Tailplane incidence

[Russell Bond]

OK, here's a question..........

We all know that about half a degree of positive incidence on the tailplane (up at the front)☺helps stop hunting in level flight.

What if we instead of the positive incidence, we gave the stabilizer more lift on the bottom, meaning it would look like an upside down wing. (Not symmetrical).
Would this work?
It's only really hypothetical as no one can see the incidence on the tailplane anyway.

Also, has anyone actually worked out why the positive incidence works? (The theory).

[john e. holliday]

Myself, I still tr to get everything on a zero plane or on the same plain/plane.   Usually for me hunting is not having the plane trimmed or controls too tight.

[Brett Buck]

OK, here's a question..........

We all know that about half a degree of positive incidence on the tailplane (up at the front)☺helps stop hunting in level flight.

What if we instead of the positive incidence, we gave the stabilizer more lift on the bottom, meaning it would look like an upside down wing. (Not symmetrical).
Would this work?
It's only really hypothetical as no one can see the incidence on the tailplane anyway.

Also, has anyone actually worked out why the positive incidence works? (The theory).

    I wrote two entire SN Columns on the topic. MY theory was that it is the result of the nose-up torque from precession and continuous left yaw motion. Interestingly, it appears to act differently if you spin the prop the other way.

    Brett

[rustler]

Probably not relevant here, but there have been F/F models on which the T/P airfoil is inverted.  Probably ones with a forward CG? Thinks - why did I post this? 

[Tim Wescott]

Probably not relevant here, but there have been F/F models on which the T/P airfoil is inverted.  Probably ones with a forward CG? Thinks - why did I post this? 

To increase your article count?

There have been full-scale airplanes with inverted airfoils on their tail feathers.  The only one that comes to mind immediately is the Pilatus Porter, which needs to keep the tail down during STOL operations.

[Ted Fancher]

To increase your article count?

There have been full-scale airplanes with inverted airfoils on their tail feathers.  The only one that comes to mind immediately is the Pilatus Porter, which needs to keep the tail down during STOL operations.

Most full scale aircraft require some "download" on the tail to maintain steady state flight at a given airspeed.  This is because CGs on most are forward of where the lift produced by the wing is centered.  For more or less positive g restricted aircraft (non aerobatic types) the designers employ some "decalage" or angle of incidence difference between the wing and the stabilizer/elevator optimized to minimize the need for trim at cruise airspeeds and thus minimize the drag that results from having to deflect the elevator or retrim the stablizer; the drag from which would, of course, reduce the design cruise speed and not look good on the sales brochures. 


Other ways of mitigating the need for deflection include the inverted airfoil referred to by Tim or, as in the case of aerobatic aircraft utilizing tail volumes that allow the CG to be moved aft while still retaining the required static margin (distance between the CG and the Neutral point of the entire aircraft).  If the CG is close to where the lift of the wing is generated the need for directed lift forces from the tail for stable hands off flight is essentially eliminated and any deleterious effect on positive versus negative G conditions about the pitch axis is pretty much a non issue as well.

Ted

[Randy Cuberly]

Most full scale aircraft require some "download" on the tail to maintain steady state flight at a given airspeed.  This is because CGs on most are forward of where the lift produced by the wing is centered.  For more or less positive g restricted aircraft (non aerobatic types) the designers employ some "decalage" or angle of incidence difference between the wing and the stabilizer/elevator optimized to minimize the need for trim at cruise airspeeds and thus minimize the drag that results from having to deflect the elevator or retrim the stablizer; the drag from which would, of course, reduce the design cruise speed and not look good on the sales brochures. 


Other ways of mitigating the need for deflection include the inverted airfoil referred to by Tim or, as in the case of aerobatic aircraft utilizing tail volumes that allow the CG to be moved aft while still retaining the required static margin (distance between the CG and the Neutral point of the entire aircraft).  If the CG is close to where the lift of the wing is generated the need for directed lift forces from the tail for stable hands off flight is essentially eliminated and any deleterious effect on positive versus negative G conditions about the pitch axis is pretty much a non issue as well.

Ted

So...if the lift point is near the CG and the stab is the right size then 0-0-0 should produce stability?...Hmmmmm...

What about the lifting characteristics of the stab...ie sharp vs round leading edge, airfoiled vs flat...surely that has to matter!

Randy C.

[Howard Rush]

So...if the lift point is near the CG and the stab is the right size then 0-0-0 should produce stability?...Hmmmmm...

Yep. Some aviation authors confuse moment with rate of change of moment due to angle of attack.  Moment alone doesn't contribute to stability.

What about the lifting characteristics of the stab...ie sharp vs round leading edge, airfoiled vs flat...surely that has to matter!

It depends on what's sizing the tail.  If it's landing flare at max flaps and forward CG, the tail probably has an airfoil that will provide a lot of downward lift. 

[Chris Wilson]

What if we instead of the positive incidence, we gave the stabilizer more lift on the bottom, meaning it would look like an upside down wing. (Not symmetrical).
Would this work?

It probably works as well as the airfoil shaped Nobler rudder but that total control surface is fixed and only active in the one direction.

What worries me is that with a movable bidirectional control surface is an asymmetrical shape going to bias the control inputs in a bad way?

In other words is the same degree of deflection of 'down' just off neutral going to much more effective than up would be ?

[Howard Rush]

Looks like there's some confusion.  Russell, do you have a sign backward?  One could use camber instead of incidence to get a little lift from the tail if that's what you want.  In either case you could expect the elevator effect to be linear for small elevator deflections.  The mechanism that causes the "hunting" is probably something weirder.  Some stab configurations are more susceptible than others to it.  See http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=133980&mesg_id=133980&listing_type=search for the lowdown. 

[Russell Bond]

Quote; What worries me is that with a movable bidirectional control surface is an asymmetrical shape going to bias the control inputs in a bad way?

This could be interesting.
We need snappy cornering at the bottom of triangles and the hourglass, could this be a way to get it.
More lift at the bottom of the stab should make it want to turn better in that direction, meaning the tail would rotate quicker for those corners?

(Pity I don't know the correct terminology) 

I know that Al Rabe had an asymmetric airfoil section on the wing of the Mustunt to make it turn snappier on those corners.
Maybe we could compine the two.  

[Doug Moon]

OK, here's a question..........

We all know that about half a degree of positive incidence on the tailplane (up at the front)☺helps stop hunting in level flight.

What if we instead of the positive incidence, we gave the stabilizer more lift on the bottom, meaning it would look like an upside down wing. (Not symmetrical).
Would this work?
It's only really hypothetical as no one can see the incidence on the tailplane anyway.

Also, has anyone actually worked out why the positive incidence works? (The theory).

I am not sure the results would be what you think.  While the plane may work very well in one direction it might be a complete mess the other way, depending on how much asymmetry you actually build in.

I may be off here but here is how I see it.

For an extreme example, if you have access to an RC simulator (they have them at our LHS and they are a blast), get a trainer plane loaded up and fly it inverted.  The amount of elevator input to keep it level is quite alot. Upright you can have a conversation with you buddy while it flys unattended into the wild blue yonder.  Of course the main wing is asymmetrical as well, and the motor down thrust is noticeable from quite a distance.  But you will see the effect right away.  When you start working with asymmetrical airfoils the trim settings wont work in both directions.  You would need one for each direction and we cant change in flight.

The up incidence in the stab with the elevators inline with that stab create a need for a tiny amount of up input at the handle to use the elevator to push the tail down a little to pick up the nose.  Then once in flight that input is backed off some.  I think Ted referenced a board in the wind in one of his articles.  Your hand out the window of a car does the same thing.  Very slightly tilt it up and your hand will rise.  Same with the stab with positive incidence.  It will want to rise and push the nose down. But we give a little up and off we go.  Of course there is a great deal of prop wash all over the place but I have done the hand trick with my hand near the side rear view mirror at speed and while you can feel the turbulence all over your hand the affect is still the same, just a bit rougher. If you notice pics of stunt planes in level flight the elevator is still up a tiny bit but the big thing you notice is the flap is down and in some cases VERY noticeably down. This is creating the camber needed in the wing to keep it in the air.  This is what happens when you add "slop" to the elevator horn.  You can input for a slight bit of up and the elevator stays very near neutral but the flap is rolled down creating a camber.  Once it is flipped over the "slop" works in the other direction as well.  It doesnt take much, just enough so you can get a camber out of the main wing and flaps while still keeping a very near neutral setting at the elevator.

The stab/elevator and wing/flaps are working together and against each other at all times while fighting off the torque of the motor as it works like hell to turn the prop.  The whole mess of parts is in a constant state of tension.  If everything were ever really at 0-0-0 and effortlessly slipping through the air it would never get off the ground and just be a cool looking tether car with really big wings.  Things on our planes can't really ever be equal.  That is what we call trimming.  Finding the sweet spat where all the pressure is equal enough on the airframe to produce lift needed yet not too much to keep in a straight flight path.

That's how I think it works, at least my trimming keeps telling me that.

[Serge_Krauss]

OK, here's a question..........

We all know that about half a degree of positive incidence on the tailplane (up at the front)☺helps stop hunting in level flight.

What if we instead of the positive incidence, we gave the stabilizer more lift on the bottom, meaning it would look like an upside down wing. (Not symmetrical).
Would this work?

Also, has anyone actually worked out why the positive incidence works? (The theory).

I've been following this thread for a while and wonder why no one has commented that the inverted airfoil thing should give the opposite effect of positive incidence in the stabilizer. I see the positive incidence not just as a trimming thing - like to counteract precession from ccw circular flight - but as a way of keeping the pilot from having to alternate between positive and negative elevator for corrections around neutral. It seems that not having to pass the elevator through neutral (thus reversing horizontal tail camber) is a simpler and gives a more delicate touch. With the positive incidence or some negative elevator, the control around neutral is just  a matter of varying up pressure on the handle. Inverted, it's then just a matter of varying negative pressure. Shouldn't this enhance the "groove," with the easier or more familiar of the two being for upright flight? Down thrust probably is a close trade-off.

I believe that Doug and others before him have it right about the flaps. Perhaps the positive stabilizer is more beneficial for flapless planes, since they have no added negative pitching moment from the flaps. 'dunno, since the flap deflection canges with the elevator's leaving neutral. It can be good for the flapped plane to have some down flap (both ways, as Doug said) so that it can fly level with the fuselage level.

Anyway, since CL flyers say that negative stab incidence is a problem, it seems that the inverted airfoil shouldn't be expected to be a benefit. Several things about CL stunt seem to call for different solutions than FF and sometimes even RC (like increased static margins  for CL).

SK

[Howard Rush]

I've been following this thread for a while and wonder why no one has commented that the inverted airfoil thing should give the opposite effect of positive incidence in the stabilizer.

But not following it too closely, it appears.

[Steve Helmick]

Russell does have it confused. The "equivalent" to a symetrical stabalizer/elevator at +.5 deg would be a lifting airfoil such as used on probably at least 99.9% of all free flights.

Free flights are weird, because CG's vary dramatically. Don Zipoy had a '50's era gassie with a long tail and large lifting stabalizer. The CG was aft of the wing TE (110% back of the LE, if I recall correctly). Many of them had the CG at the TE. Auto-surfaces typically moved the CG forward to 50% or so. Nordic Glider (F1A, my event for 15 years) designs typically have the CG around 50%, with small lifting stabalizers and very long tail moment. There was incentive to use small tailplanes on all the FAI classes, because total area was limited, and fairly high wing loading required. Anyway, few FF's have the CG as far forward as CL Stunt planes.  Steve

[Chris Wilson]

The wording was jumbled but the meaning was clear - probably what drove most of the replies previous.

[Russell Bond]

Yes, I was half asleep when I posted.
Of course I meant the other way.  

[Serge_Krauss]

But not following it too closely, it appears.

OK, Howard. I have re-read every post preceding mine, and find no criticism of the original statement that apparently saw a downward cambered airfoil as a replacement for positive incidence. I think I just wasted my time re-reading them. While there were several good points made, not one person suggested to Russel that it would not substitute for the effect. Russell has since replied, since Steve clarified what I apparently did not state clearly enough. Geeez...

SK

[Tim Wescott]

I've been following this thread for a while and wonder why no one has commented that the inverted airfoil thing should give the opposite effect of positive incidence in the stabilizer.

oops.

[Doug Moon]

... not one person suggested to Russel that it would not substitute for the effect. Russell has since replied, since Steve clarified what I apparently did not state clearly enough. Geeez...

SK

I did, in my opening line, I suggested that he wouldnt find it to do what he thinks it might do.  Then I tried to explain why.

But, one step further, I wish he would try and report back.  I would be very interested to hear the results.  I have an idea what would happen but cant say for sure as I have never tried it myself.

[Howard Rush]

We all caught the sign.  It isn't a big deal.  The idea of using a little camber instead of incidence is essentially what the Impact does, and it works.  The issue to which I tried to lead people in the reference I posted is what becomes of the transition point on whatever stab you pick.  See Igor's pictures.   Also, Frank Williams showed some stuff toward the bottom of this thread: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=371054&mesg_id=371054&listing_type=search .  We've figured out how to make a symmetrical stab with a semblance of an airfoil work and how to make the Impact configuration work.  Russell proposed using a cambered stabilizer.  A cambered stabilizer certainly could give all the moment you need to turn corners in both directions, but flying level could be an issue. You might get a clue from Profili or XFoil as to whether it would work or how to make it work.  

Igor and Brett have calculated the contributions of flow direction and prop gyroscopic moment, respectfully, in level flight.  Finding the combined effects of those for the backward-turning electric APC prop might be useful to those of us trying to get electric stunters to fly level. Anybody up to the ciphering?


Edited to respell ciphering, to expound upon the clue, and to add a  favorite malapropism.

[Russell Bond]

Slightly off topic, but I started the thread, so I don't care! 

I am making a 60" electric at the moment and it will have a removable tailplane and fin.
It will be mounted by 4 screws so I will be able to add negative or positive incidence to see what happens.
I'll test fly it without the fin finished, that way I can modify the fin to suit.
Hell, I'll probably even test fly it before I paint it. (No oil).
That way I can cut or modify after trimming and won't wreck the paint. 

[Tim Wescott]

Russel: you're still on topic.  Promise to let us know what happened, and you'll be doing just what Doug asked.

[Russell Bond]

Yes, it will be very interesting to see what happens.
I'm flying an electric at the moment and that has very slight down on the elevators compared to the flaps.
This is with a pusher prop. This plane tracks very well.

The other thing I'm going to do on the new plane is make a spare tailplane and elevators that are flat, the tail at the moment on this new plane is airfoiled. This will also be an interesting comparison.

[Tim Wescott]

The other thing I'm going to do on the new plane is make a spare tailplane and elevators that are flat, the tail at the moment on this new plane is airfoiled. This will also be an interesting comparison.

To be complete, you'll have a symmetrically airfoiled tail, a lifting tail, and a flat tail.  Then you'll get some results, and someone will say "but that's only with one plane.  Why, there's so many possible variables that you can't just tell with one plane, you need to build 100".

Which is, of course, the basic principle that fuels at least 90% of the arguments on the Engineering board.

[Dave Denison]

"Yes, it will be very interesting to see what happens.
I'm flying an electric at the moment and that has very slight down on the elevators compared to the flaps.
This is with a pusher prop. This plane tracks very well."

After following this thread and enjoying it. I would like to add some observations from my electric powered stunt ship and generate a few more questions that need to be answered.

Russell; I have an electric powered stunt ship running a pusher prop(12 X6), using a flat plate type stab. My stab is set up at zero incidents relative to the wing datum line. I have set the elevator negatively at one half rotation on a 6/32 pushrod. Given that my CG is slightly nose heavy, this airplane grooves and flies very flat, both upright and inverted and has no hint of hunting

Howard if you have an opportunity can you give us all an idea why we still seem to believe in the "old school set up" for positive incidents in the stab on today's modern stunt ships?

My background comes from over 30 years of flying pattern and thermal duration sailplanes. As I have gotten back into control line(a few years ago) this one set up in the building of our modern stunt ships has puzzled me more than any other alignment step.

Regards.
Dave
ama 41041

[Doug Moon]

"Yes, it will be very interesting to see what happens.
I'm flying an electric at the moment and that has very slight down on the elevators compared to the flaps.
This is with a pusher prop. This plane tracks very well."

After following this thread and enjoying it. I would like to add some observations from my electric powered stunt ship and generate a few more questions that need to be answered.

Russell; I have an electric powered stunt ship running a pusher prop(12 X6), using a flat plate type stab. My stab is set up at zero incidents relative to the wing datum line. I have set the elevator negatively at one half rotation on a 6/32 pushrod. Given that my CG is slightly nose heavy, this airplane grooves and flies very flat, both upright and inverted and has no hint of hunting

Howard if you have an opportunity can you give us all an idea why we still seem to believe in the "old school set up" for positive incidents in the stab on today's modern stunt ships?

My background comes from over 30 years of flying pattern and thermal duration sailplanes. As I have gotten back into control line(a few years ago) this one set up in the building of our modern stunt ships has puzzled me more than any other alignment step.

Regards.
Dave
ama 41041

Hello,

I know I am not Howard but read my post and you will see the very thing that you are doing with the negative elevator is allowing you to add camber to the wing while keeping the elevator at or near neutral.  And it grooves along nicely.  It may be an old school setup, that's because we are still flying old school CLPA.    We just use newer engines/motors props etc.  The aerodynamics still holds true.

[Brett Buck]

It may be an old school setup, that's because we are still flying old school CLPA.   

   What amuses me is that this idea was radical when I suggested it in the ancient old days of 2006, and that the explanation provided over the course of 6 pages of magazine print was insufficient. As was, apparently, the summary I posted in this very thread 2 days ago.

    Brett

[Fred Underwood]

So what happens when inverted?  The prop is also "spinning the other way," a pusher.

[Dave Denison]

Doug, Brett.

Thanks for your understanding and clarification on this.  I fully understand the "camber condition of the stab and elevator." On my last two ships I align the stab. zero zero and then dial in a turn of down elevator.....so far that set up is working fine.

Regards.
Dave
ama41041

[Brett Buck]

So what happens when inverted?  The prop is also "spinning the other way," a pusher.

  You have the same precession torque in the body frame as you do when upright. The backwards prop is much more interesting. A conventional airplane has everything in it rigged to provide nose-down pitch, even if you put the tail on straight. The backwards prop definitely creates a nose-down torque in the body frame. I would expect from prior reasoning that you might want to give it a good bit of negative incidence in the tail, with some apocryphal evidence  to support that.

   Brett

[Howard Rush]

Folks, the convention is to call down elevator positive.  If stunt people use a different sign, we'd probably better stick to up and down to describe elevator deflection. 

Howard if you have an opportunity can you give us all an idea why we still seem to believe in the "old school set up" for positive incidents in the stab on today's modern stunt ships?

Dave, look at the two links I posted above.  Brett, is your ciphering on line somewhere?  

I'm not sure whether Dave considers his own setup "old school".  Having a flat stab above and parallel to the wing centerline and having down elevator when the flaps are centered is how I've been taught to operate an Impact.  Sounds like that's what Dave is doing.  I fiddled some with elevator bias now that I'm operating an electric with a backwards prop, but it's still down some when the flaps are neutral. 

[Doug Moon]

   What amuses me is that this idea was radical when I suggested it in the ancient old days of 2006, and that the explanation provided over the course of 6 pages of magazine print was insufficient. As was, apparently, the summary I posted in this very thread 2 days ago.

    Brett

Sorry, I dont have the 2006 SN articles that you wrote. 

[Claudio Chacon]

   What amuses me is that this idea was radical when I suggested it in the ancient old days of 2006, and that the explanation provided over the course of 6 pages of magazine print was insufficient. As was, apparently, the summary I posted in this very thread 2 days ago.

    Brett

Hi Brett,
Could you please post -if you have it handy- that SN article you mention? I'm most interested in reading it and keep it as a future reference (as I do with every article written by you I can get my hands on).

Thanks in advance,
Claudio.

[Eric Viglione]

fiddled some with elevator bias now that I'm operating an electric with a backwards prop, but it's still down some when the flaps are neutral.  

Forgive my aerodynamic ignorance here but aren't we looking at a moving target here based on more than 1 cause and effect?

First, the prop effect which can vary based on direction, rpm and weight/ diameter of the prop then design asymmetry of the plane itself?

Howard, didn't you once tell me the impact required more than the usual elevator droop, like 2 to 3 degrees compared to the normal less than 1 deg? I always assumed this was because of the high stab relationship to the wing and thrust centerline?

If this is true, then it wouldn’t surprise me that going to a pusher prop on an Impact only partially negated the design asymmetry and you still have to droop the elevators a little. Or... am I all wet?

As to the positive stab incidence thing, I haven’t really convinced myself that the effect is exactly the same as drooping the elevators. Why I say this is I’m not really sure if one method has more net negative effects, like turn symmetry, more than the other.

I can say one thing with some certainty, on more than one ship of similar design, I have proved to myself that you can droop the elevators enough to cure some upright hunts, but if drooped even further than needed can cause an inverted hunt.

One last word on hunting… as of late, I have been chasing the source of most of my more minor hunts (not saying this is everyone else’s problem, lots of things can cause this…) but a pet peeve has become slop at the bell crank due to the BC rocking on the pivot wire. Assuming you have no slop in your horn, or your using ball-links, grab and lock your lead outs, then wiggle your flaps. You don’t want slop there. I know some people liked a little slop at the elevators back in the day, and some still do, but I think it’s a curse to have any slop at the short pushrod area. This may not cause the classic hunt, but it seems to make a plane much more susceptible to turbulence, where the controls are loading and unloading in level flight, and also the rise/fall upwind/downwind effect, as that slop gets taken up one way, then the other. On a calm day, such a ship may feel fine and the effect goes unnoticed. I think an ideal BC would use at least 1/8” pivot wire, maybe something thicker if we could find something light that wouldn’t bend/break, and the BC itself would be as thick as possible in the pivot area to provide a large supporting surface as possible, but not so large as to cause any drag. So far the Morris aluminum Brett copy BC is working well for me, but I still detect a little rocking there… I know some people try to save weight and use the smaller (said thinner) of the 2 phenolic BC on 60 ships, and I really don’t like the effect. The good old  4” large arrowhead Brodak sells is also a decent BC in my opinion, and is reasonably thick at the pivot and stair steps down to the leadout points. Is this one just me, or have others been chasing this issue as well?

Sorry  I rambled on so much, just all seemed worth mentioning.
I apologize for any perceived thread drift.
Thanks,
EricV

[Dennis Toth]

I did compare tractor and pusher props on the same ship. With the pusher prop on an electric conversion of my Stuka the first thing I noticed upon release was it wanted to hug the ground. I believe Brett has it right and the nose up/down moment we get is a function on the direction of rotation, direction of flight (those who fly clockwise will get the opposite), weight of the prop and rpm. The lighter the prop and slower the rpm the lower the pitching moment.

For electrics it seems that setting at 0-0 is a good place to start until you know if you will run a pusher or tractor prop, the weight of that prop, rpm and the vertical CG of the ship. Since you can run in either direction you may find that to compensate for high or low vertical CG the motor directions will help offset the roll moment. This was a problem that happened to some piped designs as the pipe was below the wing, the gear was below and the engine torque on the counterclockwise rotation rolled the ship in that added to the moment from the low vertical CG. Here a reverse rotation engine would help balance  the forces some (not perfect but better than additive forces). This can be done easy with the electric's.

Best,       DennisT

[Howard Rush]

Forgive my aerodynamic ignorance here but aren't we looking at a moving target here based on more than 1 cause and effect?

First, the prop effect which can vary based on direction, rpm and weight/ diameter of the prop then design asymmetry of the plane itself?

Howard, didn't you once tell me the impact required more than the usual elevator droop, like 2 to 3 degrees compared to the normal less than 1 deg? I always assumed this was because of the high stab relationship to the wing and thrust centerline?

Yep.  I'm hoping that somebody will calculate all that stuff for my airplane. 

Impact has 4 or 5 degrees as I recall.  As for the reason, it's because it works.  I'm still wondering why.

If this is true, then it wouldn’t surprise me that going to a pusher prop on an Impact only partially negated the design asymmetry and you still have to droop the elevators a little. Or... am I all wet?

So far, it looks like the elevator still has a down bias for the reverse-pitch airscrew. 

One last word on hunting… as of late, I have been chasing the source of most of my more minor hunts (not saying this is everyone else’s problem, lots of things can cause this…) but a pet peeve has become slop at the bell crank due to the BC rocking on the pivot wire. Assuming you have no slop in your horn, or your using ball-links, grab and lock your lead outs, then wiggle your flaps. You don’t want slop there. I know some people liked a little slop at the elevators back in the day, and some still do, but I think it’s a curse to have any slop at the short pushrod area. This may not cause the classic hunt, but it seems to make a plane much more susceptible to turbulence, where the controls are loading and unloading in level flight, and also the rise/fall upwind/downwind effect, as that slop gets taken up one way, then the other. On a calm day, such a ship may feel fine and the effect goes unnoticed. I think an ideal BC would use at least 1/8” pivot wire, maybe something thicker if we could find something light that wouldn’t bend/break, and the BC itself would be as thick as possible in the pivot area to provide a large supporting surface as possible, but not so large as to cause any drag. So far the Morris aluminum Brett copy BC is working well for me, but I still detect a little rocking there… I know some people try to save weight and use the smaller (said thinner) of the 2 phenolic BC on 60 ships, and I really don’t like the effect. The good old  4” large arrowhead Brodak sells is also a decent BC in my opinion, and is reasonably thick at the pivot and stair steps down to the leadout points. Is this one just me, or have others been chasing this issue as well?

Thanks for the tip.  I have used Windy bellcranks, but I'm making my own for the new dog.  I wonder if ball bearings would be prudent.

Your "ramblings" are pertinent and thoughtful, as usual.

[/quote]

[Tim Wescott]

Thanks for the tip.  I have used Windy bellcranks, but I'm making my own for the new dog.  I wonder if ball bearings would be prudent.

If I'm interpreting Eric's comments correctly, ball bearings will not automatically fix the problem that he perceives.  A single ball bearing with a shaft going through it will still allow the shaft (or the thing to which it is attached) to rock.  If you wanted to prevent that, you either need to get special bearings with "X" contact points or you need to mount two bearings with some distance between them.  "X" bearings are expensive, hard to get, and tend to high friction.  Mounting two bearings with some distance between them means that you'd need to make a bearing carrier to which you would mount the bellcrank.  Or, you could just make a bushing (I'd use aluminum) the same height as your putative bearing carrier, and firmly attach the bellcrank to it (probably by gluing it with epoxy, but maybe by gluing and screwing).

Someone makes a high-zoot center spar assembly for detachable wings that includes a suspended bellcrank mount; I can't remember who.  What do they do for bearings on the bellcrank?

[Brett Buck]

I did compare tractor and pusher props on the same ship. With the pusher prop on an electric conversion of my Stuka the first thing I noticed upon release was it wanted to hug the ground. I believe Brett has it right and the nose up/down moment we get is a function on the direction of rotation, direction of flight (those who fly clockwise will get the opposite), weight of the prop and rpm. The lighter the prop and slower the rpm the lower the pitching moment.

   There's really no argument that there isn't a precessional torque from the prop, or even what the magnitude or direction is in any specific case. The debatable part is whether or not this is the sole or even the primary contributor to the need (determined by trial and error) for some sort of down elevator (positive incidence, drooped elevator, etc).

   I don't have the thing in front of me*, but for my airplane and parameters, it was something like 33 in-oz. (nose up since it was conventional rotation). I figure that the torque from the thrust line offset is about half that, so you need something like 15 or so in-oz additional from somewhere to get to "neutral".

    BTW, I think there's a pretty big area where the performance is acceptable, so there's plenty of tolerance for a typical range of prop inertias, RPM, etc. I build it in because I know it works and I would have to rig some in somehow anyway, and that my tail airfoil is sufficiently forgiving to provide a decent range. That doesn't mean everybody should blindly do it.

   If I was going to build an airplane with a backwards prop* today, I would probably go back to 0-0 in the construction, then rig in some "up" elevator to start with. Then experiment from there.

   Brett

*it's not really a pusher prop, it's tractor prop that spins the other way. It's only a pusher if its pushing, i.e. behind the engine, er, driving mechanism.

*  it's the Mar/Apr 2006 SN. I remembered right, it was 33 in-oz.

[Russell Bond]

Someone makes a high-zoot center spar assembly for detachable wings that includes a suspended bellcrank mount; I can't remember who.  What do they do for bearings on the bellcrank?

Dallas (Herbie) Hanna in Sydney Australia makes them.

[RandySmith]

"Thanks for the tip.  I have used Windy bellcranks, but I'm making my own for the new dog.  I wonder if ball bearings would be prudent.

Your "ramblings" are pertinent and thoughtful, as usual."

I have helped fix many many stuntships "hunting " problems over the decades, Most of them came down to slop in the bellcrank/flap area, This is  Far worse than any slop in the elevator, The flaps ride up and down when the wind direction changes, or gusts blow the plane around, This changes the AOA of the wing by a slight amount immediately,when the flaps move up and down within the "slop range" and cause the hunt. Or the clime/dive routine.
\One of the most bizarre problem I found in hunting was a plane built extremely well with tight tolerances, the fit on the fillets to flaps and fillets to elevators were very tight and precision. What happened here was the parts never touched, but this was a problem with them being so close in that the castor oil from the side exhaust muffler filled the gap with the oil and the flap would stick as they passed the fillets, later in the flight the elevators would join in, so the hunting was non existent at the very first of the flight, then became worse 1/4 way into the flight, and progressively got worse as the flight went on, By the end of the flight it was very hard to hold level, opening up the gaps fixed the problem.

I have used ballbearings Bellcranks, they ARE NOT the answer, they are worse than plain bushings and the vibration in the bellcrank kills the ball bearings, Plus the bearings are still much of the time the plane is running, ball bearings work better, way better when moving or spinning. If I was to use any other bearings now it would be maybe needle bearings,, needle bearings would work far better than balls, and will last much longer, but I think a good tight bushing will do the job we want.

Randy

[billbyles]

I've been following this thread for a while and wonder why no one has commented that the inverted airfoil thing should give the opposite effect of positive incidence in the stabilizer. I see the positive incidence not just as a trimming thing - like to counteract precession from ccw circular flight - but as a way of keeping the pilot from having to alternate between positive and negative elevator for corrections around neutral. It seems that not having to pass the elevator through neutral (thus reversing horizontal tail camber) is a simpler and gives a more delicate touch. With the positive incidence or some negative elevator, the control around neutral is just  a matter of varying up pressure on the handle. Inverted, it's then just a matter of varying negative pressure. Shouldn't this enhance the "groove," with the easier or more familiar of the two being for upright flight? Down thrust probably is a close trade-off.

I believe that Doug and others before him have it right about the flaps. Perhaps the positive stabilizer is more beneficial for flapless planes, since they have no added negative pitching moment from the flaps. 'dunno, since the flap deflection canges with the elevator's leaving neutral. It can be good for the flapped plane to have some down flap (both ways, as Doug said) so that it can fly level with the fuselage level.

Anyway, since CL flyers say that negative stab incidence is a problem, it seems that the inverted airfoil shouldn't be expected to be a benefit. Several things about CL stunt seem to call for different solutions than FF and sometimes even RC (like increased static margins  for CL).

SK

Interesting post, Serge, for a few years I flew right wing (#3) with Team America, a 3 airplane formation aerobatic airshow team.  As we were coming from behind the crowd starting the show we set up for the formation acro: lead would call "landing light on, fuel on left tank, boost pump on, trimming for 200 knots, left echelon."  The trimming for 200 knots was rolling in down elevator trim to about 10 pounds of back pressure on the stick (our show was about 15 minutes long, depending upon the D/A so it was a huge relief to dial the trim back to neutral after the break for landing.)

The back pressure on the stick made it so that the airplane would not "wander" through up & down pitch and made it so we could fly formation acro with a spacing of about 4 feet during looping and rolling maneuvers as well as formation changes/join-ups.  Our left wing (#2) was a retired Naval aviator with many hours of formation flight and he schooled us about the down elevator trim.  Without the down trim it was scary to try to fly close formation - I only tried it once.

I can see how this would relate to the trim pressure that you mentioned above...something I had not translated before from my airshow flying experience.  Thanks for your insight.

[Serge_Krauss]

Thanks for the very interesting post, Bill. My "full-sized" air time was limitd to just some cross-countries and local flights - no aerobatics or formation flying. So that's an eye opener for me. 'must have been a bit stressful!

SK

[Howard Rush]

Is this the same situation?  For the full-scale airplane, do you use the trim bias to put some load on the controls to eliminate deadzone, or does it do something aerodynamical?

I'm not sure what Serge means by negative elevator or "negative pressure" on the handle, but I'll guess that he means positive (down) elevator and more tension on the down line than on the up line, respectively.  In that case, I don't think what he's describing is the feel you'd get with a trim bias, but just the differential line tension you'd expect with a forward CG.  Maybe we do use a trim bias-- some hinge moment one way or the other when flying straight-- and then mask it with handle setting.  Stunt is mysterious.

[Howard Rush]

The debatable part is whether or not this is the sole or even the primary contributor to the need (determined by trial and error) for some sort of down elevator (positive incidence, drooped elevator, etc).

In one of the links I posted, Igor used JavaFoil to figure the zero-lift downwash at an elevated stab.  If I get industrious, I'll do that for my airplane.  It looks easy.  I'd just as soon somebody else get industrious and do it for me, though. 

[Tim Wescott]

Is this the same situation?  For the full-scale airplane, do you use the trim bias to put some load on the controls to eliminate deadzone, or does it do something aerodynamical?

Or biomechanical?  Human senses and musculature are highly nonlinear; one may well have finer control against something with a little bit of load than against something that's completely free.

[Douglas Ames]

<snip>
The back pressure on the stick made it so that the airplane would not "wander" through up & down pitch and made it so we could fly formation acro with a spacing of about 4 feet during looping and rolling maneuvers as well as formation changes/join-ups.  Our left wing (#2) was a retired Naval aviator with many hours of formation flight and he schooled us about the down elevator trim.  Without the down trim it was scary to try to fly close formation - I only tried it once.

I can see how this would relate to the trim pressure that you mentioned above...something I had not translated before from my airshow flying experience.  Thanks for your insight.

So you were increasing the drag on the elevator with the trim tab, like a mini speed brake? Is there a dead spot in elevator response around neutral that was causing the "wandering"?

[Howard Rush]

... formation acro with a spacing of about 4 feet during looping and rolling maneuvers as well as formation changes/join-ups.

Wow.  400 feet would be scary enough.  

[Brett Buck]

In one of the links I posted, Igor used JavaFoil to figure the zero-lift downwash at an elevated stab.  If I get industrious, I'll do that for my airplane.  It looks easy.  I'd just as soon somebody else get industrious and do it for me, though.  

   Make sure and do it in inverted flight, too. I worked on the downwash theory for years. Positive incidence is in the right direction when you are upright, but definitely the wrong direction when you are inverted. That confounded me for years, I knew it worked both directions but couldn't see how.

     We will soon see what downwash does. I recently assisted a notable modeler with a plane that was absolutely evil in both upright and inverted flight, but mostly OK in the maneuvers. Could not be flown level. The model was a slight modification of a known-good design, the modification being that the stab was lowered to be just slightly above the centerline of the wing. This was on of the infamous flat stabs, but there had been MANY modifications to that with pointy LEs, turbulators, etc. with no significant improvement. Oh, and the prop was backwards, although they had tried it the other way with no significant change. My theory was that the stab was running near the downwash all the time, and when perturbed the signs of the disturbance torque started swapping around depending on slight variations in the control position.

   Since he was either going to fix it, or retire it, or back over it with the car,  I suggested moving the stab back where it belonged and making it airfoiled. Higher, in the normal spot, I think the stab will be out of the downwash and turbulence upright, inverted, and at very small control deflections you need to maintain level flight.

     Brett

[Douglas Ames]

There is a trend in R/C Pattern to use a flat elevator with radiused edges top/ bottom on the T/E. One of our club members uses them.
Supposed to make the elevator more resposive (?)