Trimming Question on my Electric Powered Legacy 40

[Crist Rigotti]

I was flying with Mike Scott today and asked him to watch my airplane for any yawing or rolling in a hard corner.  After the flight he said that the airplane was flying tail low.  Then he showed my my airplane with the flaps neutral and how much up elevator it had!  Yikes!  I've included a picture of that amount.  When I started I know I had the elevator neutral or slightly down.  Over the course of the few flights I did have to lengthen the elevator push rod to help trim out a tighter outside turn compared to an inside turn.  So, when I got back to the shop I redid the elevator so it is ever so slightly down when the flaps are neutral.

Now my trimming cheat sheet does say to lengthen the elevator push rod if the outside turn (loops) are tighter.  Now I'm wondering, if I keep doing that as before, I'll end up with the same situation.  So what does one do to cure outside loops being tighter than inside loops without lengthening the elevator push rod?  Bias the handle?  Change the CG? ETC?

Oh, BTW the stab has a + 1/32 built in incidence at the LE.

edit: added Electric Powered to heading.

[Dan McEntee]

    Hi Crist;
     If you adjust the push rod length, you might have to re-adjust the neutral at the handle. The angle the flaps are at can affect flight trim and still let the elevator have the authority you need. The other adjustments you mention should work also, and maybe slight combinations of each, but one at a time. I have made slight thrust adjustments also to help with turning issues. The easiest is biasing the handle. You can do that before each flight and if the move necessary to correct it is small enough, you can achieve that in one flying session.
    Good luck with the new model!
   Dan McEntee

[Paul Walker]

Oh, BTW the stab has a + 1/32 built in incidence at the LE.
[/quote]

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?

[Crist Rigotti]

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?

Yes, the motor is 0-0 to the wing CL.  The stab incidence works out to .5 degrees positive.

[Motorman]

Shim the motor for some up thrust, worked wonders for me.

MM

[Crist Rigotti]

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.

[Matt Colan]

At the NATS, Randy Smith said my tail was flying low on my thundergazer, and I couldn't tell from inside the circle. We trimmed it out by shortening the pushrod one turn. That was all it needed to fix it. My elevator started out level with the stab. The only thing I could feel was an incredibly small difference in neutral when flying inverted, but other than that, that was it.

[RC Storick]

I didn't mention it but this Legacy 40 is electric powered with reverse rotation prop.

Well if you were in the southern hemisphere (where the water goes down the drain in reverse) that should not matter.  

[FLOYD CARTER]

The Coriolis effect is real.  Many people don't realize that, at the equator, water doesn't spin CW or CCW.  In fact, it doesn't flow at all!  One must empty a bucket with a ladle.  (turning it upside down also works here)

[Crist Rigotti]

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?

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.

[Brett Buck]

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

[Crist Rigotti]

  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

I don't see a lot of other electric flyers and what they are doing.  I also missed the last several NATS.  So it looks like I'm still flying in the dark ages.  My guess that those who are flying with a normal rotation prop is flying one of the CF props.

I'll have to dig around my stuff to see if I have an equivalent tractor prop of the pusher prop (an APC 13 x 5.5EP cut down to 12) I'm currently using.

[Ted Fancher]

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.

[Paul Walker]

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.

Yes, I had assumed the hinge lines were sealed, thus the comment.

Paul

[Randy Powell]

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.

[Crist Rigotti]

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.

Ted,
Yes the hinge lines are sealed. I appreciate your explanation. 

[Crist Rigotti]

Yes, I had assumed the hinge lines were sealed, thus the comment.

Paul

Ok.  There are sealed and I went out this morning and tried the bias handle thing.  It didn't work.  I quit early to bring home the airplane and start over on the "bench trimming".  I'll try a standard rotation prop.  I'll have to order some.  Also thinking about cutting the stab and redoing it with zero incidence. 

Motorman suggested some upthrust.  I thought of that too.  What would be the downside of that?

As always I appreciate your comments.  Thanks.

[Crist Rigotti]

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.

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.

[Leester]

Hey Crist... I can have Helga come down and fix it for ya  Please    ouch,ouch
just kiddin.....not !!!

[Crist Rigotti]

Hey Crist... I can have Helga come down and fix it for ya  Please    ouch,ouch
just kiddin.....not !!!

Lee, I keep you in mind.  It just might take Helga's touch!

[Motorman]

I didn't mention it but this Legacy 40 is electric powered with reverse rotation prop.

That doesn't matter to up thrust.

MM

[Target]

Isn't upthrust going to create problems when inverted if the motor really is 0-0?
Or is it to possibly correct a non 0-0 situation, or to lead to some other conclusion if the results are favorable in upright flight.
R

[Brett Buck]

That doesn't matter to up thrust.

MM

   I would certainly expect it to. Numerous people have been putting in downthrust to compensate for the nose-up precession for a long time now. With a reverse-rotation you might expect to want upthrust, although I haven't tried it.

    Brett

[Motorman]

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.

MM

[Brett Buck]

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.

  It's a sign of way too much up elevator, which is exactly what he showed above.

    Brett

[Crist Rigotti]

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.

Now I need to decide if I should remove the upthrust washers.  Any opinions?

[Dan McEntee]

   I think you have found your problem. As far as taking out the thrust shims, it was still turning too tight with the shim in, so if you have made significant control movement changes, I would take it out. With the control throws more or less even now, the up thrust adjustment may swing the pendulum the other way and make outsides much harder. If you leave it in, just be ready for it and allow a bit more altitude until you are satisfied with it.
  Type at you later,
    Dan McEntee

[Howard Rush]

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.

That's what I suspected.  I hesitate to say anything about trim after Paul Walker and Motorman have weighed in, but when one of my airplanes had a control asymmetry such as you describe, it felt like it had too much elevator for the amount of flap it had in one direction and not enough elevator for the amount of flap it had in the other.  To wit, in the first direction it would rotate easily, but get kinda flaky in tight turns, as if for a given radius it had more lift coefficient than in oughta, and got beyond the linear part of the lift curve, as Igor says not to.  In the other direction it felt reluctant to turn, but turned smoothly once you got enough control deflection in to turn the corner.  If the airplane's flap-to-elevator relationship is symmetrical, putting in a elevator bias to fix this asymmetry will cause it to fly level with the same pitch angle (OK, Tim, the same absolute value of pitch angle) right side up and upside down.  If "tighter outside turn than inside turn" is the situation I described above, I'd guess that your flap-to-elevator relationship is cattywampus.  Mind you, it's possible to make your airplane so that there is some elevator bias relative to the stab when the flaps are level and pitching moment is zero.  "Jackknife" is the technical term.  My planes are like that by (PW's) design: the elevator is down four or five degrees when the flaps are neutral.  A plot of flap deflection to elevator deflection is a straight line that passes through the point of zero flap and 4.4 degrees elevator (see attachment).  You can still have symmetry around some elevator bias-- maybe up elevator in your case-- but that bias can cause other problems.

If your Legacy has the conventional arrangement of flap horn sticking up and elevator horn sticking down, you might be able to fix the geometry by moving the hole in the elevator horn aft while the flaps are in neutral and the elevator is wherever it needs to be for zero net pitching moment. 

[Motorman]

Paul Walker and Motorman have weighed in 

Howard, you've got to get both ends of the spectrum on these things.

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.

I use .010" washers for thrust adjustment. If .030" didn't do anything hmm... maybe elevator angle trumps thrust angle?

Brett, don't you need to use up elevator to compensate for down thrust in level flight.

MM

[Howard Rush]

More pitching moment simultaneously increases lift AND pitching moment.  

Ted refers to the rate of change of pitching moment with pitching moment, ∂C_m/∂C_m.  Stability and control engineers have found it best to set this value to 1 (for small values of 1).

[Howard Rush]

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. 

[Motorman]

Only in one specific case.  

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

[Howard Rush]

The flap horn would also need to be 90 degrees to the pushrod.

...why would you have down elevator with neutral flaps, precession?

The down elevator bias is maybe because of the flow angle from the wing.  Stabilizer incidence is zero on my (our) airplanes.  The stab is about .8" above the wing centerline, so maybe that puts the stab at a negative angle of attack relative to the wind blowing on it, requiring some down elevator to balance it.  It could also be because of the gyroscope effect of the rotating prop going around the circle, as you and Brett note, but the elevator bias was also down with backwards props, if I remember correctly. 

[Ted Fancher]

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.

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

[Fred Underwood]

the elevator bias was also down with backwards props, if I remember correctly. 

That is what I have.

[Crist Rigotti]

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. 

[Brett Buck]

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. 

   But, if it had to fly at an exaggerated angle but otherwise correctly, there's something else going on. 1/2 degree of positive incidence (which is roughly twice what I use) doesn't cause that all by itself.

    Brett

[Dennis Toth]

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

[Ted Fancher]

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?

[James Mills]

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

There are some pics in the electric section.

James

[Crist Rigotti]

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

Here is a picture of it.

[Crist Rigotti]

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?

Ted,
There is some up slope to the tail portion of the belly.

And yes, I would have flown it in competition Sunday.

[Doug Moon]

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

I always setup my horns to have the same total up and down travel and 0-0 to start.  Also the wing, stab, and motor 0-0 as well.
I usually end up with some down elevator by the time it's in decent flying trim. Plus I add in a little "slop" at the elevator.

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.  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 seen some pics of classic planes with small flaps in flight and it is very noticeable. But once the proper balance between the two is found it will cruise dead level with almost no input needed.  Finding that balance can take a little time.

I have had some that I could never actually get just right but they were still very good fliers.  The level flight portions of the pattern were a down right struggle. I had to work hard for those points.

[Motorman]

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.

That makes perfect sense to me, thanks.


MM

[Tim Wescott]

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.

Before Howard says it -- a symmetrical wing WILL produce lift, just not at a zero angle of attack.  That's why flapless planes fly (not to mention the B-17, which used NACA 00-- series airfoils, although I can't remember the root and tip thicknesses).

Al Rabe talked about using a slightly cambered airfoil in one of his articles -- that would accomplish the same thing, at the cost of the camber being cast in stone (well, balsa.  Unless I'm the one building it).  His reasoning was that the final turn of the hourglass was Really Important so he compromised on inverted turns a bit to gain better performance there -- but he may have been solving a different problem than he thought.

[Brett Buck]

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. 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

[Howard Rush]

  As noted, all that is required is an AoA.

All that is required is lift.

[Doug Moon]

  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

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.

[Igor Burger]

a line drawn from the LE of the wing to the TE of the flap gives the wing a positive AoA and ...

and wing can still make zero or even negative lift 

[Brett Buck]

All that is required is lift.

  And the lift on a symmetrical airfoil at an AoA of zero is.... 

   Oy!

   Brett

[Brett Buck]

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.

  That is all true, however, it works without the flaps, too. In that case the camber remains zero. Either I am misreading your previous comment, or it doesn't seem correct. You do not need camber to create lift.

   Brett

[Howard Rush]

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.

That is not a very useful way of looking at it.  In level flight an airplane needs enough lift to balance its weight.  It's more useful to look at lift vs. fuselage angle of attack (coinciding with flaps-zero wing angle of attack for stunters).  A line drawn from the wing LE to the flap TE has no particular aerodynamic significance.  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 and dynamic pressure to figure the relationship between fuselage angle and the direction the airplane is going, but nobody who does any calculation uses that convention.  

[Doug Moon]

I going to gracefully bow out. 

[Igor Burger]

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

0.11 * (alpha + c * 0.11) ?  

[Tim Wescott]

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?

[Crist Rigotti]

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?

Well Tim, I can't remember what the question was!

[Crist Rigotti]

I took it out today and no joy.  Same as before.  Time to get the #11 Xacto out.

[Crist Rigotti]

The epoxy is drying.  Then more epoxy, etc.  Then on to refinishing.

[Paul Walker]

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.

[Target]

Me too, and good luck Crist.

[Crist Rigotti]

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.

Yeah Paul, the HT back down to zero degrees.  After a few more repairs, it'll be ready to fly again.  Hopefully the weather will cooperate this weekend for further test flying.  If it won't then I'll just have more time to refinish!

[Crist Rigotti]

I just checked the weather.  Looks like Sunday afternoon for the next set of test flights.

The wing, motor, and stabilizer are all back to zero-zero-zero.  We'll go from there.  I'll bench trim it with neutral elevator too.  Stay tuned.  I'll post some pictures of what I did to lower the HT to zero a little later on.

[Crist Rigotti]

OK, some details of how I lowered the stabilizer LE 1/32".  With my #11 blade I cut above and below the fillet.  This way the cut will include the stab platform inside the fuselage sides and will only cut through the sides and the 1/4 TRI stock.  Then I cut along the LE.  This was the tough part.  I also used a hacksaw blade to cut the bulkhead in half and to have enough saw kerf of 1/32 that I needed to lower the LE.  I then took my #11 blade and cut into the vertical fin because it was glued onto the top of the stab.  I used an Exacto saw blade removed from it's holder to do this.  I think Zona Saw has one like this but I don't own one.  Once the vertical fin was cut in half, the LE could then be lowered the 1/32 that I needed.  I used 15 minute epoxy forced into the bottom slots on each side then jigged till it was cured.  I then added some thin CA to where the vertical fin was cut and filled the gaps with balsa and Gluit glue.  When dry, I cut the excess balsa away and sanded the areas with 80 grit then 320 grit.

I then used some AeroPoxy Lite filler and applied it around the repaired areas smoothing with alcohol the best I could.  In the morning, I'll sand the area smooth and reapply the filler as needed.  When done, I'll go on to the doping refinish.

The pics should show how it all worked out.

[Crist Rigotti]

The rest of the story......

[Crist Rigotti]

All done with the paint touch-up and clear.  Tomorrow I'll test fly it.

[Target]

Good luck! Hope it's damn near prefect!

[Crist Rigotti]

Thanks Chris.  Full report after test flights.

[Paul Walker]

Thanks Chris.  Full report after test flights.

It's been awful quiet since your last post....

Did things not go well?

[Crist Rigotti]

It's been awful quiet since your last post....

Did things not go well?

Paul and others,

Last Tuesday I had a chance to try it out.  The air was very bumpy but I was able to fly a few 1 minute flights.  It looked promising, but I had to leave for Cleveland on Wednesday morning.  On Friday, I was able to do several more 1 minute flights getting things better but again the air was very bumpy.  Then that evening, the air was great and I flew more full flights.  I flew it in competition on Sunday.  First flight was in nice conditions, and the 2nd flight the wind had come up but I scored a lot higher!  Bottom line is that the airplane is flying real good and in the correct attitude with equal turn rates in both directions. Lowering the stab did the trick.  Of course, there is more trimming that needs to be done.

[john e. holliday]

So did you win?

[M. Scott]

Hi Crist,
I will be out this Wed. & Thursday about sun up till noon if I can help.

[Crist Rigotti]

So did you win?

Nope, last place as a matter of fact.  The last 3 places were separated by 1.5 points.

[Crist Rigotti]

Hi Crist,
I will be out this Wed. & Thursday about sun up till noon if I can help.

Mike,

Thanks for letting me know, but I'm out of town this week.  Next week for sure we'll meet at the circle.

[Paul Walker]

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.

[Crist Rigotti]

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!

[Paul Walker]

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.

[Crist Rigotti]

That was only part one. A mere 6000 words await Bob to publish in SN for part two.

I would like to personally thank you for the trimming articles and appreciate you taking the time to share your thoughts with the rest of us.  I know it's a lot of work.  Thank you.