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Author Topic: Moving elevator to trim  (Read 488 times)
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3view
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« on: September 10, 2021, 04:12:37 AM »

I’ve heard a few times now that it is bad practice to trim a (scale) model by deflecting the elevator (up or down). Thus introducing camber. It is better to leave the tailplane section symmetrical and adjust the incidence of the whole surface?

What are your thoughts on this?

Steve
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lincoln
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« Reply #1 on: September 10, 2021, 04:30:13 AM »

I doubt if two or three degrees would be a problem. Even more might be ok. You might try looking at photos of the prototype in the air. I wouldn't be surprised if some of them have noticeable elevator trim with certain pilots.

Obviously, for complete aerodynamic perfection, it's probably better to have almost no elevator trim. But at what speed? You have to have some elevator trim if you're going to fly at different speeds.
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billdennis747
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« Reply #2 on: September 10, 2021, 04:38:52 AM »

Hi Steve
Since scale tailplanes are usually small I think it is important to have little or no deflection (certainly no up elevator) which may reduce efficiency. Using the incidences I do, I rarely need any deflection but will change the whole tail angle if necessary
Bill
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DHnut
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« Reply #3 on: September 10, 2021, 06:39:54 AM »

What is your wing incidence? There should be a 3 degree difference between that and the tail ie + 3 deg wing and 0 deg tail incidence. I use elevator to do final trim once the CG has been established and it works well. Power trim is side thrust ballanced with rudder.
There are some excellent guides on the webunder the 10 steps to trim a model.
Ricky 
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3view
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« Reply #4 on: September 11, 2021, 03:28:05 AM »

Thanks for the replies. Interesting to think that a bit of up elevator might ruin the efficiency of an already small/marginal tailplane.
I use 3 degrees of decalage and usually end up with some up elevator. Might increase this to 4 degrees?
In the future do I cut the tail off and reseat it after initial trimming or engineer an adjustable tailplane mount?

Steve
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billdennis747
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« Reply #5 on: September 11, 2021, 04:00:13 AM »

Steve, I just lash it together and chuck it at some grass. It's enough to get a good idea and big changes thereafter are never needed.
I use 3 degrees on biplanes and HWMs (of whatever colour - it seems to make no difference) but it may be that low wingers need more. I think Ivan has found this.
Bill
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lincoln
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« Reply #6 on: September 11, 2021, 06:02:47 PM »

Talking about decalage is very fuzzy unless you're going by the zero lift angles of the respective airfoils.

Barring something really weird, a tail surface with a bit of elevator trim will have the same lift curve slope as a completely symmetrical one, so the "efficiency"  of the tail shoukd be about the same, except for a small difference in drag. Unless we know the actual load on the tail in flight, we can't even say for sure that no trim is lower drag than a minor amount of trim. Furthermore, symmetrical shapes often have a non-linear lift curve near zero lift, which might require a slightly larger tail.

I suppose it's possible to have an absurdly forward cg or enough flaps, slats, etc. that the tail might stall first. However, in that case, camber or trim will work better. Check out the tail on a CH701.
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packardpursuit
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« Reply #7 on: September 12, 2021, 11:57:58 AM »

There are too many big words and math in these explanations! So I just ignore everything and do what I want. It's been sort of working, for  66 yrs. Roll Eyes

I generally use a much lower degree of angular dihedral than is considered standard. My full scale Aeronca KCA had about 2 degrees (1 deg positive at wing and 1 deg negative fixed at stab. It trimmed for take off/landing/cruise with tab on elevator. I don't think there was any noticeable efficiency difference than say an adjustable stab of Piper J-3 etc. But I sure used to wonder if there was a difference!

As a kid, one of my best flying designs was an all sheet balsa low wing,(flat sheet surfaces)with 0-o wing stab set up!! IIRC it was first time Id used 1/8 flat rubber strip, from SIG. Model was loosely based upon a Walt Mooney pre-Peanut Piper Pawnee in M.A.N. I literally wore it out testing different motor lengths and changes to balance etc. If I needed to change trim I would bend something, or add ballast. IIRC it always ended up close to original configuration, with minimal elevator trim, where it flew best. It was a whole year before I discovered mechanical stretch winding and the local FF field.


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3view
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« Reply #8 on: September 12, 2021, 04:26:03 PM »

Thanks guys for your input.

My Chipmunk refused to ROG until the model was retrimmed with a more forward CG and some up elevator.

Steve
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Yak 52
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Jon Whitmore



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« Reply #9 on: September 13, 2021, 06:12:50 AM »

What are your thoughts on this?

Hi Steve,

Most scale models will have small enough tail volumes such as they are providing a constant downforce, so up elevator is actually creating the right kind of camber. A very small tail area needs to provide a fairly high lift coefficient and may stall under certain circumstances. Cambered airfoils can 'do a bit more' than a flat tail of the same area so a little up elevator may actually be helpful. But overdoing it will be adverse (introducing more than about 5% total camber) especially in the case of very small tailplanes which are marginal. Too much camber and low Reynolds numbers are a bad combination.

The other consideration is tail airfoil 'deadband'. I won't go into detail here but a flat plate tail can sometimes suffer from this. A slight camber may move the deadband away from the 'in trim' load CL and so improve stability. Again, a very minor effect unless you happen to run into a problem with it.

All of this is pretty much fine tuning and not a big issue unless your model is the particularly difficult end of the trimming spectrum. There's a few different things going on here where the effects are all small and therefore difficult to sort out. Drag efficiency is not really a consideration (fractions of a percent of the total drag) unless you are soaring or flying rubber duration. It's another one where there is no rule - what helps with one model might hurt a different design.

To summarise - a little up elevator (like 10 degrees max) won't hurt. But big deflections can kill the tailplane effectiveness, a problem if it's already marginal.


Jon
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3view
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« Reply #10 on: September 14, 2021, 01:42:11 PM »

Thanks Guys, I guess that I'm going to stop worrying my poor little head about all this and carry on trimming with the elevator.

Steve
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lincoln
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« Reply #11 on: September 15, 2021, 05:03:09 PM »

packardpursuit:
Big words and math don't add any drag to the model, and can be fun. IMHO, school was boring despite those things, not because of them.
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packardpursuit
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« Reply #12 on: September 17, 2021, 07:16:14 AM »

"Big words and math don't add any drag to the model..."

But they can also add a great deal of un-nescesary mental wheel spinning, which can be a form of drag. Grin
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