Oh, BTW the stab has a + 1/32 built in incidence at the LE.
Wow, Crist, I am not a fan of so much incidence in the HT. Can lead to elevator issues like you have.
I would make the elevators and flaps level-level and adjust the bias on your handle to compensate for this trim issue.
You didn't say, but is the motor 0-0 with the wing CL?
Shim the motor for some up thrust, worked wonders for me.
MM
I didn't mention it but this Legacy 40 is electric powered with reverse rotation prop.
Oh, BTW the stab has a + 1/32 built in incidence at the LE.
Wow, Crist, I am not a fan of so much incidence in the HT. Can lead to elevator issues like you have.
I would make the elevators and flaps level-level and adjust the bias on your handle to compensate for this trim issue.
You didn't say, but is the motor 0-0 with the wing CL?
I didn't mention it but this Legacy 40 is electric powered with reverse rotation prop.
Flip it back the normal way! The entire notion of needing slight positive in the tail (either from building it in (like mine), or rigging it in with elevator (like an Impact)) is to compensate for the nose-up precession due to the constant left yaw rate and the normal rotation prop. Reverse the direction and I don't know what it should be.
I thought almost everybody had switched back to normal rotation - I rarely see backwards anymore.
Brett
Paul,
When you referred to "adjust the bias on your handle" do you mean to move the down line closer to the middle of the handle thus reducing the amount of down?
I appreciate your help and comments. Thanks.
Crist,
Knowing you pretty well I expect the answer to this will be yes...but just in case. Are the hinge lines sealed?
I have resolved multiple examples of uneven turn rates upon discovering the hinge lines weren't sealed and then doing so. Embarrassed to admit that the last really bad case was Lanny Shorts ship at VSC last year that almost crashed in every outside turn. Knowing Lanny was a pretty savvy guy the possibility of non sealed hinges never occurred to me until Bob McDonald mentioned it as I was standing around pretending to know all kinds of "stunt stuff". Well, they weren't and after we got them sealed up the airplane flew just fine inside and outside and became competitive.
The reason for always sealing the hinge lines (elevator AND flap) is the possibility of the gaps opening and closing as the movable surface deflects thus increasing/decreasing both the lift produced and the resulting pitching moment of the surface (tendency for the total surface to pitch in the direction of the movable surface's deflection...thus the deflected flap's pitching moment fights the effort of the stab/elevator to pitch the airplane the desired direction). More rapid outside turns could thus be the result of either/both of the elevator hinge line closing with down inputs (increasing lift produced) and/or the flap hinge line opening thus reducing pitching moment and reducing lift. As a result outside turns are much more abrupt and often have a difficult to constrain tendency to tighten.
That latter comment might sound odd but is a perfect example of how negative pitching moment of the flaps retards the rate at which the aircraft will pitch in the desired direction. More pitching moment simultaneously increases lift AND pitching moment. The result is that although you've got greater lift the rate of of aircraft turn is decreased...corners are softer and take much more deflection, thus handle input). Sort of flies in the face of the zero ounce per square foot wing loading generally considered the Holy Grail for a winning stunt ship, doesn't it.
Ted
Ted
It's worth checking out if you haven't sealed them but I suspect that Brett's suggestion might have more merit if you are, in fact, running a left hand rotation (bassakward) prop.
Yes, I had assumed the hinge lines were sealed, thus the comment.
Paul
I've dialed in down elevator to reduce hunting (after the hingelines are sealed). Helped with my plane anyway. Though I found out when using a reverse pitch props that that trim tidbit didn't work anymore.
Hey Crist... I can have Helga come down and fix it for ya VD~ VD~ VD~ VD~ Please mw~ mw~ mw~ mw~ ouch,ouch
just kiddin.....not !!! LL~ LL~ LL~
I didn't mention it but this Legacy 40 is electric powered with reverse rotation prop.
That doesn't matter to up thrust.
MM
What I'm saying is, if the outside loops are tighter, it might have too much down thrust so, shim it up a little and see what it does. If it creates some other problem, it's easy enough to take the shim out. He said it flys tail low, also a sign of too much down thrust.
Here is the latest on this thing. I installed a washer (.030) of up thrust which gave me about 1 1/2 degrees. I then flew the airplane and had to readjust my neutral on the handle. When that was done, the airplane was flying with a nice tail position. Then I tried an inside loop, nice, then the outside, way too tight! Just like before. I didn't get the chance to try it again this weekend because of our contest.
After getting home from the contest and taking a good nap, I put the plane on the bench and measured the stab incidence. Sure enough, it had some positive incidence.
I checked the plans for my electric profile that I fly and sure enough I had spec'd the same 1/2 degree positive incidence and it flys with a nice tail position.
That got me thinking so I checked the L-40 control throws. The flaps had equal amounts of throw, while the elevator had about 1/8 inch more down than up. Now the 1/8 inch was with the elevators off and measured at the elevator control horn. Up was 5/8 inch while down was 3/4 inch. Not good. This got me thinking about "differential aileron" on our R/C ships where the throw is more one way than the other. This is accomplished by tilting the control horn forward (before computer transmitters). I though that I would try this approach before I cut the stab. I would tilt the elevator horn forward when the flaps are neutral to give me more up than down. Just like biasing the lines on the control handle, right? I could tilt the horn to give equal amounts of elevator throw, but I decided to go even further and have more up than down just to make sure. About 1/8 inch more up than down with the elevators installed.
In reality, I titled the elevator horn only slightly and looking at it more closely, the bolt for the control horn is just ever so slightly forward of the hinge line.
Paul Walker and Motorman have weighed in
More pitching moment simultaneously increases lift AND pitching moment.
If you draw a line from the hinge line to the push rod attach point, that line should be at 90 degrees to the push rod when the elevator is in neutral and the push rod is mid travel. That's how you get equal elevator travel up and down.
Only in one specific case.
...why would you have down elevator with neutral flaps, precession?
Thanks Randy. If I can get the model to turn the same in each direction without being "tail low", or really in proper trim, then I'll work on the hunting thing. Right now I gotta get this thing trimmed out. As it is, I won't fly it in this weekend's contest here in Dallas....sigh. I'll keep working on it till I get it right though.
BTW, your new ship is very nice. I hope it works well for you.
the elevator bias was also down with backwards props, if I remember correctly.
Crist,
Just wondering. Was the inside/outside turn well balanced before you started worrying about the "apparent" tail low attitude? If so it might be worth evaluating what makes an apparently decently trimmed airplane "look like" it is tail low to an observer...I'm understanding you weren't aware of that issue until it was brought to your attention. If an airplane flies straight and level just fine and turns inside/outside acceptably equally I'd be reluctant to spend too much time worrying about apparent body angle in level flight...especially if the first thing that happens is what was good is now not so good.
Ted
Ted,
Yes, the airplane was flying and handling well, turned equally in both directions but the plane had a tail low attitude.
I'll keep what said about it was good before and not so good now in mind. I can always go back to the way it was. Thanks for your comments Ted.
Crist,
You didn't post a picture of the ship and paint scheme but one thing you might be seeing is the visual illusion created by the paint/trim lines (like the bending of a pencil as you wiggle it between you fingers). Some ships have lines and shapes that give this impression, some slight revision to the trim lines can correct this. An example was a P51 stunter with the BARDAHL checker broad trim on the tail. The checkers were aligned along the angle of the rudder leading edge. For me, this gave the illusion that the ship wasn't quit level and didn't appear to stop crisply. Just some thoughts for you to consider.
Best, DennisT
Crist,There are some pics in the electric section.
You didn't post a picture of the ship and paint scheme but one thing you might be seeing is the visual illusion created by the paint/trim lines (like the bending of a pencil as you wiggle it between you fingers). Some ships have lines and shapes that give this impression, some slight revision to the trim lines can correct this. An example was a P51 stunter with the BARDAHL checker broad trim on the tail. The checkers were aligned along the angle of the rudder leading edge. For me, this gave the illusion that the ship wasn't quit level and didn't appear to stop crisply. Just some thoughts for you to consider.
Best, DennisT
Crist,
You didn't post a picture of the ship and paint scheme but one thing you might be seeing is the visual illusion created by the paint/trim lines (like the bending of a pencil as you wiggle it between you fingers). Some ships have lines and shapes that give this impression, some slight revision to the trim lines can correct this. An example was a P51 stunter with the BARDAHL checker broad trim on the tail. The checkers were aligned along the angle of the rudder leading edge. For me, this gave the illusion that the ship wasn't quit level and didn't appear to stop crisply. Just some thoughts for you to consider.
Best, DennisT
consistent with Dennis' thought process...
Is the belly of the airplane tapered at all toward the tail or is it pretty much flat with all nose to tail fuse taper in the top block only? If so and the apparent tail low is only modest it might also be giving a false impression.
Last question, before being told of the tail low effect was it flying well enough that you would have been willing to fly the ship competitively at the contest in question?
I see what you mean. I've always set up everything zero zero with both horns set for equal travel up and down. If elevator and flaps are not neutral at the same time you would need something different. Maybe I didn't pick up on this before, why would you have down elevator with neutral flaps, precession?
Thanks
MM
Our symmetrical wing by itself will not produce lift without some camber. By setting a little down elevator with the flap neutral we are actually allowing there to be some down flap deflection in flight while the elevator is neutral or slightly up.
Our symmetrical wing by itself will not produce lift without some camber. By setting a little down elevator with the flap neutral we are actually allowing there to be some down flap deflection in flight while the elevator is neutral or slightly up.
I have found looking at many different pictures of models in level flight over the years the flaps will be showing a little down deflection while the elevator is either neutral or a little up. Our symmetrical wing by itself will not produce lift without some camber.
As noted, all that is required is an AoA.
As noted, all that is required is an AoA. With coupled flaps and elevator, you have to have some very small elevator deflection to get any AoA, so the flap will naturally be deflected down, creating camber as well. Its the same thing that happens in the corners, except that it's much smaller.
Brett
a line drawn from the LE of the wing to the TE of the flap gives the wing a positive AoA and ...
All that is required is lift.
Yep. With the flap slightly deflected down, with the elevator slightly up or close to neutral as possible, a line drawn from the LE of the wing to the TE of the flap gives the wing a positive AoA and the plane gets off the ground while the fuse is maintaining a level flight attitude.
Yep. With the flap slightly deflected down, with the elevator slightly up or close to neutral as possible, a line drawn from the LE of the wing to the TE of the flap gives the wing a positive AoA and the plane gets off the ground while the fuse is maintaining a level flight attitude.
The relationship between lift and a line drawn from the wing LE to the flap TE (at some place on the wing) is something you could possibly use along with airfoil shape
never mind
I knew Howard was going to get all picky.
Crist, did you get your question answered sensibly before we spun off on this (admittedly quite fun for me) tangent?
Does this mean that you cut the HT out and put it back in a 0 degrees incidence?
With electric, you don't have to make it pretty before flight testing it. Once glued in enough, I would be back out to test before spending any time refinishing.
Thanks Chris. Full report after test flights.
It's been awful quiet since your last post....
Did things not go well?
So did you win????
Hi Crist,
I will be out this Wed. & Thursday about sun up till noon if I can help.
Glad to hear that the issues have been resolved.
I have never had a plane that liked incidence in the HT.
Good luck on your final trim settings.
Thanks Paul. I've been reviewing your articles in SN. BTW, good article on flying in the wind!
That was only part one. A mere 6000 words await Bob to publish in SN for part two.