A "Classic" Tail?

Tail volume coefficients are getting some talk in recent threads. Here are some quotes from “The Father of Modern Stunt Ships” - - - -

“ . . . As far as stability goes, it is meaningless to use a tail having 22% or 24% or even 28% of the wing area unless the calculations also include the distance between the wing and tail. For the past 35 years I have used as my design goal a tail volume coefficient of .4. Since I still use the same .4 coefficient in all of my designs, the tails are bigger only because the airplanes are bigger. The size of the tails are the same relative to the size of the airplane. . . .” – Al Rabe

“. . . Not to start anything, or take anything away from the premise, but as a point of clarification my airplane has a tail volume coefficient of .46, and most are at least .45. It doesn't stand out at you because the aspect ratio is so low. . . .” – Brett Buck

“. . . Specific TVCs can be calculated by varying the chords, moments or areas within the formula. The real justification for using MAC lies in the stability calculations for airplanes with swept wings where the MAC may be a long way from the chord where the wing enters the fuselage. How many of us are going to compete with swept wing stunt ships? OK, so what? Have the vast majority of us ever calculated a TVC for our airplanes? Do we really care? Does anyone know the tail volume of a Nobler? . . . “ – Al Rabe

So, I wondered, how different are the TVCs of some older designs (Classics, OTS, and potpourri) compared to today’s TVCs? I don’t have any newer plans or models to measure, but let’s assume from what was said that at the top level, most of the today’s contenders are flying designs with TVCs in the .45 range. Here are a few older designs I calculated for comparison:

Model . . . . % Empennage . . Tail Moment . . TVC
Smoothie . . . . . 18.7 . . . . . . . . 16.99 . . . . . .296
Oriental . . . . . . 15.6 . . . . . . . . 17.70 . . . . . .287
Grn Bx Nobler . .18.5 . . . . . . . . 19.25 . . . . . .356

Some flapless designs (some newer, some older):

Ringmaster . . . . 19.3 . . . . . . . . 14.82 . . . . . .321
Flite Streak . . . . 12.0 . . . . . . . . 15.85 . . . . . .202
Doctor . . . . . . . . 22.2 . . . . . . . . 19.96 . . . . . .423
Medic . . . . . . . . . 22.1 . . . . . . . . 18.48 . . . . . .421
U-Key 40 . . . . . . . 8.3 . . . . . . . . 18.36 . . . . . .162

And, just for fun (circa 1996?)

Das “Ugly” Stunter 22.7 . . . . . . . . 19.53 . . . . . .454


Note 1: All tail moments are calculated from 25% wing MAC to 25% tailplane MAC, not root chords. For example, the 25% MAC chord of a Ringmaster or Flite Streak wing is FORWARD of 25% of the root chord. The 25% MAC chord of a Ringmaster or Flite Streak tailplane falls AFT of 25% of root chord. So, the tail moments of both are longer at both ends than what they appear.

Note 2: The % empennage and TVC are both scalable, but the tail moment is absolute. That is, you cannot compare tail moments directly between models of different sizes. For example, the Medic is a scaled down Doctor, with nearly identical % empennage and TVC, but an inch and a half different in tail moment. That is one of the advantages of using TVC as a measure.

Note 3: We seemingly know less of how TVC affects flapless designs than flapped ones. Ted Fancher has stated that he would reduce the empennage area of the Doctor to 15% or 18% of the wing area if he were designing it today. In a number of recent threads Ted Fancher has discussed some of the flapless issues in detail, including TVC and CG location. The CGs are generally farther forward of the wing MAC on flapless designs than on flapped ones of the same TVC.

Note 4: The MAC formulae for various wing / tailplane shapes were stolen shamelessly from http://www.nasascale.org. Curved outlines were calculated in a variety of ways, depending on the shape. Uhh, for the Oriental tailplane I resorted to “graphic calculus” – ; ) ; )
There are lots numbers and measurements involved. If any of the results look goofy, that could easily be my error. If you have conflicting info, let me know.

OK, now tell me what, if anything, this all means!

Larry Fulwider

>OK, now tell me what, if anything, this all means!
>
>Larry Fulwider
>

"If it looks right, it'll fly right". What could be more obvious?

>"If it looks right, it'll fly right". What could be more obvious?

That's often the case. HOWEVER, it also depends on WHO is looking at it. What looks right to an aero engineer may not look right to you - and vice versa! I always thought the Piper Cherokee and Beachcraft Muskateer were ugly...and how about the "A-10"? The Marske sailplane designs I've felt to be most beautiful, are called "stubby" or "funny lookin'" by one of my ex-aeromodeling friends.

So it's also often enough NOT the case.

Then there's the question of what it is to "fly right." Everything I think flies well or "right" would probably not satisfy the top fliers. But they go up, down and around. Maybe that's "right".

'think I'll reply to Larry...

SK

I did some calculations on freeflight designs some years ago. I found that the range of CG position between marginally stable and perhaps too stable, was about 10% of MAC.

You can get the same TVC using different moments and areas. The results aren't always the same. I've kind of found that for any given tail moment a larger tail always has more positive control and better feel for neutral. A TVC of .4-.5 is certainly a very good target. Getting more TVC by making the fuse longer usually helps too, but it is easier to get to a point where the longer fuse takes too much time to kick out and makes it hard to turn sharply. The longer fuse also adds weight faster than making the tail larger, causing other problems.

When the tail moment gets down in the range of the avg. chord you can also start to run into problems, no matter how big the tail is. Stabilators seem to work OK, but the classis stab/elevator doesn't work as well when the fuse is very short. the stab become less effective, maybe due to turbulence from the wing, and you can run into controlability problems- non-linear response, variations with speed, gust sensitivity.

I would add that a longer tail increases what I see is precise controlability. While the planes I've built with very long tail moments and very large tail planes don't really seem to turn more or less sharply, I can say clearly that they stop turning with more authority.

>You can get the same TVC using different moments and areas.
>The results aren't always the same. I've kind of found that
>for any given tail moment a larger tail always has more
>positive control and better feel for neutral. A TVC of .4-.5
>is certainly a very good target. Getting more TVC by making
>the fuse longer usually helps too, but it is easier to get to
>a point where the longer fuse takes too much time to kick out
>and makes it hard to turn sharply.

Certainly, this has been my experience as well. Too long is too long, for the reasons you state. Even if you had a TVC of 1, if it's too long it will inhibit maneuvering. It goes back to the same sort of damping argument that we discussed in the "add weight to both tips" plan.

Note also that this is where the "scaling" of the same "numbers" to fit various size airplanes breaks down. A 16" tail moment has some of the same characteristics on a half-A that it does on a 91- powered plane.

Brett

Well, here we go again...BUT THIS TIME I COPIED MY POST. When I pressed "post message just now, this whole thing disappeared again! Here goes for the second time.

Edit: My apologies for the edits. I've found enough misworded passages to occupy me for too long.

>OK, now tell me what, if anything, this all means!

I see that so far response has been sparse (mid-day Wednesday), perhaps because so many of us are still thinking about this. I almost wrote something similar a few days ago, because this is an interesting and not fully explored question. I too have measured and computed TVC's and tail area ratios for interesting or significant models I've encountered. Unfortunately, I have not been keeping my errant slips of computations organized. I'd intended to sit back and read what came in, but I think that Ted, for instance, has said about all he wants to in his interesting posts. So here are some musings - as usual, FWIW...

First, a note on how I view the roles of stab and elevator. The stabilizer functions by exerting a moment contrary to any unintentional displacement. For instance, if the wing suddenly lifts more in a gust and pitches the nose up, the stab also pitches up (moves down, pitches UP). In a conventional configuration this creates lift behind the c.g. and creates a restorative (nose-down) pitching moment. So the greater the stab area, the greater the auto-restoring force. Likewise, drag on the stab and elevator tends to keep the tail centered by creating its own restoring moment, whenever the tail is displaced. This goes for parasite as well as induced drag. So a low aspect ratio is also stabilizing for the tail.

The elevator exerts control forces that increase with its area, and within limits, its deflection. These are caused by changing the camber of the horizontal tail surfaces, the "lift" being distributed unequally along the entire horizontal tail, stab and elevator both. The amount of deflection and position of the hinge determine the new lift center. Overall, the effective tail length (a.c. to a.c.) for flapped planes diminishes with flap and elevator deflection, due to longer rearward c.p. movement along the wing.

I think horizontal stab and elevator influences differ from flapped to unflapped models. First, the need to overcome negative pitching moments from wing flaps requires a greater tail moment. So tail area must be greater and/or the tail length must be greater for flapped models. If the tail moment is too small, the pitching rate has to suffer for flapped models, and the downward force causing any pitch up must be greater, creating the need for greater wing lift. So there is a good argument for increasing the tail length. This also increases the longitudinal stability by increasing the restorative moment from the stabilizer. An opposing factor might be that downwash from flaps may actually increase the tail's efficiency, although asymmetry due to relative vertical positioning of thrust line and wing and tail chords may cause trim problems. Ted has pointed out the advantages in stability/C.G. placement from moving the a.c. of the whole plane rearward with long tail arms and greater horizontal tail areas on flapped planes.

A limit to this trend, long before the plane becomes a canard, probably comes from two factors: inertia (proportional to the tail's mass and the square of its distance from the c.g.), and the need for ever greater elevator areas and deflections to overcome the increasingly stabilizing relative wind due to "circular airflow" (Frank Zaic's term), as the horizontal tail becomes less tangent to the path of any maneuver. For example, in an inside loop, the tail assumes an increasing angle of attack (independent of the elevator) as the tail length increases. This must be overcome by greater elevator deflection, which creates increased stabilizing drag too.

The final nice thing about flaps is that the wing can be made to create enough lift with the fuselage still tangent to the path of a maneuver. For a flapless plane, the fuselage assumes the wings aoa.

What I think I know about flapless planes...They can produce plenty of lift, but the tail must hold them at a greater pitch angle for the same weight (although they can be built lighter). Their tails require less area and deflection to do this, since they don't need to fight the flaps' negative pitching moment, AND the effective tail length might actually lengthen slightly some with elevator deflection in the absence of rearward c.p. movement along the unflapped wing chord (I'm not sure about the movement of the l.e. stagnation point). From what I've read here, their tail volume limit seems to come from the need to have a more forward c.g. for "feel", i.e. hinge moment. Placing tail surfaces closer to the c.g. (and therefore the wing) reduces the need for forward weight and certainly reduces the polar moment of inertia of flapless types through mass and arm reductions at both ends (but limits fuel-tank location/capacity). I wonder whether short tail moments give the horizontal tail a stronger wing down-wash to work in. Tails on flapless planes would thus benefit from being both more tangent to any maneuver and perhaps operating in a more advantageous relative wind. Thus destabilized, they would need a larger ratio of stab to elevator areas. Larger horizontal tails may still have higher limits to their pitch rates due to their slower required rotational speeds at such short radii from the c.g. and their lower inertial moments. The question then is how to stabilize them.

Perhaps this is why we see such low aspect ratios on wings of some unflapped planes; their pitch damping may come from the wings themselves. Combat types probably need/desire a lot less pitch damping.

So...it looks as though flaps require and benefit from larger stabilizer-elevator areas, with the elevator needing to be greater in chord and deflection as tail length increases. Unflapped models need smaller horizontal tail areas with less elevator deflection and a lower ratio of elevator to stab area. The best compromise may be a slight increase to tail areas on models like the Streak designed for combat, but considerably larger (16%-18%) horizontal tails and greater tail lengths, like those Phil, Mikey, and now Ted have suggested, for stunters. As I recall, all their suggestions came from experience, with Phil and Mike actually varying the tails until they found flight performance they wanted. Thicker wings for stunt at lower speeds also require more tail area to be as effective.

One thing I think we should consider about the data on older flapless types is that they were designed to fly faster. Therefore their wings are often too thin for stunting at present speeds, unless the planes are very light. Those tiny tails were effective in proportion to their speed, and new tails designed for precision aerobatics would need to be larger, even at some increased tail lengths.

I like Al's way of writing TV coefficients. Instead of (At x Lt)/(Aw x Cw), I like to think of it as (At/Aw)x(Lt/Cw), where cw is the MAC of the wing. This lets us see that this coefficient comes from the ratio of areas and is proportional to the tail length expressed in wing chords. I think this recognizes the contribution of the wing's chord to rotational inertia and the role of the tail moment arm in dealing with this. It incorporates its own scaling factor, and to me is an essential tail moment consideration that accounts for different sized aircraft; they require different tail moment arms - no magic number for all stunters. So wider wings benefit from longer moments. It has been posted here too that tail moment should be squared here too, that is, that it is a more powerful influence. That might be another limiting factor.

I'll think more about this, but - who knows?

**************************************

Here's some more calculated data from some scraps lying around (just realize that the sources of measurements are questionable in their degree of accuracy):

Igor Burger's flapped "Middle" (from converted measurements and measuring on-screen images):

Wing Area: approx. 424 in^2
Stab/Elev Area: approx. 114 in^2 (27%)
About 19% flaps and A/R = about 5.3
L (from monitor screen): approx 20"
MAC (from measuring chords on monitor and guessing MAC location): approx. 9"
TVC: approx. .60


Charles Mackey's flapped "Bluebird (computations from measurements of scale magazine plans with inconsistencies, some guestimates):

Wing Area: 374 in^2; A/R: 4.72; flap area: approx. 63 in^2 (only moveable part; 16.9% of wing area), MAC: 9.3" (guestimate from area, span and chord position approximation)
L(1/4 chords): 15.1"
Stab/Elev Area: 55.2 (14.8%)
TVC: approx. : .24

An un-flapped Peacemaker measured from bare-bones photo on SSWF (REALLY Approximate) and printed out magazine page plans. These are all in actual print-out measurements (cm) - NOT full-scale and in the photo, NOT perfectly from above:

Wing Area: 52.8+ and 58.24; Lt: 5.9 and 6.2; Tail Area: 4.3 and 5.62(treated as ellipse, 8.3%, 9.6%); MAC: 3.55 and 3.7
TVC's: .135, .162 (probably closest)

Circus King measured from print-out of distorted plan image (I think the print-out is fairly correctly proportioned compared to the monitor image):

Wing Area: 368 in^2; Stab/Elev Area: 63 in^2 (17%); Wing MAC: 9.4"; Lt (1/4-1/4 MAC): 14.9"
TVC: .27

Ted's "Imitation" (my figures vary slightly from his published ones). These are from scaling magazine plans from internet printouts and using actual plan chords and published span (plans were apparently distorted in copying):

Area: 616+ in^2; A/R: 5.65 MGC: 10.9" (from Palos Verdi, using measured tip rib and guessing lower), Ted: 10.8"); Lt (1/4 - 1/4) = 22.3", At: 140 in^2 (22.7%); TVC: .465

I have calculated several others, like the P-Force, Venus, Ares, Streak, my own designs, etc., from accurate plans, but can't dig these out, if they still exist.

This is taking a long time to type. I wonder how many will have respomnded by the time I post it. I may re-read and revise what I've said, but this run-on thing is what I could come up with for now.

SK

Serge –

A clear, understandable note!

The raison d’etre (at least for me) for all this messing about with numbers is simply to better plagiarize, copy, and blatantly steal ideas from the good and great designers. In any effort, the better we can use mathematical tools to describe the real world, the easier it is to extrapolate the results from one set of data to another, and a different situation. In other words, the purpose is not to attempt to do something creative or innovative, but to copy.

“. . . but I think that Ted, for instance, has said about all he wants to in his interesting posts. So here are some musings - as usual, FWIW. . .”

Yes, everyone has been very open about sharing results and insights. We have a lot of information to work with!

“First, a note on how I view the roles of stab and elevator. The stabilizer functions by exerting a moment contrary to any unintentional displacement. For instance, if the wing suddenly lifts more in a gust and pitches the nose up, the stab also pitches up (moves down, pitches UP). In a conventional configuration this creates lift behind the c.g. and creates a restorative (nose-down) pitching moment. So the greater the stab area, the greater the auto-restoring force. . . .”

“The elevator exerts control forces that increase with its area, and within limits, its deflection. These are caused by changing the camber of the horizontal tail surfaces, the "lift" being distributed unequally along the entire horizontal tail, stab and elevator both. The amount of deflection and position of the hinge determine the new lift center. . . .”

I didn’t record separate stab and elevator areas. As you know, for TVC calculations, it is one big tailplane, a possible shortcoming of using it as a general guideline for “copying” someone elses’s design. On flapped stunters, there does not appear to be much variation in elevator area to stab area. You may have hit on something for unflapped ones, though.

“I think horizontal stab and elevator influences differ from flapped to unflapped models. First, the need to overcome negative pitching moments from wing flaps requires a greater tail moment. So tail area must be greater and/or the tail length must be greater for flapped models. If the tail moment is too small, the pitching rate has to suffer for flapped models, and the downward force causing any pitch up must be greater, creating the need for greater wing lift. So there is a good argument for increasing the tail length. This also increases the longitudinal stability by increasing the restorative moment from the stabilizer. An opposing factor might be that downwash from flaps may actually increase the tail's efficiency, although asymmetry due to relative vertical positioning of thrust line and wing and tail chords may cause trim problems. Ted has pointed out the advantages in stability/C.G. placement from moving the a.c. of the whole plane rearward with long tail arms and greater horizontal tail areas on flapped planes. . . .”

“What I think I know about flapless planes...They can produce plenty of lift, but the tail must hold them at a greater pitch angle for the same weight (although they can be built lighter). Their tails require less area and deflection to do this, since they don't need to fight the flaps' negative pitching moment, AND the effective tail length might actually lengthen slightly some with elevator deflection in the absence of rearward c.p. movement along the unflapped wing chord . . . their tail volume limit seems to come from the need to have a more forward c.g. for "feel", i.e. hinge moment. . . .”

“So...it looks as though flaps require and benefit from larger stabilizer-elevator areas, with the elevator needing to be greater in chord and deflection as tail length increases. Unflapped models need smaller horizontal tail areas with less elevator deflection and a lower ratio of elevator to stab area. The best compromise may be a slight increase to tail areas on models like the Streak designed for combat, but considerably larger (16%-18%) horizontal tails and greater tail lengths, like those Phil, Mikey, and now Ted have suggested, for stunters. . . .”

Or maybe we should say we know that unflapped models need smaller elevators; and that the ratio of stab to elevator area is likely to be different for flapped and unflapped models; and that total TVC is may or may not be different with the relatively smaller elevators. If we keep the TVC on unflapped airplanes the same as on flapped designs, maybe we need only put a lot more of that area in the stab, with a correspondingly smaller elevator. Maybe someone has tried that approach, I don’t know.

“I like Al's way of writing TV coefficients. Instead of (At x Lt)/(Aw x Cw), I like to think of it as (At/Aw)x(Lt/Cw), where cw is the MAC of the wing. This lets us see that this coefficient comes from the ratio of areas and is proportional to the tail length expressed in wing chords.”

Agree. If you write it Al’s way, it’s readable, and you say to yourself, “Well sure, everyone knows that!”

I see all this as very applicable to anyone wanting to build “look-alikes” of older models, but with more modern performance.

Larry Fulwider

Well, we're really getting in a rut on these lost postings. I just tried to post this twice, and it failed to appear. Fortunately, again, I've copied the text. So here goes... AGAIN!

Larry-

I am a little disappointed that your post did not get more attention - and response! I was going to watch to see whether anyone had a truly quantitative description of how tail area should vary with length on a flapless design - i.e. what the TVC should be for a flapless stunter, as opposed to a combat/sport derivative like the Flight Streak. So far the areas seem to go from 16% to 20%. The varience of per-cent elevator with moment arm would also be interesting.

I suppose that there have been insufficient "experiments" like the Skyray, P-Force, and Derrick Moran's west-coast stunter to form "rules". I should go out and measure my SkyRay and P-Force (should be 18% horiz. tail, I think) and compare the numbers to Derrick's. I did just look up Derrick's post via the search function, and his horizontal tail is greater than Phil Cartier's or Mike Pratt's findings (16% - 18%) at 20% of the wing area. His TVC comes out at .42.

>If we keep the TVC on unflapped airplanes the same as on flapped designs, maybe we need only put a
>lot more of that area in the stab, with a correspondingly smaller elevator. Maybe someone has tried
>that approach, I don’t know.

That was my approach in designing the plane pictured in my "polishing" post near the bottom of the page. My numbers are really a lot like Derrick's - except WEIGHT, my higher aspect ratio, and my smaller percent elevator. I don't know whether my data will be fair to the design with all that weight, and I'm probably a couple percent high on horizontal tail area. If I find success at the flying field next summer, I'll post that. Ha! Anyway, the guys who know say that Derrick's plane flies really well. It looks great. You can see it if you do a search on "flapless". If I go there to copy the URL, I'll lose this post - experience. For some reason, using his or my names with the word "flapless" gets no real results. Anyway, his pictures and dimensions are on my post about tails for flapless stunters from at least a couple years ago. You can get some quantitative info from Phil and Mikey's posts through the search function.

EDIT: OK, I'm going to add a couple URL's here, now that this thing's finally posted:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=133145&mesg_id=133145&listing_type=search

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=91834&mesg_id=91834&listing_type=search#91835

Look for the posts from "mpa" near the bottom.

EDIT 2: Since I posted that material, I trimmed some from the stab, getting it closer to 20%. Looking at it now, I guess it really is close to 60/40, so not so different from Derick's.

Anyway, thanks for posting a good question. Maybe we'll learn more later.

SK

Serge --

Thanks! I'll check those specicic archives!

The good news is that if someone wanted to compete in intermediate with a .40 size flapped ship and no pipe, we can take most any of the classics, boost the TVC to .45 - .46, move the CG back a tad, and have some assurance it would be easier to fly and will do better in gusts than the original.

It seems to be only on the flapless we can't do quite the job of "stealing" well.

Larry Fulwider

One thing, though..

Everyone talks about a "negative pitching moment" from the flaps. Is this really the case? As has been pointed out elsewhere, on a real plane (GA, airliner, whatever) if you deploy the flaps without touching the elevator, does the nose pitch up or down? I'm pretty sure it pitches up, ask any full scale pilot out there. So deploying the flaps on an airplane with a tail results in a PITCH UP MOMENT, not pitch down. Tailess, that's another story.

The wing itself has that negative moment which requires greater tail input for equal pitching; that's a measured certainty in myriad reports. That's what happens when camber is increased without reflex. The stabilizing function of the horizontal tail must just keep the tail enough in line that the added lift of the wing still pulls the nose up relative to the tail. I'm not convinced that this always happens on full sized planes, but it may. I have read posts where the model has been described as pitching down when flaps were deflected without elevator input though. Obviously tailless combat types do pitch down and do tight outside maneuvers with negative elevator (flap) deflection.

SK

P.S. Hitting the road in a few minutes - again!!

>One thing, though..
>
>Everyone talks about a "negative pitching moment"
>from the flaps. Is this really the case? As has been pointed
>out elsewhere, on a real plane (GA, airliner, whatever) if you
>deploy the flaps without touching the elevator, does the nose
>pitch up or down? I'm pretty sure it pitches up, ask any full
>scale pilot out there. So deploying the flaps on an airplane
>with a tail results in a PITCH UP MOMENT, not pitch down.
>Tailess, that's another story.


Nearly all airplanes pitch nose down as the flaps extend. Without getting into arguments about CP travel with the changing shape of the airfoils, this effect is at least partly due to slowing as the flaps extend and loss of downforce on the tail as the airplane slows. Without refering to technical references, I'll just say that in my last 28,000 hours or so, I've encountered few airplanes which pitched up as flaps extended. (and I fail to seee the significance of the question)

Al

Al-

I was pretty sure I remembered my Beachcrafts pitching down with flaps deployed, but several here disagreed with me. I wonder whether flight techniques might differ....???

SK

>Al-
>
>I was pretty sure I remembered my Beachcrafts pitching down
>with flaps deployed, but several here disagreed with me. I
>wonder whether flight techniques might differ....???
>
>SK

Just because it is fun talking and understanding this stuff.

There are airplanes that appear to pitch up with flap extension but the reason has little to do with the pitching moment of the wing. I think it is safe to say that cambering an airfoil will always result in a negative pitching moment as we've discussed. That pitching moment is only one of the things that happens when the flaps are extended, however. What happens to the airplane to which it is attached depends on other factors as well.

Here's a few of them.

When the flaps are extended (or "deflected" in the case of our "stunt" flaps) the trailing edge, of course, goes down. If the deflection is a lot the trailing edge goes down a lot. The angle of attack of a wing is defined as the relationship of the ambient airflow to a straight line connecting the leading edge to the trailing edge. Thus, the deflection of a flap, by definition, increases the angle of attack of the wing to which it is attached. True for any airplane from a model to a 747.

The relationship between the angle of attack of the wing and that of the horizontal tail (stab and elevator) is referred to as "decalage" or, sometimes, longitudinal dihedral. In most airplanes the Chord line of the tail is set at an angle relative to that of the wing. The leading edge of the tail will generally be slightly "down" so that the tail will produce the download necessary to "stabilize" the wing.

(A good place to see this in real life is to watch an airliner like a 747 taxiing out to take off. Take a close look at the horizontal stabilizer and you'll see that it is very noticably angled leading edge down. This is the result of the stabilizer "trim setting" required to "stabilize" the wing in the takeoff configuration which is generally with a "moderate" amount of flap extension. If you could see the tail at landing you'd note that the angle is even greater, in part due to the greater flap deflection and lower airspeed on approach and landing.)

Now, you can see that on a "normal" airplane the act of extending the flaps increases not only the angle of attack of the wing through increasing the mean "camber" of the wing (which produces the negative pitching moment we've discussed)but also greatly increases the "decalage" between the wing and tail. Depending on a number of factors (including the size of the tail and the CG location relative to the CL as we've discussed previously) the net result might well be enough download on the tail to overcome the negative pitching moment and cause the aircraft itself to pitch up.

The DC-8, for instance, balloons pretty dramatically with flap extension and requires a pretty firm down elevator input to maintain altitude. Still, a lot of things go into that phenomenom, most of which is the result of the increased lift from the increased camber. As the airplane slows down the "eight" -- like all conventional configurations -- requires nose up trim as it slows to approach speeds. A topic for another day.

In our stunt ships, for the most part, with the controls neutral there will be little or no "decalage" because, in general, we set our ships up "zero, zero" with the symmetrical wing and symmetrical tail at the same "zero" angle relative to the centerline and thrustline.

Of course, we all understand that our stunters have to drive the wing to some tiny angle of attack in level flight to generate the lift for level flight. What does this is a tiny bit of control input that provides the decalage necessary to produce a tiny angle of attack and a tiny bit of tail download to keep the nose from dropping and, "voila" level flight. That tiny bit of down flap and up elevator actually produces a "tiny" bit of decalage.

The negative pitching moment is the force that will cause a flapped airplane to dive under the panicked "up" control the pilot gives when the pushrod fails between the flap and elevators. The reason the resulting dive is comparatively shallow is because of the decalage that results between the wing with the deflected flap and the stab elevator that now "faires" in neutral as a result of the failed pushrod.

If the tail were "lost" (departed the airplane) the rate of pitch from the deflected flaps would be much more rapid and the dive a lot more dramatic. The difference in pitch in the "broken pushrod" case and the "lost" tail case is the result of the decalage in the first case and the lack of decalage in the second.

At any rate, the bottom line is that an airplane may react in a variety of ways when flaps are deflected depending on "conditions".

Ted

Al is correct. Flaps down pitches the nose downward. Here is why. The CG is in front of the flaps. The plane rotates about the CG or MAC (can't remember). Flaps are lowered to increase lift. This upward force rotates the nose downward and drag is increased.

Bob Smiley

>One thing, though..
>
>Everyone talks about a "negative pitching moment"
>from the flaps. Is this really the case? As has been pointed
>out elsewhere, on a real plane (GA, airliner, whatever) if you
>deploy the flaps without touching the elevator, does the nose
>pitch up or down? I'm pretty sure it pitches up, ask any full
>scale pilot out there. So deploying the flaps on an airplane
>with a tail results in a PITCH UP MOMENT, not pitch down.
>Tailess, that's another story.


Hi All

Just for grins I will throw in a little info about this from my experience

My 152s both pitch up when the flap deployed

my 172s pitch nose up when the flaps are deployed

The 182s went nose up when the flaps went down

My Pipers pitched up when the flaps were lowered

The Beechcrafts I have flown pitched up when the flaps went down

206 TU goes nose up when the flaps go down

Gruman Tiger went nose up when the flaps went down etc etc

maybe pitching down only happens to passenger jet aircraft



The nose up pitching happened if you held the yoke perfectly still, or if you relaxed and let it push itself, In order to stop the nose up you had to push down fairly hard on the stick.

Regards

Randy

Yeah, Ted's explanation is what I'm getting at. Deploying the flaps doesn't just increase the camber of the wing, it increases the angle of attack with respect to the tail. If the tail weren't there, yes, you'd get a pitch down moment - that's caused by the flaps acting pretty much like "down elevator". But since the tail is there, now you've got, in essence, more "up elevator" due to the changed relationship between the wing's zero lift line and the tail's.

Since Al says most (or perhaps, all) the planes he's flown pitches down when flaps are deployed, perhaps things don't work the way I thought they would after all, but I've come across quite a few people (full size pilots) who say otherwise. I came across a discussion about this elsewhere (can't remember where, might've been Usenet) and the consensus seemed to be that deploying the flaps would cause the nose to pitch up.

I think an interesting experiment would be to take something simple with flaps (maybe a modified ARF Flite Streak, or ARF Super Clown) and fix the tail so that the elevator doesn't move. Set up the flaps to be the only moveable control surface. Take off and see which way it turns when the flaps go down. I read about someone who was flying a racer set up like this - fixed tail, only very small flaps for maneuvering.

While not quite the same thing, there were also some RC gliders (for slope flying) set up with fixed tails and all-flying (oxymoron?) wings that rotated in unison for pitch and in opposite directions for roll.

>>One thing, though..
>>
>>Everyone talks about a "negative pitching
>moment"
>>from the flaps. Is this really the case? As has been
>pointed
>>out elsewhere, on a real plane (GA, airliner, whatever) if
>you
>>deploy the flaps without touching the elevator, does the
>nose
>>pitch up or down? I'm pretty sure it pitches up, ask any
>full
>>scale pilot out there. So deploying the flaps on an
>airplane
>>with a tail results in a PITCH UP MOMENT, not pitch down.
>>Tailess, that's another story.
>
>
>Hi All
>
>Just for grins I will throw in a little info about this from
>my experience
>
>My 152s both pitch up when the flap deployed
>
>my 172s pitch nose up when the flaps are deployed
>
>The 182s went nose up when the flaps went down
>
>My Pipers pitched up when the flaps were lowered
>
>The Beechcrafts I have flown pitched up when the flaps went
>down
>
>206 TU goes nose up when the flaps go down
>
>Gruman Tiger went nose up when the flaps went down etc etc
>
>maybe pitching down only happens to passenger jet aircraft
>
>
>
>The nose up pitching happened if you held the yoke perfectly
>still, or if you relaxed and let it push itself, In order to
>stop the nose up you had to push down fairly hard on the
>stick.
>
>Regards
>
>Randy

Let me take my above comments a step further to make it clearer.

One of the things we did as kids was to "recycle" any salvagable wings from wrecks and make flying wings out of them. Everything from Ringmasters, Yaks, P-51s to even a Smoothie.

Without exception, when we flew these formerly "flapped" wings as flying wings (with no tails at all), when we deflected the "flaps" up the airplane went up and when we deflected them down the aircraft pitched "down". This occurred because of the pitching moment of the wing when it was "cambered" by deflecting the "flaps".

Most of us built flying wings as youngsters and we probably called the "flipper" on them an elevator ... because it seemed to work just like the elevator on our Ringmasters and All Americans. Although they seemed the same and the result was the same the result came from different aerodynamic forces.

It is worth noting that the guys that used to used flying wings in competition (combat flyers)quickly learned that they could get much better performance out of their airplanes if they moved that "flipper" off the trailing edge of the wing and made a "tail" of some sort out of it, either a stab/elevator or a flying tail.

The reason these airplanes flew (turned) so much better was because now the wing was acting as a wing (providing the lift required for maneuvers) and not trying to do double duty as both lift provider and attitude adjuster. A flying wing is a miserable configuration for maneuvering because it can only maneuver by making itself less efficient at its primary job ... producing lift.

To make a long story short, the only proof you need of the existence of negative pitching moments produced by cambered airfoils is that simple little Omega or Half Fast from the olden days of combat. The only reason they turned at all was the existence of negative pitching moments as a result of "cambering" the airfoil! And the reason you've gotta go to the olden days is because the guys who do that stuff really good quickly learned it wasn't a good way to do the job.

Ted

However, what I was writing about wasn't the pitching moment of the wing alone, it was the effect on pitch of lowering the flaps on a plane that also had a tail. I agree, the wing alone would pitch down. Combined with a tail, however, lowering the flaps would cause a pitch up because you have, in essence, tilted the wing up in relation to the tail.

Hi All
>
>Just for grins I will throw in a little info about this from
>my experience
>
>My 152s both pitch up when the flap deployed
>
>my 172s pitch nose up when the flaps are deployed
>
>The 182s went nose up when the flaps went down
>
>My Pipers pitched up when the flaps were lowered
>
>The Beechcrafts I have flown pitched up when the flaps went
>down
>
>206 TU goes nose up when the flaps go down
>
>Gruman Tiger went nose up when the flaps went down etc etc
>
>maybe pitching down only happens to passenger jet aircraft
>
>
>
>The nose up pitching happened if you held the yoke perfectly
>still, or if you relaxed and let it push itself, In order to
>stop the nose up you had to push down fairly hard on the
>stick.
>
>Regards
>
>Randy

Let me take my above comments a step further to make it clearer.

One of the things we did as kids was to "recycle" any salvagable wings from wrecks and make flying wings out of them. Everything from Ringmasters, Yaks, P-51s to even a Smoothie.

Without exception, when we flew these formerly "flapped" wings as flying wings (with no tails at all), when we deflected the "flaps" up the airplane went up and when we deflected them down the aircraft pitched "down". This occurred because of the pitching moment of the wing when it was "cambered" by deflecting the "flaps".

Most of us built flying wings as youngsters and we probably called the "flipper" on them an elevator ... because it seemed to work just like the elevator on our Ringmasters and All Americans. Although they seemed the same and the result was the same the result came from different aerodynamic forces.

It is worth noting that the guys that used to used flying wings in competition (combat flyers)quickly learned that they could get much better performance out of their airplanes if they moved that "flipper" off the trailing edge of the wing and made a "tail" of some sort out of it, either a stab/elevator or a flying tail.

The reason these airplanes flew (turned) so much better was because now the wing was acting as a wing (providing the lift required for maneuvers) and not trying to do double duty as both lift provider and attitude adjuster. A flying wing is a miserable configuration for maneuvering because it can only maneuver by making itself less efficient at its primary job ... producing lift.

To make a long story short, the only proof you need of the existence of negative pitching moments produced by cambered airfoils is that simple little Omega or Half Fast from the olden days of combat. The only reason they turned at all was the existence of negative pitching moments as a result of "cambering" the airfoil! And the reason you've gotta go to the olden days is because the guys who do that stuff really good quickly learned it wasn't a good way to do the job.

Ted


Hi Ted

I of course know what your saying and I was not disputing it,I do realize the negative pitching moment happens , I was strickly addressing the question of Isky's about what happens in a real airplane when the flaps go down. And of course since these have extended fuselages with stabs rudders and Vertical stabs, well that would be the differance.

Regards

Randy

Since Ted Fancher and I are the only one's I've heard about seriously designing unflapped planes, here are some pertinent experiments. This is not to discount some of the excellent flights I've seen with unflapped planes. Kaz Minato put in a heck of a flight at Brodak's a few years ago with a Humongous, which shows that in good hands an unflapped plane is not at any real disadvantage. Larry Draughn(sp?) has also put in some excellent flights with both the Humoungous and Jamison Spl.

Photo #1 is Big Foot which started out as an oversize Scrapper. I used a spare set of Giles cores and laid out the smallest all balsa fuse I could, which turned out to weigh almost as much as much bigger foam core fuse. You may be able to pick up in the photo where I grafted 2.5 in. onto the tail moment. Specs: 60 in. span 740 squares 42 oz. Wing Loadin 17.6 sq.in./oz 108sqin. stab(15%) Tail moment 20(orig) 23(final). TVC=.23 The elevator was also slimmed down to 2 in. wide x 24 in span. The orginal 2.5 in. elevator was just to stiff if the plane picked up any speed.

Photo #2 is the current Giles. This one came in at 44 oz. Span 64 in. area 780sq.in. stab span 28 in. 168 sq.in. 22.5 in. moment. Wing Loading 17.7 sq.in./oz. TVC=.39 An excellent flying combination.

Photo #3 is the Interceptor design, higher aspect, with much more taper. Span 63 in. Root Chord 15.5 in. Tip Chord 6 in. Area 680 sq.in. Weight 45 oz. Wing Loading 15.1 sq.in./oz. Stab 176 sq. in. 28 in. span. 25 in. moment TVC=.59

The results are pretty much in line with my expectations. The BigFoot flys a lot like an old tyme combat plane like the Scrapper. Quick in and out of turns. Should turn a lot tighter, but it came out too heavy with all the paint. Speed limit combat planes have wing loadings around 27 sq.in./oz. and turn much tighter. Not terribly stable or pointable, but it flys well. With an 11 in. stunt prop there is a lot of yaw from precession.

#2 is a honey. Flys much like any flapped competition stunter. Competitive corners, pretty solid starting and stopping turns. It likes to fly on 62.5 ft. lines at 52 mph. Slows to about 45 mph in full control maneuvers, which you can do almost any where you like in mild breezes. It really needs the Rabe rudder to control the prop precession. The lower aspect wing, with the fairly broad tips, is susceptible to gusts. They will bounce you around.j

#3 also flys very well, but doesn't turn quite as tight. The excess weight is mostly in the fuse wood. Turns very sharply(for its weight) and is very steady, with the relatively huge TVC. The shallower fuse profile is less sensitive to prop precession, but it still needs the Rabe rudder. Very pointable with very solid level flight.

Like I said above, it's hard to see any upper limit for TVC at this point. As long as the tail moment and stab area are balanced for the wing, and you work out how to limit the control loads so the plane will will still turn in windy weather, excellent flying planes are possible. The highly tapered wing on the Interceptor really tames it down in gusty weather, with little boucing around. I'm now trying to apply all this to a more manabeable size plane. Something in the 500 sq.in. range.

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Hey, great info, and too good looking for "Test-Beds"!

>
>Photo #2 is the current Giles. This one came in at 44 oz.
>Span 64 in. area 780sq.in. stab span 28 in. 168 sq.in. 22.5
>in. moment. Wing Loading 17.7 sq.in./oz. TVC=.39 An
>excellent flying combination.
> . . .
>#2 is a honey. Flys much like any flapped competition
>stunter. Competitive corners, pretty solid starting and
>stopping turns. It likes to fly on 62.5 ft. lines at 52 mph.
>Slows to about 45 mph in full control maneuvers, which you can
>do almost any where you like in mild breezes. It really needs
>the Rabe rudder to control the prop precession. The lower
>aspect wing, with the fairly broad tips, is susceptible to
>gusts. They will bounce you around.
> . . .
>I'm now trying to apply
>all this to a more manabeable size plane. Something in the
>500 sq.in. range.
> . . .

Please post your results with the 500 size. I am assuming you are scaling down #2, with maybe more wing taper?

>#3 also flys very well, . . .
> . . . The shallower fuse profile is less
>sensitive to prop precession, but it still needs the Rabe
>rudder. Very pointable

Never heard of the relationship between shallow fuse and precession resistance! Thanks for that tip also.

Can't wait to see the 500 size!

EDIT: I forgot to ask -- % stab area / elev area of total tailplane area?


Larry Fulwider

thanks Larry, they look better in pix 'cause you can't see all the bloopers. Rustoleum over the plastic covering, primer, and then paint. I don't even try for a 20 pt finish, just something nice to look at and not too hard to mask. If I don't count sanding for painting, a plane like this takes me about 20 hrs to get ready to paint, and a couple hours for final assembly and controls after painting. Not too bad for trying out ideas.

These all use elevators that average about 2 in. wide and about 40% of the total stab/elev area. That is about the max for a 3 in. bellcrank. I'm trying a 4 incher next.

As far as the fuse profile and precession, I'm not sure if it is the tall rudder on the Giles or the overall shape of the fuse and tail, the longer tail moment, or something else, like weight. It can't turn as tight, so it doesn't turn as fast, reducing the precession effect. The Bigfoot also has a lot of precession reaction, but there is no place to put a decent moving rudder. The Interceptor flies with minimal yaw reaction and has a smaller rudder with less movement than the Giles. I have noticed that there aren't too many regular stunters that have much more than a token fuselage. They generally don't have tall fuselages or tall rudders- Al Rabe's stuff is a big exception.

I did screw up some of my info. Tommy Luper has been cleaning up with the Humongous, not Larry Draughn, although I'm pretty sure Larry has put up some excellent flights with similar planes. Clay Smith also reminded me I skipped his Teosawki design. They have done really, really well in the Southeast. The picture is from Brodak's last year. You may recognize the faces, but I can't pull all the names out of the memory banks.

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edit for spelling

Attachment #1, (jpg file)

Phil,

Cool airplanes. I like a lot of the ideas. In particular, I think you did the right thing with the reduced chord elevators. One of the things I've done with the Doctor is to cut off some of the elevator trailing edge which actually improved the corner. I also used a three inch crank and it simply didn't have the requisite authority to displace the elevator under low tension conditions. Reducing the chord improved that.

One thing that surprised me were your comments about precession and the need for a moveable rudder. I couldn't see a spinner or any noticable weight on the crankshafts. On airplanes as big as these I wouldn't have expected much in the way of precession effects. What engine and props are you using on them? What kind of RPMs? What are the symptoms that lead you to suspect precession as the culprit?

Also of interest were your comments about less corner than desired on the #1 ship. In my world (strictly stunt)the wing loadings you're talking about are pretty darn low. I'm curious what the limiting factor on corners is. Does it simply not turn or is it actually stalling from excessive angle of attack?

Neat airplanes.

Ted

As a flight instructor, I agree with Randy's summations of airplanes pitching up when flaps are deployed. Al is just going to have to fly a light airplane.

Which makes me wonder why the heavy metal will pitch down when the flaps are deployed. Are they somewhat different than light planes? I can think of three differences, and a possible fourth.

1) On an airliner, deploying the flaps also increases the wing area. A lot. Segmented/Fowler flaps. Not sure why this would result in down pitch, though.

2) The flaps are segmented, with big gaps in between the elements. Lots of drag (especially in landing configuration), and the flaps go down a lot, especially when landing. Drag beneath the thrustline?

3) Ted wrote once, long ago, that the most efficient arrangement for airliners was an unloaded tail. They achieve this by adjusting fuel fore and aft until the tail is just barely loaded. Again, not sure of the significance. On light planes, I imagine there's a fair amount of down force on the tail.

4) How about the size of the tail as a percentage of the wing? Does it differ much from a light aircraft?

Ted, Phil,
Very interesting discussion!

>One thing that surprised me were your comments about
>precession and the need for a moveable rudder. I couldn't see
>a spinner or any noticable weight on the crankshafts. On
>airplanes as big as these I wouldn't have expected much in the
>way of precession effects. What engine and props are you
>using on them? What kind of RPMs? What are the symptoms that
>lead you to suspect precession as the culprit? Ted

Phil,
And a follow-up question to that last one: have you tried flying the same plane both with and without the rudder wiggle? I understand that trimming a moveable rudder involves the interaction of several variables. At the same time, this issue seems to involve such a confused mix of science and black magic that it might be helpful to have smoking gun evidence from the same plane, properly trimmed and then retrimmed, with and without the wiggly. My 2 cents.

Kim Mortimore

Kim, I did fly the Giles first without the moveable rudder. It flew well, but getting smooth corners was tricky, trying to get the radius right, getting the control input smooth an fast enough but not pulling it too tight and getting the tail wiggle. I've read all Al's stuff on the moveable rudder, and Brett's comments that adjusting a full size rudder one turn on a 2-56 clevis made a big difference in yaw. So I sized a rudder about 1/3 of the scale size, chopped it out, and hinged it, hooking it up with ball links. Per Al's advice I set it up with most of the movement to the right on down(about 20 deg) and just a touch of left movement on up(about 2-3 deg).

It worked so well on the first flight I've hardly touched it since. I didn't have any black magic problems. About the only thing is not to use a "full scale" rudder. It will be way to big on a standard stunt plane and need very little movement, which is hard to set and trim. Something in the range 1.5 in. chord and 3-4 in. tall works pretty well.

Precession is kind of funny. It depends mostly on the diameter and weight of the prop(moment of inertia),the rpm, and how fast the plane turns. Adding just a foot to the turn radius can make a big difference.

The other thing is how the plane handles the swinging of the wing tip from precession. On a heavier plane, which is pulling harder, the freqency of the yaw is higher and lower in amplitude, so it is easier for the pilot to adjust to. These planes are pretty light so the preccession yaw can get pretty violent.

I put Matt Kania Perky, Ringmaster, etc. tail feathers on just about everything I build -- I like the looks -- and I size them to "look right."

I enjoy the PROCESS of building. Over engineering is not part of my process. I don't know for sure but I don't think Kania or Saftig were into much slide rule design.

>I set it
>up with most of the movement to the right on down(about 20
>deg) and just a touch of left movement on up(about 2-3 deg).
>Phil

20 degrees of right "mini-rudder" at full down elevator?

Kim Mortimore

That is correct. Actually, you got me interested and I did some rough measurements. The elevators move about 22 deg(and I do use it all at times) The rudder moves right to 30 deg on full down and left about 3 deg to neutral on full up.