Engine offset, 2 degrees?

I see that stunt models have the spinners tight with the fuselage shap.

That is, the offset is brought into the fuselage so the spinner lines up with the offset.

What is the offset most used because it looks as if it's never changed.

My guess is 2 degrees. Just a guess.

Charles

>I see that stunt models have the spinners tight with the
>fuselage shap.
>
>That is, the offset is brought into the fuselage so the
>spinner lines up with the offset.
>
>What is the offset most used because it looks as if it's never
>changed.
>
>My guess is 2 degrees. Just a guess.
>
>Charles
>
>

A lot of people now use 0 or 1/2 a degree. I'm using 0 engine offset. There is a reason for it, but will leave that up to the experts to explain.

I have heard that 2 degrees is about the accepted maximum, but even if you set the model to fly at the perfect tangent to the circle with 0 degrees of engine off set you will find that the models path will still angle away from the pilot.

Now who is good at trigonometry and can figure out what the flight path angle is with a zero degree offset engine?

Generally with the big planes, use 0. If you have engine offset, the plane will fly nose out which causes drag. You don't want drag. As the plane slows in certain maneuvers, the nose will go farther out, causing more drag, and slowing further, greatly reducing line tension, particularly in the top of the vertical 8, hour glass and overheads.

As the plane flies more tangentially, this nose out tendancy is reduced, less drag, doesn't slow, which give more line tension. Somewhat backwards to older convention and counter intuitive but can be demonstrated consistently.

So, no engine offset and little to 0 rudder offset for the same reason. My last 2 ThunderGazers have 0 engine offset, and about 1/2 turn inboard rudder, because it pulls too hard in certain situations.

Since our stunters fly pretty much in a crab all the time, I've wondered if a little port engine and/or rudder offset might actually be in order.. with proper trimming of course.

I know that seems really counter intuitive, which might mean it would work. ;->

We know the position of the bellcrank axis with respect to the wing tip leadout exit is of no (or tiny) consequence (per flap/bellcrank 3D geometry and/or leadout friction, say). However, it would seem that the CG of the model could influence its yaw, and the more its misalignment the greater the fluctuation of yaw with the resultant effects on drag, etc.

Wider fuselages, presenting thicker airfoils for more orthogonal "lift" (a la our stunter wings) and thereby generating more line tension is an idea! Is it a good one?

Comments..



"My milkshake brings all the boys to the yard,
And they're like: It's better than yours,
Dam.n right it's better than yours,
I can teach you,
But I have to charge" -Kelis

In the early days of CL flight, there seemed to be a paranoia about keeping the lines taut during flight. Results were the heavy use of engine and rudder offsets. We've learned a lot since then, such as:

Those trim features more often screw up a model's performance than really help it. CF, with a cleanly aligned model is usually ample.

If we have the model trimmed so that the pull force(*1) from handle to CG is at right angles to the fuselage long axis, we are flying "tangent" to the flight circle path, right?

The flight path is round; the fuselage axis is straight. The forward fuselage has a small 'nose-out' angle - from the flight path - when we fly "tangent" as described above. Adding out-thrust, with regard to the fuselage axis, adds to this "natural" angle.

Same-same, a slight, but larger, 'tail-out' angle is present back at the fin/rudder location. Adding rudder offset, again, adds to the "natural" angle.

IF the line pull aims correctly through the leadouts to the CG ('pull' being the actual force carried in the lines along their actual shape during flight) we get the fuselage centerline tangent at the CG.

That is the yaw attitude with the least undesired yaw, overall.

Very slight quibble, Larry? I don't consider a CL model 'crabbed' when it flies tangent. "Crabbed:" I think of a straight fuselage aircraft flying a straight path that is not aligned with the fuselage.

The curved airflow _path_ meets the fuselage nose-out forward of the CG and tail-out aft of it. The angles to the airflow probably are similar to what we'd have if we bent model's path straight, and curved the fuselage centerline outwards at the flight radius.

Didn't Hal deBolt's Speedwagon use a curved fusleage centerline - with "in-thrust" - to fit the pieces to the flight path curvature? Might have helped ease the pull of a .60 speed ship... Plenty of CF from simple airspeed/laptime...

(*1) - The 'pull force direction' is NOT there in the lines or any other physical parts of the model. It is along an ideal straight line between the handle and the CG. Physically, there is always some sag aft due to the air drag of the lines (see: the LINE*** apps), and a much smaller sag downward from their weight. These influences 'resolve' into a single force which 'shapes' the lines in flight.

(edited)

>The flight path is round; the fuselage axis is straight. The forward fuselage has a small 'nose-out' angle - from the flight path - when we fly "tangent" as described above.
>Adding out-thrust, with regard to the fuselage axis, adds to this "natural" angle.

Lou-

I'll disagree on just this one point. While the nose-out angle exists relative to the air direction ('relative wind') ahead of the c.g. of a plane flying tangent to the circle without engine offset, that would be only with respect to the future flight path. The nose (thrust line) is directly on the line tangent to the circle, and the thrust acts tangent to the circle at the plane's c.g., whose position defines the position of the circle. So the thrust is directly along the flight path of any plane flying with fuselage tangent to the circle, unless the plane employs out thrust (engine offset) otherwise balanced to hold the fuselage tangent to the circle.

SK

A propeller is not a rocket acting in a vacuum. The main phenomenon causing a side force from a propeller is from it changing the direction of air passing through the prop plane at an angle. I discovered this when I felt the difference that a couple of degrees of engine offset made to line tension in a combat plane flying level.

A 60%-efficient prop on a combat plane with a 1 HP engine flying at 110 mph has 2 lb. of thrust. Thrust * the sin of 2 degrees is 1.14 oz. Two degrees of engine offset gives a whole lot more incremental line tension than that.

A corollary is that two degrees of offset will reduce thrust more than (1 - cos (2 deg.)) x thrust = .02 oz. That's maybe why engine offset isn't a good idea if your engine is puny. Electric power will enable us to measure a lot of stuff like this. You can do a trade study to see how much battery weight an improved hourglass corner will cost you.


Howard Rush
Bellevue, WA

Serge, to your #9...

Of course you describe the thrust line relationship correctly. I did not address that in my comments. Thanks for adding it to the thread.

I described the airflow as it meets the fuselage.

The effect is slight, and I imagine Hal deBolt's Speedwagon had a bit more curve than needed to 'fit' the flying circle curvature. Consider his All American stunters... and I believe he flew clockwise? Any engine torque reaction to the mounts would also have tended to roll the model out (in upright flight.) His SWAG at the amount of extra inboard panel area needed was "a bit on the high side," no?

I did use an AA,Sr with a left-hand Fox 35 as long as the model lasted. Worst effect was my tendency to punch the pavement when flipping the prop clockwise. Flew about identically to any AA,Sr, but didn't need a wart to fly level. ...took out the tipweight, too, BTW.

Howard and Lou:

Agreed!

As a side note, one of our experienced club members put about 2 degrees of "out thrust" into an "Oriental" a few years back and discovered several seemingly unrelated problems, as well as insufficient line tension in take-offs and certain maneuvers (I've forgotten the details). After following my suggested lowering of the flaps by a degree (right-left rod thread twist adjustment) without success, he finally realized that he'd marked the crutch for an upright engine, thus building in a couple degrees of IN-thrust. Among other things, that probably created an inboard yaw moment aggravated by wing asymmetry inherant in the design. Needless to say, it flew a lot better, after he'd reversed the engine mounting bias.

SK

>In the early days of CL flight, there seemed to be a paranoia
>about keeping the lines taut during flight.

In the early days outboard wing tip weight was often not considered so other methods of 'trim' may have been exaggerated in an effort to compensate.

The plane that finished 2nd at the Nats and Team Trials had 2 degrees offset built into it. Made the third corner of the hourglass better!

Good luck!

Are those running L.H. props using ' engine ' offset ?

P Walker,

Some degree or degrees make since to me. No degrees doesn't sit well with me, although, what's said does make some since. I know so little.

Do you know the 2 degree model and the builder.

OK, what about windy days?

Is that adjustment made at the wing tip with an adjustible leadout guide?

Charles

>Do you know the 2 degree model and the builder.

I've heard a rumor that P. Walker has slept with that guy's wife.

Howard Rush
Bellevue, WA

>P Walker,
>
>Some degree or degrees make since to me. No degrees doesn't
>sit well with me, although, what's said does make some since.
>I know so little.
>
>Do you know the 2 degree model and the builder.
>
>OK, what about windy days?
>
>Is that adjustment made at the wing tip with an adjustible
>leadout guide?
>
>Charles


As a matter of fact, that was my plane.
Several years ago I ran a series of experiments with respect to engine offset. This was of course on electric planes with tractor props. I found that 2 degrees was optimal. I then built all electric planes after that with 2 degrees offset.

Now, the plane used at the TT this year has a pusher prop on it. Have I repeated the experiment to see if it is needed for a pusher prop....Well no. Too many other things to do to get ready.

After returning from the TT's, I have retrofitted an older Impact and flew it with a pusher prop. That plane has no engine offset. It flies fine as is. I have tried it with a tractor prop and it clearly didn't fly as well. The difference was a surprise to me. It could easily be the difference in the two props, but they were supposed to be very similar. 13 * 4.5, tractor and pusher.

Both planes weighed 62 ounces as flown. The Impact with no offset had more line tension than did the Impact XS with 2 degrees offset at the same lap speeds. Probably more to do with the side area distribution of the two fuselages. The Impact has more side area aft than does the Impact XS.

So where does this leave you? I would suggest that it is not a very significant effect. If you feel you "need" it, it's likely not going to hurt. If you feel you "don't need" it, it's likely not going to hurt you.

Do what you think you want to do. It will work.

P.

The best thing to do actually, is follow what others have done for best performance.

Pointless and foolish to reinvent the wheel.


Charles

But they change their minds faster than I build.

Howard Rush
Bellevue, WA

Howard,

I build slow also.

AADHD. And I'm not kidding.

OK,

What do we have here?

A trick photo?

Or some "offset" about to happen on this Stiletto?

CB

[photo not recovered: 40123.jpg]

Attachment #1, (jpg file)

>OK,
>
>What do we have here?
>
>A trick photo?
>
>Or some "offset" about to happen on this Stiletto?
>
>CB
>
>[photo not recovered: 40123.jpg]
>

Looks like the "good old days". Back then we put one or two washers under the front mount on a profile. Or we cut wedges to put under the engine. Brodak sells wedges for this exact purpose. Oh, and we flew them that way and had lots of fun.

But they aren't needed. When you are attempting to trim the plane accurately (and we didn't know what that was back then) they get in the way. Just mount it at zero unless...

Ah, yes, the exception to the rule. If you have a lot of assymetry (inboard wing longer than the outboard) then a bit of offset may be a help. The farther outboard you mount the engine the more tendency there will be to turn the plane inboard, and a bit of outthrust can balance this. Now, remember, I said "a bit". It is not like as "if a little is good, then maybe a lot is better". It isn't.