I have a Continental that flys like ***t

[Andrew Borgogna]

I have a problem, my Continental does not fly very well and I have no idea what the problem is.  When I move the CG forward enough to stop the hunting the plane is so unresponsive that flying the pattern is almost impossible.  When I move the CG to where it will fly a decent pattern it hunts like a starving wolf.  My good friend and Zen Master Larry Renger thinks the problem might be related to the elevator flap ratio is 1:1.  Larry thinks that if I increased the elevator throw to 1.5:1 and move the CG to where the plane does not hunt might solve my problem.  Before I cut into the plane I would like to hear some opinions from out there in cyber space.  The plane does have a sharp leading edge, per the plans and I think this could be adding to the hunting problem at least I have heard some people say that.

OK people give me some help on this.
Andy

P.S. My Stunthanger tea shirt arrived today and yes it was worth the wait.

[Steve Helmick]

Is this the Warden or deBolt "Continental", or something else? Here's some ideas I had: Are the hingelines sealed? How heavy is it/what's the wing area? How big is the bellcrank? What handle spacing? Is the CG where the plans call for? Where are the LO's, relative to the CG? Is the LO guide a slider? Is the slider tight? Did you do a good job on "bench trimming"? Did you put a touch of positive incidence in the stabalizer? Do you have some down trim in the elevator?

If you increase the elevator travel, don't forget to narrow the handle spacing. If you dial in down trim, don't forget to adjust handle for neutral. I moved the slider on my SV-11 and managed to break it out of the soft 1/16" balsa rib...very exciting flight characteristics resulted! Stuff like this needs to be beefed up a bit, but it's typically not shown on plans. A couple of short vertical pieces of 3/16" square is plenty, one on each end of the track.  Steve

[wwwarbird]

 An easy thing to try is to add just a hint of downthrust to the engine.

[Al Rabe]

Move the CG forward to stop the hunt and add line spacing at the handle up to as much as 6".  All of my airplanes fly with 5 1/2" - 6" line spacing at the handle.  Do this before cutting the airplane.  I wouldn't worry about the leading edge radius but really hate the thought of increasing the elevator travel and reducing flaps.  This sounds like an invitation to get stall buffet in the corners.

Al

[Doug Moon]

Down thrust is a good first option and easy to do if you have the spinner clearance.  One washer under each front lug.  Then fly.  If the hunt goes away or is very reduced then you could have a thrust issue.  Or it could help mask the problem like the nose weight and forward CG will do.  

With the engine mounted normally, no down thrust added. Locate the CG near or right on where the plans say it should be and head out for some test flights.  Take the model up high, 35-45 degrees should be high enough and put your hand what would be level for that altitude and let it fly where it wants to, meaning no input.  It should go up OR down on its own.  This will tell you if you have the push rod too long, hunting up, or to short, hunting down.  From here you will know what to do.

Once you have the rod the right length you should be able to final trim it out from there.

Hope this helps.  Let us know how it goes.  

[Doug Moon]

Move the CG forward to stop the hunt and add line spacing at the handle up to as much as 6".  All of my airplanes fly with 5 1/2" - 6" line spacing at the handle.  Do this before cutting the airplane.  I wouldn't worry about the leading edge radius but really hate the thought of increasing the elevator travel and reducing flaps.  This sounds like an invitation to get stall buffet in the corners.

Al

I second what Al says about increasing elevator to overcome a nose heavy CG setting.  The nose heavy CG in this case is masking the hunt which is caused by something out of alignment. 

[Larry Fernandez]

Do what Doug says.
I had a Thunderbird a couple of years back that displayed all the symtoms you speak of.
I tried many thing but the final solution was a bit of down thrust, Just one washer under the front motor lug did the trick.
Try it and let us know how it worked out.

Larry, Buttafucco Stunt Team

[builditright]

Let's think back during your construction for a second.
Is/was it a kit or scratch built?
Did you check your wood and wing and tail cutouts during construction?
It could have been an unseen issue with a piece of wood that may have
been bowed, so you also want to check your incidences.

Oh! and like Steve said; make sure you have bench trimmed it and seal the hingelines.

[Dennis Toth]

You didn't indicate what the ship is powered by, if you are running a pusher on electric you may need to use some upthrust by shimming the motor. A couple of the usual things to check: Set the CG per the plans and set the leadout position about 1/2" behind that for a starting point. Fly the ship and if the hunting is still there move the leadout 1/8" further rearward and fly again, if the hunt is still there go 1/8" forward of the 1/2" start point and fly. Whichever way it flies best leave it. Next move the CG forward in 1/8" increments and see were it reduces the hunting the best, leave it there and readjust the leadouts. If it still hunts take a length of 0.030" music wire and tape it to the center of the leading edge of the stab. This gives a break point to the airflow over the stab and has helped on some ships if the stab has a round leading edge. If all this doesn't help there are two last options to consider. You indicated it has a shape leading edge to the wing, this is likely the root of the problem and you could simply break the edge a little to get better flow over the LE. This might be less trouble then cutting into the fuse, the last option is to add slop to the elevator horn.

Best,      DennisT

[Andrew Borgogna]

OK let me give you some data on the plane.
RSM version of the Tom Warden's Continetal.
CG, there was no call out on the plans for the CG.  I contacted Tom Sizemore and he gave me the location where is set it for the protype build.
Weight I believe is 61 ounces, but I will recheck.  Right now i am nurseing an illness and model airplanes are taking second place.
Thanks for all the advice, I noticed nobody was concerned about the sharp leading edge so I will put that on the back burner for the time being.
Andy

[EddyR]

Leadout rake sometimes will cause hunting. Just move it a 1/16 sometimes corrects it.
Ed

[Matt Colan]

Do you have negative incidence in the stab and elevator? That would also induce a hunt. That would be the only other thing I would look at along with what everybody else has mentioned.

[John Miller]

An easy thing to try is to add just a hint of downthrust to the engine.

 

We've discussed this before, but there's been enough time passed by that it probably bears repeating.

RJ, AKA Alpha dog, penned a post several years ago entitled, "Things that always work".

It's my opinion that every model airplane builder should get a copy, laminate it in plastic, and pin or tape it above the building boards.

To paraphrase what he wrote, among other items, he extolled on the desirability for a degree or 2 of down thrust when using a counter clockwise rotating prop. (This is the normal rotation style prop we've used for years). You don't really have to understand the reasons, just know, it works. Here's a hint, it has to do with gyroscopic precession, and the resulting asymmetrical thrust.


Coupling the above practise, with a slight positive incidence on the stab-elevator, has allowed me to have planes that were easy to trim, and experienced a lack of hunting characteristic's. No sagging elevator, (droop), or loose controls,allowing the making of better intersections, and more accurate shapes.

 There are few who can figure out the trim needed for a competition airframe,  just by watching it fly, but Bob is one of those gifted ones.

Barring flying in front of someone like Bob, at least try a little down thrust.   

[Avaiojet]

Could someone be so kind as to define "hunt," "hunting."

Exactly what does this mean?

Charles

[John Miller]

Could someone be so kind as to define "hunt," "hunting."

Exactly what does this mean?

Charles

A plane is said to be "Hunting", when it won't settle into a groove. The pilot has to make constant adjustments even when flying level between the "tricks".  You must make all these corrections, as the plane seems to have a mind of it's own, when it's hunting.

[Steve Helmick]

Somewhat akin to "hunting"...possibly mistaken for "hunting"...I watched a fella flying his plane at ata contest. About half a dozen times per lap upright, he yanked on some up elevator, because the plane was diving toward the ground, instead of grooving at 3.9' to 5.9'. I looked at his handle position during level flight, and he was holding "up".

That's a really dangerous situation, because he could easily smack it into the ground at the bottom of any of the squares or triangles, spreading parts all over the circle.  Not wanting to see the carnage, I mentioned to him that his handle was misadjusted, and yet saw the same thing the next round. This is one of the (many) reasons why it is crucial to have a dedicated set of lines and handle for each plane. It also confirms the old adage that you can lead a horse to water, but you can't make it drink.  Steve

[wwwarbird]

 

RJ, AKA Alpha dog, penned a post several years ago entitled, "Things that always work".

It's my opinion that every model airplane builder should get a copy, laminate it in plastic, and pin or tape it above the building boards.

 I would love to see Alpha Dog's post, sounds like some good stuff.

[Russell Bond]

Is this the one?

[John Miller]

Yessir, that's the one. following Bobs suggestions will all but eliminate hunting in our models.

[john e. holliday]

Thanks, I now have it printed off and will file it away where  I will forget it.

[Randy Cuberly]

Thanks, I now have it printed off and will file it away where  I will forget it.

Yeah,
That's one of my problems too Doc.

Of course here in Tucson we do have a slight advantage...Bob is always at the field and ready to help trim a problem airplane.  He definitely is one of the best at doing it.  Magic?  I dunno but he always seems to be able to sort out the problems and get it working.
I'm not the worst at trimming but Bob is definitely the best I've come across...Talent?  Yeah I suspect that's the answer.

Randy Cuberly

[Leo Mehl]

I have had a couple of planes that I built would not fly the pattern at all or not very good  One was my Actic Fox I can't remember what I did to trim it but the plane would like to dive in level flight. I finnaly figured out that the leadouts were sticking so when I oiled the leadout guides it quit hunting right away but I had no line tension at some places in the pattern so I addes tip weight which stopped the lose line tension. I also went to more line spacing on my handle which made it easier to correct any problems with it staying in the grove very rapidly. It also took away my fear of loosing the plane in a tight manuvouer. All in All it turned out to be one of my best stunt designs. I also built a Vector that would almost fly a good pattern but it did pretty much stay in the groove but It did have the wrong flap to elevater throws so I canged it to a two to one ratio. It then wanted to stall  and hinge on the triangle and the hourglass. I figered it had too much tip weight so I took 1 ounce of the weight out and that stoped the hinging. The plane was a little tail heavy so I added one half ounce to the nose so it would track a little better and I went from a two blades 11X6 APC to a three bladed 10X 6 prop and the turn was there and with the hinging stoped it flew a good pattern but the rounds were hard to control so I went to a hard point handle with wide handle spacing and everything smooted out and flew the way I wanted it to. These two airplane were a lot of work but the main thing is not to give up on the trim process untill you know it will fly the way you want it to. I think if you have to open up the plane for adjustments you could make the elevaters with a little slop in them and that would help your level flight. The main thing is not to give up on it and get help from other flyers that may have more experience in trimming than you do. Lots of luck Andy. and have a good winter of flying.

[wwwarbird]

Is this the one?

 Thanks for digging that up Russell.

[Chuck_Smith]

Interesting take on the aerodynamics. It fails to take into account ALL the moments on the aircraft working around the CG, such as the fact that the wheels contribute a lot more moment on than the stab/elevator, that the wing is generally below the CG, and most important of all, the wing's downwash which is giving the stab an effective angle of attack.
 

And finally, the upwash and downwash areas of the flow field around the wing and their effects on the aircraft's fuselage contribution to trim condition are completely ignored.

But where it misses the mark is it is talking about static stability and not dynamic.

This is misleading because static stability DOES NOT guarantee dynamic stability. The good news is, dynamic stability DOES guarantee static stability.

Also, I fail to see how p-factor affects pitch. Yaw 'fer shure. Same with precession, the precessive force is normal to the rotation as angular momentum is conserved.  Precession is defined as the torque normal to the applied moment, so that doesn't make sense to me either... need to think out it a bit.

Finally, he writes about the stab's drag contribution, but really guys, 1 degree? I'm pretty sure sin(1 degree) is pretty much zero, so the delta in drag will be insignificant.


But back to the "hunting" issue.


Look, we have two effects here: Most people can get their heads around static stability, and that's what the article skirts with but doesn't quite get it right. But static stability is almost theoretical anyway. Unless you're flying a perfectly trimmed sailplane in wave at 20'000 feet you will never have the conditions where the aircraft has no disturbances in the air or control inputs.

Dynamic stability is how an airplane responds to any disturbance, be it atmospheric turbulence, control inputs, or some screwy things like separation bubbles moving around.

Dynamic stability requires the slope of the pitching moment curve for THE AIRPLANE to have a negative slope and that the the Cm(AoA) be zero at a positive value of AoA, and that gets designed in.

The dynamic stability of an airplane is by definition, the time it takes the airplane to reach an amplitude of one-half of the original disturbance.

We want to balance the damping of this amplitude with the ability to turn a sharp corner. We do this by picking the best dCm/dAoA. Again, designed in.  

Now, let's think about this... if we put positive incidence into the stab, (assuming that LE up is positive) we can't move the neutral trim point, because that is a function of lift and airframe geometry, and we don't change the damping. So other than changing the required angle of the elevator attached to the stab to get back to the aircraft's trim point, what did we accomplish?

Well, I'll tell you. We moved the flaps up slightly and changed the wing's  Cl(AoA) curve, giving us a differential flap movement between inside and outside. That's what happens. Baaaa Zinnng! But we can do that with linkage geometry too which is much more elegant.

Like all things stunt, I think there is a pretty big "placebo effect" to many of the little things guys do to their ships. I have a solid belief that many flyers prefer a ship that turns better outside than inside. Why? Because the "corners of death"  in the pattern to a newb are the outside ones. The outside square, the intersection of the eights, heck, the reverse wingover pullout on the first half...all of these are nose-pointed-down outside corners, and I believe that anything newcomers can do to make a ship turn better outside gives them confidence, even at the cost of the last turn of the hourglass. That habit gets instilled and that's how they like their ships to fly. Nothing wrong with that either.


I've built many a model with 0-0-0 settings and they fly just fine. Combat ships turn both ways pretty good as do guided missiles, neither needing a stab correction.

Hunting, once the CG has been determined to give adequate static margin, IMVHO, means you either have a design problem or more likely... your stab is:

1) in turbulence from the wing

2) too small (not likely!)

3) Too thin (highly likely)

4) Has a problem with the LE

or that something is going on at the wing ...usually related to the flap gap... that is causing instability in the flow. Remember, what is happening at the flap gap is affecting what is happening in front of the wing! Really. So if a small flap movement causes a change in the way things are leaking through the gap - entirely possible - the airplane's neutral trim point starts moving around and you get a tired pilot.

Always bear in mind that we fly stunt at such ridiculously low Re numbers that there isn't sufficient energy in the flow to keep things moving straight, so it's very difficult to keep the flow attached. Anything sharp (TE's excluded!) or inconsistent can play havoc with trim.

YMMV, that's totally cool.

[EddyR]

Mr Sith
 I don't know you but you have opened the door
Ed

[Randy Cuberly]

Interesting take on the aerodynamics. It fails to take into account ALL the moments on the aircraft working around the CG, such as the fact that the wheels contribute a lot more moment on than the stab/elevator, that the wing is generally below the CG, and most important of all, the wing's downwash which is giving the stab an effective angle of attack.
 

And finally, the upwash and downwash areas of the flow field around the wing and their effects on the aircraft's fuselage contribution to trim condition are completely ignored.

But where it misses the mark is it is talking about static stability and not dynamic.

This is misleading because static stability DOES NOT guarantee dynamic stability. The good news is, dynamic stability DOES guarantee static stability.

Also, I fail to see how p-factor affects pitch. Yaw 'fer shure. Same with precession, the precessive force is normal to the rotation as angular momentum is conserved.  Precession is defined as the torque normal to the applied moment, so that doesn't make sense to me either... need to think out it a bit.

Finally, he writes about the stab's drag contribution, but really guys, 1 degree? I'm pretty sure sin(1 degree) is pretty much zero, so the delta in drag will be insignificant.


But back to the "hunting" issue.


Look, we have two effects here: Most people can get their heads around static stability, and that's what the article skirts with but doesn't quite get it right. But static stability is almost theoretical anyway. Unless you're flying a perfectly trimmed sailplane in wave at 20'000 feet you will never have the conditions where the aircraft has no disturbances in the air or control inputs.

Dynamic stability is how an airplane responds to any disturbance, be it atmospheric turbulence, control inputs, or some screwy things like separation bubbles moving around.

Dynamic stability requires the slope of the pitching moment curve for THE AIRPLANE to have a negative slope and that the the Cm(AoA) be zero at a positive value of AoA, and that gets designed in.

The dynamic stability of an airplane is by definition, the time it takes the airplane to reach an amplitude of one-half of the original disturbance.

We want to balance the damping of this amplitude with the ability to turn a sharp corner. We do this by picking the best dCm/dAoA. Again, designed in.  

Now, let's think about this... if we put positive incidence into the stab, (assuming that LE up is positive) we can't move the neutral trim point, because that is a function of lift and airframe geometry, and we don't change the damping. So other than changing the required angle of the elevator attached to the stab to get back to the aircraft's trim point, what did we accomplish?

Well, I'll tell you. We moved the flaps up slightly and changed the wing's  Cl(AoA) curve, giving us a differential flap movement between inside and outside. That's what happens. Baaaa Zinnng! But we can do that with linkage geometry too which is much more elegant.

Like all things stunt, I think there is a pretty big "placebo effect" to many of the little things guys do to their ships. I have a solid belief that many flyers prefer a ship that turns better outside than inside. Why? Because the "corners of death"  in the pattern to a newb are the outside ones. The outside square, the intersection of the eights, heck, the reverse wingover pullout on the first half...all of these are nose-pointed-down outside corners, and I believe that anything newcomers can do to make a ship turn better outside gives them confidence, even at the cost of the last turn of the hourglass. That habit gets instilled and that's how they like their ships to fly. Nothing wrong with that either.


I've built many a model with 0-0-0 settings and they fly just fine. Combat ships turn both ways pretty good as do guided missiles, neither needing a stab correction.

Hunting, once the CG has been determined to give adequate static margin, IMVHO, means you either have a design problem or more likely... your stab is:

1) in turbulence from the wing

2) too small (not likely!)

3) Too thin (highly likely)

4) Has a problem with the LE

or that something is going on at the wing ...usually related to the flap gap... that is causing instability in the flow. Remember, what is happening at the flap gap is affecting what is happening in front of the wing! Really. So if a small flap movement causes a change in the way things are leaking through the gap - entirely possible - the airplane's neutral trim point starts moving around and you get a tired pilot.

Always bear in mind that we fly stunt at such ridiculously low Re numbers that there isn't sufficient energy in the flow to keep things moving straight, so it's very difficult to keep the flow attached. Anything sharp (TE's excluded!) or inconsistent can play havoc with trim.

YMMV, that's totally cool.

Mr Smith,
I also don't know you but I think your tretise above is a complete over simplification of the aerodynamic problems involved with a CL stunter flying in a tethered position in circular motion.  You fail to realize it seems that most stunters especially those built by less than expert fliers/builders and not perfectly aligned and a lot of the rim functions performed as in Mr Whitelys article, are to correct thos misalignments and problems without cutting things apart.  Maybe not perfect but they do work.  I've trimmed a lot of airplanes and it usually is not as simple as you seem to think...full scale aerodynamics aside.
Incidentally precession does effect stability due the constant crabbed position and angle of the propellor to the direction of flight...

Randy Cuberly

[Chuck_Smith]

Randy,

I don't really think so. We are leaving out dutch roll response, yaw and roll response and only looking at pitch from the stability aspect, which is purely aerodynamic ( for small excursions for straight and level) and for straight-and-level flight it is perfectly acceptable to ignore any coupling. Also, if you do the math you'll find the slip angle on a controline plane isn't as much as we like to think it is.  

The hunt has got to be due o one or combination one of three things:

1) The phugoid damping is insufficient (pretty unlikely on a CLPA ship if the CG correct)
2) The neutral point is moving due to aerodynamic effects,
3) Aeroelasticity

Gotta be.

OK, let's look at it. Damping, if the CG is right no problem there.  Aeroelasticity... not usually problem on a well-built model. So that leaves us with what most likely suspect. The neutral point is moving because the Cm is being affected by something.   (But you never know, like Holmes said, when you eliminate...)

And that something has been, in my experience and yours certainly may very, something with the flaps. Linkage, gap, bending...something is altering the flow field and changing the moment. When you think about it, what other surface has enough effect on the wing to cause problems like that???

So think...Something disturbed the plane from straight and level... the controls moved to correct...a vortex gets shed...something as a result of the control movement upset the flow field...the trim point moved...the elevator needs to move to compensate...the flaps are tied to it, they move...the TE sheds a vortex...the vortex travels back and hits the horizontal tail...

So, it's not an oversimplification, and it's all based on real science and aircraft stability and control experience.

Now, if you want to look at real fun think about trying figure this out with compressibility effects at high-alpha, low Rho, in a descending turn while rotating the weapons bay to open with three tons of GBU-31 Mk84s attached to it... makes writing the equations for pitch-only for a controline plane flying straight-and-level fairly straightforward.

Anyway, it's fun to hash this stuff out while the dope is drying.







 

[Andrew Borgogna]

Some more information.  I had a phone call from Denny Adamisin the other day and we discussed this plane.   I left out a couple of pieces of the pie that could relate to this problem.  The plane is electric powered with a pusher prop.  Denny suggested reversing the motor and put a tractor prop on it.  Pointing out Tom never flew or even tested the plane with  pusher prop and revers rotating engine.  Some have suggested the slight addition of some down thrust, given the way I built the plane that's just not possible.  This was a very early attempt at electric before I started gaining knowledge in the art.  After starting the project I realized I didn't know enough about electric power system so I hung the framed up airplane in the garage for the better part of a year and worked on learning electric power systems.  The flap, horizontal stabilizer, elevator and center line of the motor are all set to zero degrees relative to the wing center line.  I have rechecked this several times with digital and mechanical levels.  Another thing Denny mentioned was having a heavy battery up front has caused him to have hunting issues in the past.  I know the original Continental never had the mass up front that my does, even with a full tank of fuel. One last thing both the flap and elevator hinge lines are completely sealed, that's not one of the problems. 

Again thanks for all the help and suggestions, when I am finally over this viral infection I have been fight for a week I will start applying some of the suggested changes and see what happens.
Andy

[Chuck_Smith]

Cool. I like a mystery like this. Do you know where the CG is as a percentage of MAC?

Or even just where it is behind the leading edge and the overall root chord with the flaps is a good starting point.

Also what is the weight of the motor and do you know the armature weight? How many RPM?

[Brett Buck]

Some more information.  I had a phone call from Denny Adamisin the other day and we discussed this plane.   I left out a couple of pieces of the pie that could relate to this problem.  The plane is electric powered with a pusher prop.  Denny suggested reversing the motor and put a tractor prop on it.  Pointing out Tom never flew or even tested the plane with  pusher prop and revers rotating engine.  Some have suggested the slight addition of some down thrust, given the way I built the plane that's just not possible.  This was a very early attempt at electric before I started gaining knowledge in the art.  After starting the project I realized I didn't know enough about electric power system so I hung the framed up airplane in the garage for the better part of a year and worked on learning electric power systems.  The flap, horizontal stabilizer, elevator and center line of the motor are all set to zero degrees relative to the wing center line.  I have rechecked this several times with digital and mechanical levels.  Another thing Denny mentioned was having a heavy battery up front has caused him to have hunting issues in the past.  I know the original Continental never had the mass up front that my does, even with a full tank of fuel. One last thing both the flap and elevator hinge lines are completely sealed, that's not one of the problems. 

Again thanks for all the help and suggestions, when I am finally over this viral infection I have been fight for a week I will start applying some of the suggested changes and see what happens.
Andy

   Changing the flap-elevator ratio will likely not make it any better as far as hunting goes. Depending on the weight it could be very detrimental to the turn performance, but without seeing it I wouldn't speculate.

     Under normal circumstances I would suggest tweaking in a little down elevator at neutral flap (maybe 3/32" at the elevator). But with a backward prop it's a different ball game. I would still experiment with the flap/elevator neutral but in both directions. One change should make it worse or better, telling you which way to go.
 
    But almost all hunting is caused by some sort of binding or friction in the control system, not an aerodynamic or other dynamics problem. Even the slightest binding anywhere will cause this, even dirty lines. 

Note that it's not a pusher prop - it's a backwards prop. A pusher would have to be behind the motor, if it is in front it's a tractor. It's labelled pusher because with conventional engine it would have to be installed as a pusher.

     Brett

[Bob Whitely]

Brett has it right. Absolutely positively  the controls must be free
and silky smooth before anything else can be done otherwise there
is no point going any further.  RJ

[Brett Buck]

The hunt has got to be due o one or combination one of three things:

1) The phugoid damping is insufficient (pretty unlikely on a CLPA ship if the CG correct)
2) The neutral point is moving due to aerodynamic effects,
3) Aeroelasticity

Gotta be.

     I don't think so. Hunting, at least the classic up/down limit cycle, is only very rarely a function of the aerodynamics. I have seen one design (a moderately popular kit from the early 90s) that appeared to have an aeroelastic issue with the stabilizer. Every one of them that I saw/flew did almost exactly the same thing, pretty drastically. And when I talked one of the owners into replacing the stab with a solid balsa that had a better structural layout, it went away completely.

    Being misaligned can make it unstable or otherwise have a trim issue that must continually be corrected but that's usually not referred to as hunting. Of course I can't tell for sure in this case, since I haven't seen the airplane fly.

    But in virtually every other case, the classic hunting was eventually traced to some sort of mechanical binding in the control system due to stick/slip effects.

      Brett

[Howard Rush]

...such as the fact that the wheels contribute a lot more moment on than the stab/elevator...

When I read this, I wondered if you'd ever seen an airplane.  Then I read Bob's piece and saw that you are reacting to his statement about the pitching moment due to stabilizer and elevator drag.  You mean to say that pitching moment due to wheel drag is more than pitching moment due to stab & elevator drag.  

Bob knows how to trim an airplane.  This is one case where he departed from "because it works" to make up a theoretical reason for it working.  Yes, pitching moment due to stab & elevator drag is trivial.  

 

And finally, the upwash and downwash areas of the flow field around the wing and their effects on the aircraft's fuselage contribution to trim condition are completely ignored.

Actually, probably not.  Bob didn't approach this from a theoretical analysis, but from flying stunt planes.  Any downwash effect is included in the stab and elevator positions that experience has taught him work.
 

Same with precession, the precessive force is normal to the rotation as angular momentum is conserved.  Precession is defined as the torque normal to the applied moment, so that doesn't make sense to me either... need to think out it a bit.

Or put a hefty prop on a stunt plane and fly some hourglasses.

We want to balance the damping of this amplitude with the ability to turn a sharp corner. We do this by picking the best dCm/dAoA.  

And by picking the best values for a dozen other things.

Now, let's think about this... if we put positive incidence into the stab, (assuming that LE up is positive) we can't move the neutral trim point, because that is a function of lift and airframe geometry, and we don't change the damping. So other than changing the required angle of the elevator attached to the stab to get back to the aircraft's trim point, what did we accomplish?

Well, I'll tell you. We moved the flaps up slightly and changed the wing's  Cl(AoA) curve, giving us a differential flap movement between inside and outside. That's what happens. Baaaa Zinnng! But we can do that with linkage geometry too which is much more elegant.

Like all things stunt, I think there is a pretty big "placebo effect" to many of the little things guys do to their ships. I have a solid belief that many flyers prefer a ship that turns better outside than inside. Why? Because the "corners of death"  in the pattern to a newb are the outside ones. The outside square, the intersection of the eights, heck, the reverse wingover pullout on the first half...all of these are nose-pointed-down outside corners, and I believe that anything newcomers can do to make a ship turn better outside gives them confidence, even at the cost of the last turn of the hourglass. That habit gets instilled and that's how they like their ships to fly. Nothing wrong with that either.

This might describe rookie-level stunt planes, but Bob writes about subtleties of competition-level stunters.  Indeed, I was surprised to come very close to the ground when I turned the first outside corner of a reverse wingover with a plane that had recently won a prize at the world championships.  It was less sensitive in down than in up.  I have concocted a theoretical explanation for why one would want that.

Airplanes that are hard to fly level usually have some nonlinear problem (although, as you say, too small a stab will do it, as was probably the case with my last airplane).  As Brett said, friction is the usual culprit, hence my search for smooth leadout wire.  I suspect your diagnosis of "...some screwy things like separation bubbles moving around"   is correct for rigid airplanes that have the usual anomalies like flap gaps fixed.  I think it explains the screwy combination of stabilizer and elevator positions at neutral that makes Impacts work right.  The current world stunt champion posted awhile back a CFD analysis of stabilizer boundary layer transition that's worth looking up.  





[/quote]

[Howard Rush]

But with a backward prop it's a different ball game.

Indeed.  Now with electrics, given a mirrored pair of props, we can see the effects of all that asymmetric stuff that people worry about (qualitatively).  There's also a whole new set of mysteries. 

[Brett Buck]

Indeed.  Now with electrics, given a mirrored pair of props, we can see the effects of all that asymmetric stuff that people worry about (qualitatively).  There's also a whole new set of mysteries. 

  Indeed. Should be able to provide tremendous insight with careful experimentation. For instance, does it take a different flap/elevator relationship on your Impact with the prop going the wrong way?

     Brett

[Howard Rush]

...for straight-and-level flight it is perfectly acceptable to ignore any coupling.

You'd think so.  I thought so, too.  My airplane had a tendency to dive while flying level inverted.  The local Varsity stunt guy told me to take out some tip weight.   It fixed the diving tendency.  

Also, if you do the math you'll find the slip angle on a controline plane isn't as much as we like to think it is.  

I did that math, and it kept me from moving the leadouts on my new plane as far aft as they needed to go.  The dog is flying pretty well now, but it's sure going through the air sideways.

[Howard Rush]

Indeed. Should be able to provide tremendous insight with careful experimentation. For instance, does it take a different flap/elevator relationship on your Impact with the prop going the wrong way?

I'll bet it does.  I doubt if the best relationship is among those I've tried so far.  Good thing it has a removable tail which doesn't have to have the joint taped to keep oil out. 

[Howard Rush]

Sorry for the diversions, Andy.  I'm no help with your airplane that flies like ***t.  My own airplane, which I could see and feel and calculate and fiddle with flew like ***t until I got extraterrestrial help.  Good thing there's not a trimmer-of-the-model rule. 

[Douglas Ames]

<snip> When I read this, I wondered if you'd ever seen an airplane.  Then I read Bob's piece and saw that you are reacting to his statement about the pitching moment due to stabilizer and elevator drag.  You mean to say that pitching moment due to wheel drag is more than pitching moment due to stab & elevator drag. 

Is there a pitching moment of the verticle fin that offsets the landing gear drag?
Is this why competition models have minimalized fins?

[Howard Rush]

I doubt if landing gear drag has much pitching moment either, but that's a thought.  I'll try to cipher it, but first I need to get in on a sale that only lasts until 6:00. 

[Brett Buck]

I doubt if landing gear drag has much pitching moment either, but that's a thought.  I'll try to cipher it, but first I need to get in on a sale that only lasts until 6:00. 

  I think you will find it a lot bigger than you think, particularly if you assume an unstreamlined wire. It's still nothing compared to the pitching moment from a tiny elevator deflection, of course.

    Brett

[Al Rabe]

Airplanes with weak longitudinal stability will frequently "hunt".  Airplanes become more stable as the distance between the center of lift and the center of gravity is increased by moving the CG forward.  As stability is increased control response from deflected controls will lessen.  Larger control deflections generally increase apparent control sensitivity.  It is frequently possible to retrim a nose heavy airplane by increasing the throw of the controls and adding line spacing at the handle to restore the apparent control sensitivity.  Larger control deflections typically increase available lift as an effect of larger flap deflections.

I"m not any kind of engineer but 27.000 hours in the cockpit (more than 6,000 as an instructor) have demonstrated on occasion that loading an aircraft near the aft CG will sometimes cause an aircraft to "hunt" or "porpoise" and the controls may indeed appear more sensitive.  Also, In my years of stunt flying I have experienced the most favorable trim to be found by balancing CG location and control deflections with line spacing at the handle.

Al

[Brett Buck]

Airplanes with weak longitudinal stability will frequently "hunt".  Airplanes become more stable as the distance between the center of lift and the center of gravity is increased by moving the CG forward.  As stability is increased control response from deflected controls will lessen.  Larger control deflections generally increase apparent control sensitivity.  It is frequently possible to retrim a nose heavy airplane by increasing the throw of the controls and adding line spacing at the handle to restore the apparent control sensitivity.  Larger control deflections typically increase available lift as an effect of larger flap deflections.

   I haven't found many cases where moving the CG forward actually cured hunting, only masks it. The sensitivity effects of excess nose weight can sort be overcome with larger handle spacing but, particularly on classic airplanes with teeny bellcranks, you run out of line tension to move the controls pretty soon in the process. Also with classic airplanes with teeny tails, you just run out of control authority at some point from lack of lift in the tail. That's why the original "lawn dart" airplanes came to be - create an "irresistable force" tail to overcome the "immovable object" of excess nose weight. Then we all learned to use the same giant tail as, strangely, a stabilizer, so we could move the CG back and obviate a lot of the limited control authority and relieved us from having to manufacture large amounts of line tension.

    The allowable CG range on something like a Continental will be pretty small. Too far aft, unstable, too far forward, will not turn (regardless of the spacing).

     Brett

[Ted Fancher]

  clip

    The allowable CG range on something like a Continental will be pretty small. Too far aft, unstable, too far forward, will not turn (regardless of the spacing).

     Brett

Tiny tails!!!  Exactly why my plans for Warden's Continental (a beautiful classic eligible airplane) remain in pristine condition in my files and are not covered with notes, glue drops and "under construction" components.  A tiny tail and a forward Cg is simply a non starter for me to justify the effort necessary to build any airplane with which I want to compete.

for  the first VSC I built a nice version of Billy's published 1959 Ares, another gorgeous airplane but, alas, hindered by a tiny 15% tail.  Flying the wingover with a full gas tank took courage because response was so sluggish that a five foot pull out looked way too close to the ground.  In the middle of the pattern in flew pretty nicely.  By the time the tank was near empty at the back of the pattern it responded like a combat ship with the streamer chopped off.  Twitchy would be a kind description.  If you think it was just me, ask PTG who flew it a number of years later at VSC and was not particularly thrilled with my generosity.  Don't get me wrong, it could be finessed and score well but doing so took considerable effort and more than a little TLAR (That Looks About Right) from both the pilot and the judges.

Al's earlier comments about using six inch handle spacings to drive three inch bellcranks to compensate for forward CGs and large flap/elevator deflections highlights the validity of my previous comments on CG location, large control deflections and available line tension.  The location of the CG with respect to the Center of Lift of the mainplane  (wing in this case...the "tail" in the case of a canard, for instance) is not a reliable predictor of stability.  The relation of the CG to the Neutral Point of the entire airframe (the point at which all of the aerodynamic forces acting on the airframe are concentrated) is what predicts the willingness of the airplane to proceed unmolested in the direction desired.

This isn't just Ted picking a fight with Al (a man whose accomplishment I admire and applaud), by the way.  The simple truth is that there are hundreds (if not thousands) of extremely competitive, stable but agile stunt ships around the world today using large tails, four inch bellcranks, aft CGs, smaller than historic flap and elevator chord ratios  AND handle spacing of four inches and less being flown competitively with primarily finely nuanced finger and wrist inputs.

I would make note that it is entirely possible to move the CG of a stunt ship so far forward as to require all the available up elevator only to maintain level flight.  This might, in fact, mask the hunt to a desirable degree but flying a competitive stunt pattern would be a bit more iffy.

Finally, my suggestion to Andy would be to take Al's advice...up to a point.  Figure out the area of the tail relative to the area of the wing and locate the "dry" CG (unfueled) at that percentage of the average chord(including flaps, of course).  If you've tried to fly it with the dry weight CG at the now popular 25% of the average chord you are almost certainly asking for a skittish and, perhaps, hunting airplane.  There just isn't enough tail to allow you to move the neutral point of the ship aft far enough for an adequate "static margin" with the CG at 25%.  If the airplane still hunts start looking elsewhere for the cause.

IOW, I agree with Brett (quel surpris) that a forward CG isn't, in and of itself, a no cost panacea for hunting.

Ted

[Larry Renger]

One thing that Andy failed to mention, is that his Continental is E-Powered.  CG shift is non-existant!  So if he gets it right, it stays right. 

[john e. holliday]

You telling  me you don't lose weight as the electrons disappear or is used up.

[Al Rabe]

First, since returning to stunt in 1999, I have posted many thousands of words on stunt forums describing in detail every facet of my designs and their construction.  Nowhere is there any suggestion that 3" bellcranks were appropriate or in use in any of my airplanes built in the last 45 years.

Next, I take issue with Brett.  CG location does directly affect affect stability.  Some aircraft flown with CGs at or behind the allowable CG range "porpoise" which is sorta the full size equivalent of stunt "hunting".  I have also experienced "hunting" on stunt ships of mine which were happily cured with noseweight.  I have also used noseweight and wider line spacing to increase available lift to tighten corners of marginally heavy airplanes.

Finally,  my post here was simply to suggest that a bit of noseweight and increasing line spacing at the handle might help the builder of the Continental to make his airplane more flyable without drastic modifications to the airplane.

Some of these replies make me wonder if it is worth the effort to try to help.

Al 

[Gene O'Keefe]

Al,

please keep the effort going....because guys like me absorb every word you say that'll help me fly a better model

Thanks for your valuable knowledge.

 Geno

[Howard Rush]

If you're flying a model at the same CG that its designer used successfully and it "hunts", the cause is probably some nonlinearity.  Moving the CG forward of the design point will mask that particular problem, but you're probably better off finding and fixing it.   

[Dennis Adamisin]

If you're flying a model at the same CG that its designer used successfully and it "hunts", the cause is probably some nonlinearity.  Moving the CG forward of the design point will mask that particular problem, but you're probably better off finding and fixing it.   

AMEN