blanking" effects of the flaps on the elevator for down controls.

Now I understand there is more involved, including the fact that the wing's flight attitude must be non-zero.

I'd like to see another current discussion of this subject.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"I put instant coffee in a microwave oven and almost went back in time." -Steven Wright

> I think all the airplanes built recently around here
>have had between .25-.5 degrees of positive incidence built
>in. David's experiment was for his model only, but all three
>of my airplanes built since 1988 have had it, and I think
>Ted's new model does (or will), too. It seems to make no
>detectable difference in inside/outside turn, but it does
>seem to help the tracking quite a bit. If nothing else, it
>pretty much gurantees that the stab is not at a *negative*
>incidence, which seems to be the kiss of death for tracking.
>

Where was the article that talks about it? If not in a mag then where can i get a copy of his findings? Dana, [email protected]

I should know this but.... here's a dumb question:

What's incidence and what do you consider positive and negative incidence?

LAST EDITED ON Sep-30-03 AT 09:56 PM (CDT)
 
>I should know this but.... here's a dumb question:
>
>What's incidence and what do you consider positive and
>negative incidence?
>

Incidence, in this context, is the angular "alignment" of the tail to the wing (from the side view, of course). The traditional stunt plane alignment is to have the chord line of the wing, and the chord line of the tail to be parallel to each other. This is called 0-0 (zero zero).

Negative incidence in the tail meanz the tail is set to give a "nose-up" torque. This means the leading edge is "below" the trailing edge. This is the way most full-scale airplanes are set up. It's normal for a general aviation airplanes to have maybe 2-3 degrees of negative incidence in the tail (also called decalage) because this makes it stable with regards to airspeed.

What I and several others have been doing is just the opposite - positive incidence, where instead of parallel, the chord line is set so the LE of the stab is slightly above the TE, which might be expected to pitch the nose down.

Lots of incidence can effect the relative turn characterstics between inside and outside turns. Many airplanes, layed out the normal way with the stabilizer well above the wing. turn better inside than outside. Many airplane wind up with a significant amount of down elevator with neutral flap to even this out. Al Rabe built a few degrees of negative incidence into his entire tail to correct this. I don't want to get into the argument about why this happens - I just am not sure at all, probably related to "downwash", but not definitively.

What we (or me and Larry, among others) were talking about was a teeny-weeny bit. Not nearly enough to significantly effect the turn rate. I have been building in about .25 degrees, which is about 1/64" over the length of the fixed part of my stabilizer. David can adjust his, and he wound up with .6 degrees. The observation is that this makes the airplane more stable both upright and inverted. Experiments have seemed to suggest this. For sure, every time we have tried it, anything that looks like negative (like "up" elevator at neutral flap, or accidentally or intentionally tilting the LE of the stab down) makes many airplanes go completely ape - I gave an example above.

Of course, to reliably put in 1/64" of anything pre-supposes a certain level of consistency in construction. But it's useful to know that just about any negative is bad, but a fair amount of positive doesn't seem to hurt anything much. When you are gluing that tail into the fuse, you can shoot for 0-0, but make sure that if you are a little uncertain, the uncertainty goes in the direction of positive incidence.

Even if you get it a little wrong, as long as you can adjust the flap-elevator pushrod length, you can pretty much fix it in trimming. I strongly recommend making a provision for this, and even if your airplane seems pretty good, adjust it a little in either direction just to see if it's at it's optimum for that particular airplane.

My reasoning for the airfoiled stab was a bit less complex. It allowed me to carry the CG considerable farther aft than a flat stab. Using very long tail moments, the CG was going to be aft anyway, even with pretty light construction. So, I used airfoiled stabs to support the aft shift of the CG. Seemed to track well and the corner was, ah, pretty good.

How do you figure that?

>My reasoning for the airfoiled stab was a bit less complex.
>It allowed me to carry the CG considerable farther aft than
>a flat stab. Using very long tail moments, the CG was going
>to be aft anyway, even with pretty light construction. So, I
>used airfoiled stabs to support the aft shift of the CG.

Huh?

Brett and Howard,

It's magic. What, you guys don't use magic?

>Brett and Howard,
>
>It's magic. What, you guys don't use magic?

What do you mean, I don't use magic? I stuck a pin in a Randy Powell doll just this afternoon. How have you been feeling, by the way?

I note that Howard shared my puzzlement on the topic of airfoil (curved, or not).

Having an aft CG and a long tail would load the stab pretty lightly, so maybe the airfoiled tail would behave better than the flat one for the reason you wrote about earlier. I suspect Randy made the right choice, but just explained it weird.

Howard,

That's basically it. I suppose I shouldn't respond to stuff when I really should be sleeping. I was pretty tired.

> What do you mean, I don't use magic? I stuck a pin in a
>Randy Powell doll just this afternoon. How have you been
>feeling, by the way?

I think last week someone had a Godzilla action figure somewhere, stuck it in the toilet and repeatedly flushed. I was making more mucous than Jaba the Hut...

Great thread BTW. You guys are so smart.

I plan to "PAul Walkerize" the Dr. Jekyll surfaces to see if there are some results. This will be a definative change from Mr. Hyde. I also plan to add incidence to the tail.

Hey Ted, how do the down incidence in the motor relate to the up in the tail?

>I think last week someone had a Godzilla action figure
>somewhere, stuck it in the toilet and repeatedly flushed. I
>was making more mucous than Jaba the Hut...

Thanks for sharing.

>> What do you mean, I don't use magic? I stuck a pin in a
>>Randy Powell doll just this afternoon. How have you been
>>feeling, by the way?
>
>I think last week someone had a Godzilla action figure
>somewhere, stuck it in the toilet and repeatedly flushed. I
>was making more mucous than Jaba the Hut...
>
>Great thread BTW. You guys are so smart.
>
>I plan to "PAul Walkerize" the Dr. Jekyll surfaces to see if
>there are some results. This will be a definative change
>from Mr. Hyde. I also plan to add incidence to the tail.
>
>Hey Ted, how do the down incidence in the motor relate to
>the up in the tail?
'ZILLA,

CAN'T ANSWER THAT. THE FINAL EDITION HAS THE BUILT IN DOWNTHRUST AND STILL REQUIRED THE MOD TO A SHARP LEADING EDGE TO SETTLE DOWN IN LEVEL FLIGHT. I'VE ONLY BUILT THE TUCKER WITH THE STAB INCIDENCE AND DON'T FIND IT TO BE DEFINITIVE...IT'S JUST A SO-SO FLYER. THE SQUISHED NOBLER, ON THE OTHER HAND, FLEW LIKE IT WAS ON RAILS AND WAS ZERO ZERO.

I THINK THEY ARE POSSIBLE SOLUTIONS TO TWO DIFFERENT PROBLEMS. I BELIEVE THE TAIL INCIDENCE IS A FUNCTION OF THE INTERPLAY BETWEEN THE WING AND THE STAB AIRFLOWS AND IS THEREFORE SENSITIVE TO THE RELATIVE POSITIONS OF SAME. THE T.P./STARGAZER DESIGN USES A VERY THICK WING AND A MORE OR LESS OCNVENTIONAL LOCATION OF THE STAB RELATIVE TO THE CENTERLINE OF THE WING. I THINK THERE IS LIKELY A DOWNWASH RELATIONSHIP BETWEEN THE TWO THAT DOESN'T REACT WELL WITH THE FLAT STAB/ROUNDED LEADING EDGE COMBINATION. THIS DESIGN IS A REPEAT OFFENDER WHEN BUILT WITH THAT CONFIGURATION BUT, WHEN THE MODS ARE MADE ITS A NATS WINNER. THE NEW ONE WILL HAVE THE TAIL RAISED A HALF INCH AS WELL AS AIRFOILED TO SEE IF I CAN FIND SOME SORT OF LEVEL FLIGHT NIRVANA.

THE ENGINE DOWNTHRUST IS EMPLOYED (BY ME) TO THEORETICALLY "LOAD" THE CONTROLS MORE OR LESS LIKE THE AEROBATIC TEAMS SUCH AS THE BLUE ANGELS DO SO THAT THERE IS ALWAYS SOME CONTROL INPUT REQUIRED...EVEN IF SO SLIGHT THAT YOU CAN'T TELL YOU'RE DOING IT...TO MAINTAIN A CONSTANT PITCH ATTITUDE, I.E. LEVEL FLIGHT OR STEADY STATE PITCH CHANGE AS IN ROUND MANEUVERS. THIS SEEMS TO BE VERY SUCCESFUL AND WAS INITIALLY USED "AFTER THE FACT" ON THE ORIGINAL TRIVIAL PURSUIT BY THE USE OF SPECIAL .005" WASHERS UNDER THE FORWARD HOLD DOWN BOLTS.

I THINK IT IS IMPORTANT TO REITERATE THAT THESE ARE PRETTY ESOTERIC TRIM FEATURES THAT WON'T NECESSARILY TRANSLATE EFFECTIVELY TO ALL DESIGNS. I THINK THE DOWNTHRUST (ABOUT 1/8" IN 12'" OF MOTOR MOUNT) CAN BE EMPLOYED WITHOUT FEAR OF NEGATIVE CONSEQUENCES. I'M LESS SANGUINE ABOUT THE STAB INCIDENCE. IF YOU CAN HONESTLY KEEP THE INCIDENCE TO NO MORE THAN 1/32" ANY NEGATIVE EFFECTS SHOULD BE FIXABLE WITH ELEVATOR/FLAP TWEAKING. LARGER AMOUNTS WOULD EXPOSE ME TO MORE POSSIBLE PROBLEMS THAN WOULD BE COMFORTABLE.

AS BRETT SAYS, YOU CAN PRETTY MUCH ACCOMPLISH THE SAME THING BY TWEAKING THE ELEVATOR. THAT SOUNDS LIKE A SAFER APPROACH TO ME.

LAST EDITED ON Oct-01-03 AT 04:36 PM (CDT)
 
I BELIEVE THE TAIL INCIDENCE IS A FUNCTION OF THE
>INTERPLAY BETWEEN THE WING AND THE STAB AIRFLOWS AND IS
>THEREFORE SENSITIVE TO THE RELATIVE POSITIONS OF SAME. THE
>T.P./STARGAZER DESIGN USES A VERY THICK WING AND A MORE OR
>LESS OCNVENTIONAL LOCATION OF THE STAB RELATIVE TO THE
>CENTERLINE OF THE WING.

My latest are basically modified Impacts. The wing and stab locations are both from the Impact layout, even though my wing postion is shifted forward about .75".

I simply noticed the the amazingly consistent angle of attack the you and David both had, especially in the rounds, and your airplanes do not float into the inverted turns (same for Brett too).

Mr. Hyde and Shear Panic both have about 5 to 6 degrees of down incidence to achieve good tracking, and similar turn rates. The more down incidence the closer the turn rate.

Al Rabe simply explained his use of incidence as simply "splitting the difference" between his apparent need for about 4-5 degrees of down incidence to achieve equal turns and good tracking. 2 to 2.5 degrees of incidence was simply a solution to reduce the down incidence required. That is my plan too.

>Mr. Hyde and Shear Panic both have about 5 to 6 degrees of
>down incidence to achieve good tracking, and similar turn
>rates. The more down incidence the closer the turn rate.
>
>Al Rabe simply explained his use of incidence as simply
>"splitting the difference" between his apparent need for
>about 4-5 degrees of down incidence to achieve equal turns
>and good tracking. 2 to 2.5 degrees of incidence was simply
>a solution to reduce the down incidence required. That is
>my plan too.

'zilla,

I think you might be mixing your metaphors just a bit. correct me if I've misunderstood you.

Incidence is a specific word which relates to the angle at which a fixed surface is mounted in an aircraft relative to some datum...we pretty much use the thrustline although that would be modestly different on a design which uses built in downthrust so maybe we should just agree to use the centerline of the wing at the root...which generally is parallel to the thrust line.

When you speak of 4-5 degrees of incidence I suspect what you are actually measuring is elevator deflection relative to the stab (with the flaps at neutral) that you utilized to equalize the turn rate. Is that correct?

If so, you can relate Al's comments as referring to the incidence as I described it, i.e. raising the leading edge of the fixed stab a couple of degrees. Then, with the elevators in the neutral "trail" position relative to the stab the entire surface is angled at appx. two degrees relative to the datum...in this case the wing centerline.

The result is a fairly predictable similarity in effect between drooping the elevators a given amount compared to realigning the entire surface about half as much.

Does this describe what you're planning to do? If so, it should work out just fine although the 2-3 degrees seems a lot compared to the experiments that resulted in much less than that for David andBrett. Remember, Al's big semi-scales had a lot more vertical divergence of both the wing and tail from the datum and the effects might well be exagerated in those cases.

An Impact derivative would, I would think, be more akin to that needed by a similar design such as the TP. Move carefully in the direction of much more than one degree would be my suggestion.

I use the flat stab. It is the easiest to get lined up correctly. I set it all at 0-0. Then I deflect the elevator down a bit until it grooves right in. it usually has no effect on the turn rate.

What is the adavtage of building in postive incidence over using just down deflected elevator?

Seems to me it could be a way to mess it up more than fix it if you could just as easily do it with the adjustable pushrod and the stab built in at a 0. Instead of trying to get .6 or .4 of a degree.

I understand if you have to roll in to much elevator it can affect the turn rate. I have not run into that and one of my planes is over and 1/8" at the ele trailing edge with all kinds of slop and loose loose controls. Still flys well.

Ted you are understanding Brad correctly.

Just a further comment on terminology.

Incidence is, as I described the angle at which a fixed surface is mounted relative to a specified datum.

There is another term that is widely used that is also based on the angles of incidence of the wing and tail. That term is decalage or, as it is sometimes termed, longitudinal dihedral. This is a measurement of the angular relationship between the chord lines of the wing and its stabilizing surface...either the conventional stab/elevator, a flying stab at neutral, or a canard on tail first designs.

The term longitudinal dihedral is really the more descriptive term in that it states what the relationship is intended to accomplish, stability in the pitch axis. Most full scale aircraft (pretty much all that aren't aerobatic oriented or computer controlled for stability) utilize decalage to cause the tail to produce a download to counteract the wings natural negative pitching moments and/or a pitching moment caused by a CG forward of the aerodynamic center of the wing.

Take a tail off such a plane and it will want to pitch over and eventually return to mother Earth. When we mount the stabilizing surface (talking conventional wing-first tail-last stuff here) we do so with the leading edge lower than the trailing edge (negative incidence). Thus, the tail is producing a down force which counteracts the previously mentioned pitching moments.

The angular difference that results between the chord lines of the wing and tail is the decalage.

When we build a stunt ship with a raised leading edge on the stab we are, in fact, building in the equivilant of "negative" dihedral statically. In actuality, when you consider the downwash off the wing we are really simply moderating the amount of decalage that would result from that downwash. ANother subject for another day.

I know, boring aerodynamic stuff. Sorry.

>I BELIEVE THE TAIL INCIDENCE IS A FUNCTION OF THE
>>INTERPLAY BETWEEN THE WING AND THE STAB AIRFLOWS AND IS
>>THEREFORE SENSITIVE TO THE RELATIVE POSITIONS OF SAME. THE
>>T.P./STARGAZER DESIGN USES A VERY THICK WING AND A MORE OR
>>LESS OCNVENTIONAL LOCATION OF THE STAB RELATIVE TO THE
>>CENTERLINE OF THE WING.
>
>My latest are basically modified Impacts. The wing and stab
>locations are both from the Impact layout, even though my
>wing postion is shifted forward about .75".
>
>I simply noticed the the amazingly consistent angle of
>attack the you and David both had, especially in the rounds,
>and your airplanes do not float into the inverted turns
>(same for Brett too).
>
>Mr. Hyde and Shear Panic both have about 5 to 6 degrees of
>down incidence to achieve good tracking, and similar turn
>rates. The more down incidence the closer the turn rate.
>
>Al Rabe simply explained his use of incidence as simply
>"splitting the difference" between his apparent need for
>about 4-5 degrees of down incidence to achieve equal turns
>and good tracking. 2 to 2.5 degrees of incidence was simply
>a solution to reduce the down incidence required. That is
>my plan too.

6 degrees? I find that bordering on inconceivable - even assuming you just mean the elevator, and not the whole stab. Sounds more like a compensation for something else that makes it turn better inside, like uneven control rates or the old tilt-forward handle neutral.

> 6 degrees? I find that bordering on inconceivable - even
>assuming you just mean the elevator, and not the whole stab.
>Sounds more like a compensation for something else that
>makes it turn better inside, like uneven control rates or
>the old tilt-forward handle neutral.

I know, weird huh? Maybe 5 degrees.

You have to factor in the accuracy of the measuring instruments also. Will check again, but I am pretty convinced that the elevator down-thrust is about 4-6 degrees down from level flap.

Everything Ted said is correct. I meant elevator down-thrust. The stab incidence would be 2 degrees.

>You have to factor in the accuracy of the measuring
>instruments also. Will check again, but I am pretty
>convinced that the elevator down-thrust is about 4-6 degrees
>down from level flap.
>
>Everything Ted said is correct. I meant elevator
>down-thrust. The stab incidence would be 2 degrees.

I guess that's not as far off as I initially thought. I have seen a lot airplanes with 1/8-3/16" of elevator droop.

Half year ago or so I did a calculation of flow direction over the wing at 0 AoA at place where the stab is placed. It will be somwhere in older threads. Its angle very well matches those 0.5 deg. So I can only agree what Ted wrote:

>I BELIEVE THE TAIL INCIDENCE IS A FUNCTION OF THE INTERPLAY BETWEEN THE WING AND THE STAB AIRFLOWS AND IS THEREFORE SENSITIVE TO THE RELATIVE POSITIONS OF SAME.<

I thik that is it. However there are lot of another forces like gyro moment from prop, or "side p- effect" from prop (with side wind). But all of them are too little and 0.5 AoA on stab is too much to ballance it. The engine thrust line and drag of landig gears are enough.

That's good, Igor. I have wondered about that, but don't know how to calculate it.

LAST EDITED ON Oct-04-03 AT 05:01 PM (CDT)
 
Look at the table in the "html" attachement – it is flowfield from JavaFoil past of the TE of a symmetric 18% airfoil at 0 AoA. The first column is x coordinate – “0” means LE, “1” mean TE, the table goes to 1.55 so more than half of chord past the TE. The second column is vertical “y” coordinate in the same units. The vx and vy is the flow direction. The last column is angle of flow derived from vx and vy. It is not our real airfoil, and it is too close to LE, but I think it makes clear what I mean.

I regret that I can't read the html file. I'll look at JavaFoil. Thanks.

LAST EDITED ON Oct-05-03 AT 02:45 AM (CDT)
 
I do something wrong, I can not post HTML file, so I put the table here ... sorry for long and wrong formatting text

x......y........vx......vy.......angle

1,0600 -0,4395 0,9954 0,0372 2,1418
1,0600 -0,3809 0,9907 0,0411 2,3766
1,0600 -0,3223 0,9845 0,0451 2,6217
1,0600 -0,2637 0,9760 0,0488 2,8636
1,0600 -0,2051 0,9644 0,0517 3,0702
1,0600 -0,1465 0,9485 0,0524 3,1609
1,0600 -0,0879 0,9261 0,0468 2,8932
1,0600 -0,0293 0,8994 0,0231 1,4692
1,0600 0,0293 0,8994 -0,0231 -1,4692
1,0600 0,0879 0,9261 -0,0468 -2,8932
1,0600 0,1465 0,9485 -0,0524 -3,1609
1,0600 0,2051 0,9644 -0,0517 -3,0702
1,0600 0,2637 0,9760 -0,0488 -2,8636
1,0600 0,3223 0,9845 -0,0451 -2,6217
1,0600 0,3809 0,9907 -0,0411 -2,3766
1,0600 0,4395 0,9954 -0,0372 -2,1418
1,1300 -0,4395 0,9917 0,0324 1,8684
1,1300 -0,3809 0,9871 0,0347 2,0149
1,1300 -0,3223 0,9812 0,0367 2,1431
1,1300 -0,2637 0,9737 0,0379 2,2280
1,1300 -0,2051 0,9644 0,0375 2,2239
1,1300 -0,1465 0,9533 0,0341 2,0466
1,1300 -0,0879 0,9412 0,0255 1,5535
1,1300 -0,0293 0,9321 0,0099 0,6067
1,1300 0,0293 0,9321 -0,0099 -0,6067
1,1300 0,0879 0,9412 -0,0255 -1,5535
1,1300 0,1465 0,9533 -0,0341 -2,0466
1,1300 0,2051 0,9644 -0,0375 -2,2239
1,1300 0,2637 0,9737 -0,0379 -2,2280
1,1300 0,3223 0,9812 -0,0367 -2,1431
1,1300 0,3809 0,9871 -0,0347 -2,0149
1,1300 0,4395 0,9917 -0,0324 -1,8684
1,2000 -0,4395 0,9894 0,0276 1,5999
1,2000 -0,3809 0,9852 0,0288 1,6766
1,2000 -0,3223 0,9801 0,0294 1,7206
1,2000 -0,2637 0,9742 0,0291 1,7086
1,2000 -0,2051 0,9673 0,0271 1,6061
1,2000 -0,1465 0,9601 0,0229 1,3668
1,2000 -0,0879 0,9535 0,0157 0,9456
1,2000 -0,0293 0,9494 0,0057 0,3424
1,2000 0,0293 0,9494 -0,0057 -0,3424
1,2000 0,0879 0,9535 -0,0157 -0,9456
1,2000 0,1465 0,9601 -0,0229 -1,3668
1,2000 0,2051 0,9673 -0,0271 -1,6061
1,2000 0,2637 0,9742 -0,0291 -1,7086
1,2000 0,3223 0,9801 -0,0294 -1,7206
1,2000 0,3809 0,9852 -0,0288 -1,6766
1,2000 0,4395 0,9894 -0,0276 -1,5999
1,2700 -0,4395 0,9882 0,0234 1,3549
1,2700 -0,3809 0,9846 0,0238 1,3827
1,2700 -0,3223 0,9805 0,0235 1,3748
1,2700 -0,2637 0,9759 0,0224 1,3146
1,2700 -0,2051 0,9710 0,0200 1,1819
1,2700 -0,1465 0,9663 0,0161 0,9574
1,2700 -0,0879 0,9624 0,0106 0,6318
1,2700 -0,0293 0,9602 0,0037 0,2218
1,2700 0,0293 0,9602 -0,0037 -0,2218
1,2700 0,0879 0,9624 -0,0106 -0,6318
1,2700 0,1465 0,9663 -0,0161 -0,9574
1,2700 0,2051 0,9710 -0,0200 -1,1819
1,2700 0,2637 0,9759 -0,0224 -1,3146
1,2700 0,3223 0,9805 -0,0235 -1,3748
1,2700 0,3809 0,9846 -0,0238 -1,3827
1,2700 0,4395 0,9882 -0,0234 -1,3549
1,3400 -0,4395 0,9878 0,0197 1,1419
1,3400 -0,3809 0,9848 0,0196 1,1384
1,3400 -0,3223 0,9815 0,0189 1,1024
1,3400 -0,2637 0,9780 0,0175 1,0235
1,3400 -0,2051 0,9745 0,0152 0,8916
1,3400 -0,1465 0,9714 0,0119 0,6998
1,3400 -0,0879 0,9689 0,0076 0,4497
1,3400 -0,0293 0,9675 0,0026 0,1554
1,3400 0,0293 0,9675 -0,0026 -0,1554
1,3400 0,0879 0,9689 -0,0076 -0,4497
1,3400 0,1465 0,9714 -0,0119 -0,6998
1,3400 0,2051 0,9745 -0,0152 -0,8916
1,3400 0,2637 0,9780 -0,0175 -1,0235
1,3400 0,3223 0,9815 -0,0189 -1,1024
1,3400 0,3809 0,9848 -0,0196 -1,1384
1,3400 0,4395 0,9878 -0,0197 -1,1419
1,4100 -0,4395 0,9879 0,0166 0,9619
1,4100 -0,3809 0,9854 0,0162 0,9402
1,4100 -0,3223 0,9828 0,0153 0,8912
1,4100 -0,2637 0,9802 0,0138 0,8090
1,4100 -0,2051 0,9776 0,0118 0,6889
1,4100 -0,1465 0,9754 0,0090 0,5296
1,4100 -0,0879 0,9738 0,0057 0,3347
1,4100 -0,0293 0,9729 0,0019 0,1146
1,4100 0,0293 0,9729 -0,0019 -0,1146
1,4100 0,0879 0,9738 -0,0057 -0,3347
1,4100 0,1465 0,9754 -0,0090 -0,5296
1,4100 0,2051 0,9776 -0,0118 -0,6889
1,4100 0,2637 0,9802 -0,0138 -0,8090
1,4100 0,3223 0,9828 -0,0153 -0,8912
1,4100 0,3809 0,9854 -0,0162 -0,9402
1,4100 0,4395 0,9879 -0,0166 -0,9619
1,4800 -0,4395 0,9882 0,0140 0,8123
1,4800 -0,3809 0,9862 0,0134 0,7810
1,4800 -0,3223 0,9842 0,0125 0,7277
1,4800 -0,2637 0,9822 0,0111 0,6493
1,4800 -0,2051 0,9803 0,0093 0,5439
1,4800 -0,1465 0,9787 0,0070 0,4121
1,4800 -0,0879 0,9776 0,0044 0,2576
1,4800 -0,0293 0,9770 0,0015 0,0877
1,4800 0,0293 0,9770 -0,0015 -0,0877
1,4800 0,0879 0,9776 -0,0044 -0,2576
1,4800 0,1465 0,9787 -0,0070 -0,4121
1,4800 0,2051 0,9803 -0,0093 -0,5439
1,4800 0,2637 0,9822 -0,0111 -0,6493
1,4800 0,3223 0,9842 -0,0125 -0,7277
1,4800 0,3809 0,9862 -0,0134 -0,7810
1,4800 0,4395 0,9882 -0,0140 -0,8123
1,5500 -0,4395 0,9888 0,0119 0,6889
1,5500 -0,3809 0,9872 0,0113 0,6534
1,5500 -0,3223 0,9855 0,0103 0,6005
1,5500 -0,2637 0,9840 0,0091 0,5286
1,5500 -0,2051 0,9826 0,0075 0,4374
1,5500 -0,1465 0,9814 0,0056 0,3280
1,5500 -0,0879 0,9806 0,0035 0,2035
1,5500 -0,0293 0,9801 0,0012 0,0690
1,5500 0,0293 0,9801 -0,0012 -0,0690
1,5500 0,0879 0,9806 -0,0035 -0,2035
1,5500 0,1465 0,9814 -0,0056 -0,3280
1,5500 0,2051 0,9826 -0,0075 -0,4374
1,5500 0,2637 0,9840 -0,0091 -0,5286
1,5500 0,3223 0,9855 -0,0103 -0,6005
1,5500 0,3809 0,9872 -0,0113 -0,6534
1,5500 0,4395 0,9888 -0,0119 -0,6889

>I do something wrong, I can not post HTML file, so I put the
>table here ... sorry for long and wrong formatting text

I don't know what you did, Igor, but I checked the file that you uploaded and I come up with a blank Excel sheet. Even though you gave it HTML extensions, it doesn't seem to be formatted properly internally.

A normal HTML file should easily fit into the 50K limits. I don't know what happens when you convert a long Excel program to HTML, however.

Yes Len, I did an Excell sheet and exported it as HTM file. It is usefull by internet explorer, but if I attached it message on forum, I can not read it also, and if I try to "edit" it windows starts Excell!!. It looks Microsoft uses some dirty tricks.

Brad...oops...Godzilla said:
>I plan to "PAul Walkerize" the Dr. Jekyll surfaces to see if
>there are some results. This will be a definative change
>from Mr. Hyde. I also plan to add incidence to the tail.

This experiment has been tried here in the NW. Fortunately, yes, very fortunately it was a removable tail. I believe it was in the neighborhood of 0.50 degree LE up (right Chris?). This plane would not fly at all (well, it went round and round, but hardly where you wanted it to). The entire gambit of trim changes were made, to no success. It just would not fly properly. Very disappointing. But, many, many flights later the "shim" was removed and lo-and-behold, it flew fine....At 0-0.

Brad. If you do this, make sure the stab is removable so it can be set to 0-0 if the positive stab set-up doesn't work.

>Brad. If you do this, make sure the stab is removable so it
>can be set to 0-0 if the positive stab set-up doesn't work.

Good enough for me. 0-0 it is... always worked before anyway.

Randy, how much aft? If aft means between 20 and 25% than you UNLOAD the tail. (I note tail makes negative lift)

I suspect that 1/64" elevation on the LE of the stab is much more significant aerodynamically than a larger value of droop on the elevator TE.

[photo not recovered: 3f7b86b60de71da7.jpg]

Here is a scale drawing of the setup I use to get a Mo'Best to turn similar inside and outside loops, drooping the TE by 1/8" with the flaps held zeroed on neutral. However, note that I use a 3/8" thick stab and a 1/4" elevator which tapers to 1/8" on its TE.

One of the theorized benefits of a thinner elevator is that it has a small range around neutral where it is not in the airstream and therefore not (very) effective. Something like the neutral "slop" idea..

I'm not sure what drooping the elevator TE does to this hypothetical neutral slop zone. But notice that the bottom surface of the elevator is very close to being in parallel with the flat bottom surface of the stab.

In my mind's Analog Modeling System I can surmise smoother airflow over a curved stab surface than a flat one; the improved laminar flow idea makes sense to me. And it seems to follow that the control effects would be smoother and more positive.

That said, I've observed a couple of models with 1/2" or thicker flat stabs which flew beautifully. (We have to be careful - it's not particularly scientific to make such judgements. I'm probably grading the pilot!)

I may be getting away with the flat stab on the Mo'Best simply because it is only 3/8" thick. Dave's article suggested that physically thinner flat stabs do not suffer so many problems as the thicker ones.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"While theoretically and technically television may be feasible, commercially and financially I consider it an impossibility, a development of which we need waste little time or dreaming." -Lee De Forest

Interesting diagram with the 3/8 stab and 1/4 inch elevator.

Observation. The elevator is in the shadow of the stab and does not have much effect in the neutral attitude.

Lancairs are designed where the elevator and flaps have a slightly larger radius on the leading edge as compared to the trailing edge of the stab or wing. This allows the airflow to re-attach to the flap and elevator and provide more positive control. Whether stuntships need positive control in neutral flight or not may not be necessary and or causes hunting as the plane flies down and upwind.

Has anyone used a thicker elevator than the stab in a stuntship? If so what was the results?

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler

BB: Thanks for a very detailed and informed response. Currell

LAST EDITED ON Oct-02-03 AT 11:42 PM (CDT)
 
In regard to Brett's comments about seeing airplanes with 3/16 down elevator trim and airfoiled tail surfaces, I wrote about this in my Mustang article in Flying Models in 1978. As far as I can determine, this article was the first published mention of the use of intentional incidence in the horizontal stabilizer. I have always used airfoiled tail surfaces on my competition stunters.

My observations were completely the result of practical experience as opposed to theoretical insight. I had observed that a number of my stunt ships had about 3/16" of down trim in the trailing edge of the elevators to make the airplanes turn equally inside and outside. These observations were made mostly on two Sea Furys and a couple of Mustunts, still, the trim seemed necessary for equal turns. When I designed the Mustang "Snaggletooth", actually "Easy Two Sugar", the first of the molded Mustangs, I decided to design in the trim I had previously found necessary in other airplanes. To this end I raised the leading edge of the stabilizer 3/32" and drooped the trailing edge of the elevators 3/32" for the necessary trim and to "fair" or align the two surfaces. This 3/32" of incidence in the leading edge of the stab amounted to about 2 degrees and I have used this 2 degrees in every airplane I have built since with good effect.

My personal choice is to build 2 degrees into the stab and use exacting procedures to accurately align all of the surfaces and thrust line of my airplanes. With precise alignment, I have found no need for adjustable controls. Neither have I found any need to "tweak" surfaces, though I would be willing to do so in the interest of optimum trim if necessary. It just hasn't been necessary.

I sell a VHS tape on the construction of very light weight, very strong airfoiled tail surfaces featuring a spruce I beam stab spar. 2/hours, $15 plus $2 S&H. I also sell a one hour VHS tape on the precision alignment of a stunt ship. $10 plus $2 S&H. My new e-mail is, [email protected]. Maybe I could have picked a better e-mail. It is als(underline)crit(underline)[email protected].

The use of intentional stab incidence originated, as far as I know, with me, and a reprint of that 1978 article on the design of the Mustang V was in the March April 2000 issue of Stunt News. The design also featured other original design concepts such as minimal asymmetry with the outboard flap widened at the tip to balance flap areas, a "canted" engine, "canted" tank, an original airfoil and the use of flaps having 1/4 the total wing area. The construction featured many new ideas including molded fuselage shells, and a home made internal muffler. It finally flew with a home made engine. I thought then and now the design was evolutionary and revolutionary. I am presently updating the design of "Snaggletooth" to incorporate modern engines with rear exhaust and internal mufflers and laser cutting of parts including forms for molding balsa shells for sections of the airplane's skin. The molded Mustang design had two 3rds, a 1st at the NATs, Walker trophy and a 2nd at 1978 World Champs. I tink, if updated, it would still be as competitive now as then. We'll see.

LAST EDITED ON Oct-03-03 AT 02:37 AM (CDT)
 
> Flat came into vogue in the early 80's when a number of
>people noted the good tracking and very reliable corner
>exits of Gid Adkisson's "Laser". The "Laser" had a very thin
>(3/16-1/4") flat section with a radiused LE, a very low
>aspect ratio (something like 3.5:1) and lots of functional
>bracing wires to keep it straight. It was also dead easy to
>align. Several airplanes, notably the Impact, used a
>somewhat thicker, somewhat higher aspect ratio tail in an
>attempt to reproduce the laser's corner exit. It had to be
>thicker because nobody wanted to use the bracing wires.

Interesting, that. I remember reading an article about a different Laser (Lou Loudenslager's) and it had a flat stab, too.

Re: negative incidence on the stab. Interesting, with respect to this thread:

http://www.clstunt.com/htdocs/dcforum/DCForumID1/6050.html

Al, didn't you write you use asymetric airfoil on wing? I think that is the reason you need 2 degrees incidence.

LAST EDITED ON Oct-03-03 AT 09:09 AM (CDT)
 
Igor,

Yes, I sometimes use asymmetric aiarfoils on my models but don't think this has anything to do with the need for incidence. There are many observations in this thread of flyers needing droop in their elevators for equal turns. I too experienced that characteristic. Once I recognized that as a somewhat general characteristic, I designed my tail surfaces with what seemed an appropriate amount of "up" in the leading edge of the stab and a like amount of "down" in the elevators to anticipate the need for trim and to do it in such a way as to "fair" or align the stab and elevators for a neat appearance.

As for asymmetry in the airfoils, as long as it is limited to a change in thickness from top to bottom and done without change to other factors like high point and leading edge radius, I am unable to see any effect on the handling characteristics of the affected airplanes. The use of asymmetry is to simply reduce the overall thickness of the wing, for appearance only, by removing unnecessary thickness of the bottom half of the airfoil. See the Sea Fury article or its reprint for a fuller explanation.

At the time the molded Mustang V was designed, stunt ships typically had wings with about one inch of asymmetry in the spans. Given flaps with equal dimensions of the tip and root flap chord and longer length of the inboard flap, nearly all stunt ships of the time had more flap area in the inboard flap. I built the Sea Furys with equal span panels and thought they flew well and trimmed easily, so much so, I favored the configuration. The only negative characteristic of equal span panels was the need for a lot of tip weight. The Sea Furys used about 3 ozs. This was fine with me as I believed at the time, and still do, that tip weight is the easiest and most direct road to line tension. In the design of the Mustang V wing, I returned to asymmetry to reduce the tip weight required but tried to retain some of the characteristics of airplanes with equal span wing panels. To this end, I used 1/2" of asymmetry and widened the tip of the outboard flap to have flaps of equal area. I think I used the same root chord and about 1/8" wider tip chord on the outboard flap. This configuration was unique at the time but has been adopted by many designers since. The extra area allows more tipweight to be carried for easy trimming and good line tension. The larger inboard flaps of the time (and Palmer's "diferential flaps") all rolled the airplane outward aerodynamically. I used equal, of slightly larger outboard flaps to provide the same slight outward roll in maneuvers but done mechanically with tip weight instead of flap asymmetry. This was explained in the Mustang article and its reprint. The concept, and its rational, were original at the time and published to aid others in the design of better flying easier trimmed stunt ships. I still use 5/8" asymmetry on my larger stunt ships and 1/8" wider tip chord on the outboard flap.

There is a lot of Sea Fury and Mustang V in many current stunt ships, not the least of which is the use of the sliding block leadout adjustment, reversed bellcranks, movable rudders, tailored asymmetry and unique airfoil designs based on tests. Ideas become commonplace, their orgins lost in time and many are reinvented.

Al, if you have one side of airfoil thicker, you have asymetric = undercambered airfoil. I actually looked to a 18% thick airfoil with chamber only 1% what is very little - means almost symmetric. The 0 lift you get at -1.2 deg. If you add necessary 0.5 deg for symetric airfoil, you are close to those your 2 degrees.

LAST EDITED ON Oct-04-03 AT 06:13 AM (CDT)
 
Not exactly "undercambered". Undercambered means with a concave underside. Semi-symmetrical would be the correct term (and yes, semi-symmetrical airfoils have camber, unlike symmetrical ones, which have zero camber, but this does not imply undercamber).

In the real world experience of test flying my Lancair an exciting event revealed an interesting observation.

On one test flight after changing the oil filter I landed at Arlington Airport to show the plane to a friend. We walked around the right side of the plane to find gobs of oil smeared along the fuselage. Apparantly the oil filter seal blew out after 10 hours of flight do to the fact that it was sufficently tight to "Not leak" during the inspection but loose enough to fail after 10 flight hours.

The oil pattern revealed the airflow along the fuselage. At the nose it swooped up and over the wing (like an exaggerated air foil pattern) then gently dropped downward behind the wing and then swooped up under the H-stab to provide positive pressure on the underside of the stab. Does this pattern vary with respect to speed differentals? Probably. Is this pattern uniform across the entire flying surface of the H-stab? Probably not. Not being an aeronautical engineer I can only speculate.

It is probable that the propeller is swirling the airflow around the plane in a clockwise manner (when observed from the front) so on the left side the pattern may be somewhat different causing different loads on the left H-stab surface.

In any case, We assume generally that air flow is uniform along a flight path. Such is not the case. There is an air flow effect within the propeller arc which probably expands outward at an angle dependant upon air speed, thrust load, prop rpm and other factors (speclation on my part) and then there is an airflow effect from the wing as well as clear undisturbed air. These different air flow patterns are probably experienced by the H stab at the same time and the effects vary as attitude adjustments (control imputs or Newtonian forces are applied.

My understanding for negative incidence on real general aviation aircraft is to provide nose up stability in the slower flight regimes like approach to landings to provide positive control before touch down so that the nose will not drop and cause an incident. Elevator authority is mandatory at slow speeds and if you lose it, the ground smacks you in the nose. That can hurt, hence negative incidence to keep the nose up.

As to why positive incidence is preferred in models; I defer to the more experienced modelers in this thread.

As an aside; the Lancair has a -.6 degree negative incidence WRT the fuselage center line. The wing is 0 degrees WRT fuselage center line. In normal level flight the elevator is deflected downward slightly to compensate for the negative stab incidence as exibited by the elevator counterweight being raised above the H-stab about 3/16 inch. This is typical in other 360s.

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler.

>It is probable that the propeller is swirling the airflow
>around the plane in a clockwise manner (when observed from
>the front) so on the left side the pattern may be somewhat
>different causing different loads on the left H-stab
>surface.

This would almost certainly be the case, I think. Lots has been written about the effects of propwash on free flight models, and trimming thereof. Note the many configurations involving wing pylons, high thrustlines, and vertical stabilizer placement.

Bob
The oil streaks are always moving to low pressure; top of wing bottom of tail.
The elevator being larger at the hinge than the stabilizer is to reduce drag to a value almost as low as not having a hinge in the middle of the airfoil. You will find a similar installation on a P-51

Andrew Raney

Hi Airacobra

I've been off the map for a while due to moving ughhhh!

But I built my Intrepid X/L with about 1/16" Leading Edge incidence and also brought my leading edge to somewhat of a point rather than rounding it off as is call for on the plans. The airplane tracks excellently - I just need to get a little better at hitting intersections and landing it.