How can you cure bad airplane habits?

[RPM racing]

I have a Great Planes Extra 300 .40. When you apply a lot of elevator the plane will want to drop the wing or roll at the bottom of a loop. I am interested in building another kit like this but with bigger surfaces for 3D manuvers. I plan on putting on a bigger motor so it has plenty of power. I am also going to increase the size of the wing to compensate for the extra weight. But if it cannot handle the small elevator a bigger one will make things worse. Does anyone have any suggestion on how to fix this problem?

[Sparky Paul]

Two things... the c.g. is too far back, and there's too much elevator control. Cut down the throw, use dual rates, EXPO, moderate control input....
Fix these, it'll fly OK.

[DeaninMilwaukee]

Sparky:

Could you explain the mechanism behind why rearward CG causes the rollout from high G's?

I have this exact problem on a 42 oz 1/12 scale foamie p-51, but whats confusing is that all the other signs of tailheavyness are'nt there.
The plane flies very nicely at any time your not pulling high G's, no twitchyness in pitch at all, rails properly through lesser G turns, able to slow way down for final aproach, and glides extremely well with throttle off and prop folded back.

However, if you try to pull a pylon type turn, or pull out too hard a the end of a loop, you may very well find yourself inverted. Not fun at all. :(


Thanks,
Dean in Milwaukee

[raptor22]

Rearward cg makes the plane stall at slower speeds, but the same aoa. However, when you begin a maneuver it will naturally steepen. This provides better maneuverablility, but easier stalls.

Get a rectagular wing, washout, transition to lower speed 'foils at tips of wings (like funtana .40).

--Alex

[Ollie]

The stall is purely about angle of attack and can occur at any speed when the stall angle of attack is exceeded. The term "stall speed" refers to the minimum speed at which a stall can occur.

A rearward CG reduces yaw and pitch stability, lengthens the time it takes for the plane to recover from a disturbance without pilot input and makes the pitch and yaw controls more effective (sensitive).

[Sparky Paul]

What Ollie and Raptor sed..
Some planes with a sufficiently rounded nose to the wing will "ratchet" around a loop in a stalled condition.
It's possible to fly a plane with that snap potential, but it's not fun, knowing it can happen, but never being sure of when..
The last turn to final on landings is a popular place for too much elevator, and the consequent snap.
Moving the c.g. forward lets you keep the elevator travel high, for those tight manuvers.

[DeaninMilwaukee]

"Some planes with a sufficiently rounded nose to the wing will "ratchet" around a loop in a stalled condition."
"

Interesting, as thats actually what it feels like in a loop, and it is indeed no fun to fly.

I made two changes to the plane, both of which did help some. I blunted the relatively sharp LE, and I took about 20% out of the elevator travel. Elevator travel is quite small.

The plane is now flyable safely as long as you are content with keeping G's low.

I will give sliding the batt forward a try this spring. Maybe the plane will finally be fun to fly.

Dean in Milwaukee

Plane in question: jk aerotch p-51, 7 rc 1700's, endo 2.5:1 9x7 folder. Is quite fast. :)

[Sparky Paul]

A P-51
.
No wonder!
Those things need so little elevator you can't imagine how little, until you try one.
Why this is so, I don't know, but something about the layout makes it responsive with that little elevator and low surface displacements.

[pda4you]

I have a Great Planes Extra 300 .40. When you apply a lot of elevator the plane will want to drop the wing or roll at the bottom of a loop.

BTW - that is a very common characteristic of highly aerobatic typically rearward CG aircraft. The real one does it too!

Nothing is broken - you have a keeper!

Well to be honest, you will have an airplane with will do the most awesome snap rolls and will be very repsonsive.

I flew a CAP21 (notorious for just that characteristic) for a season until I tried to slow the landing down a bit too much. It snapped on approach and I rekitted it. :eek: :rolleyes:

It was an awesome fun time while it lasted. I had my CG set for neutral inverted flight, but it was before the days of expo radios (that I could afford!).

Expo or dual rates are the way to handle. Moving the CG up will help, but my Cap even with reduced throws would Snap (and let me tell you it was vicious with no warning).

You also must "fly" this airplane all the time. As long as you don't use too much elevator it won't snap. You just have to find the perfect combo of Expo, CG and throws to make an airplane fly correctly. It is all part of the fun of creating a great flying aircraft.

By the way if you want a more forgiving (but still very aerobatic) airframe try flying some pattern ships for a bit. They really teach you about aerobatics and can be nearly as forgiving as trainers.

Mike

[Sparky Paul]

Yes!
A Tiger II or Tiger 60 will make you look like a much better flier than you are! :)
Much more relaxing to fly than an Extra or CAP... I think the CAP licenses the snap roll to all the other designs, it has a hammer-lock on it! :(

[Ollie]

"Is quite fast," explains a lot. Control response increases as the square of the airspeed. When you double the airspeed the controls become four times as effective. That's why exponential controls or dual rates come in handy for planes with a wide speed range. Without exponential or dual rates, and a wide speed range, the controls are sluggish at low speeds or sensitive at high speeds depending on the set up.

Moving the CG forward some will desensitise the elevator response. If you move the CG too far forward the elevator response will even become sluggish at low speeds.

[Brian Nixon]

I have this exact problem on a 42 oz 1/12 scale foamie p-51, but whats confusing is that all the other signs of tailheavyness are'nt there.

Dean, I fly the JK Aerotech P-51 in 2610 combat and the last thing you want is an unexpected high-speed stall and snap maneuver in the heat of a sortie. (You think yours is quite fast? Try a Norvel .25 and an 8x6 prop!). To me, your observations point to too much elevator throw. Dial it out and the plane will become tame. Test it out with your dual rates (this gives you two shots at the throws--read Dub Jett's "Crap Trap" series about tuning a high-speed plane) until you've got it dialed to your taste. I've not encountered a snap problem with my planes (and JK Aerotech uses a real model-friendly airfoil, the Eppler 203). I do keep the elevator throws pretty reasonable. Like Sparky said, it doesn't take much to get the plane to do what you want it to do.

Good points from Ollie about speed, too. I've got a Quickie 500 plane, and the low rates for elevator throw on it is about 1/8 of an inch each way. It doesn't take much at 110 mph.

[DeaninMilwaukee]

"Dean, I fly the JK Aerotech P-51 in 2610 combat and the last thing you want is an unexpected high-speed stall and snap maneuver in the heat of a sortie. (You think yours is quite fast? Try a Norvel .25 and an 8x6 prop!). To me, your observations point to too much elevator throw. Dial it out and the plane will become tame. Test it out with your dual rates (this gives you two shots at the throws--read Dub Jett's "Crap Trap" series about tuning a high-speed plane) until you've got it dialed to your taste. I've not encountered a snap problem with my planes (and JK Aerotech uses a real model-friendly airfoil, the Eppler 203). I do keep the elevator throws pretty reasonable. Like Sparky said, it doesn't take much to get the plane to do what you want it to do.
"


I have only about 1/8th travel each way as it is.

I have built two other jk conversions, the zero and the ta-152. Neither does this and both have far more throw.

I use expo, and to be clear, the elevator is NOT touchy. Its easy to do smooth stable non acro flight. You don't realize anythings wrong at all till you pull some G's.

I will try going forward on cg and see what happens.

BTW, what rpm do the norvel .25's turn that 8x6?


Dean in Milwaukee

[Brian Nixon]

I didn't suggest your elevator was touchy, Dean. However, if the CG feels right and you're plane's snapping, you're likely pulling too much elevator. Are the ailerons correctly positioned (no reflex or flap effect)? Your plane might be a tad heavy, too, at 42 oz.

A Norvel .25BB turns an APC 8x6 at 16,500-17,000. Most combat guys use a 9x4 prop (they say it pulls a plane better through tight turns). I've run both and tend to prefer the 8x6. Add a Kentucky mousse can muffler and the RPMs go to 19,000 or so. (Haven't played with that yet. My view is it opens up a can of worms in terms of fuel-foaming and vibration issues.)

An 8x6 prop is fairly standard for club-level .25-size racing.

[RPM racing]

I originally posted this question because my Great Planes Extra is an excellent aerobatic plane but it will not do 3D manuvers. I have a Morris Hobbies Sledge 540. It is a good 3D plane but it is not as fun to fly in normal aerobatics. I thought if I could enlarge the surfaces on the Extra to do 3D manuvers I could have the best of both worlds. Has anyone done this? Will it work? Does anyone have some suggestions?

[BigDave]

I have a GWS P-51 which has had two different owners, me being owner number two. First owner's complaint = "she will snap on you at the top of a loop".

I bought the plane and put a much more powerful brushless motor in it which seems to have helped with the snap-at the-top-of-the-loop scenario; HOWEVER, I continue to see the sudden snap-stall now at the bottom of the loop (or at the bottom of a dive) when pulling out with authority.

As I recall from full scale days, this is called an "accelerated stall" which basically means the wing will stall at most any speed if the angle of attack gets too high. Appears that this is a nasty little characteristic of the P-51 no matter who kits it. Interesting that other P-51 kits have the same little mean streak in them.

BigDave

[Sparky Paul]

Those are good scale models! They duplicate the bad characteristics of the full-scale, which was never considered a good dog-fighter!

[DeaninMilwaukee]

Of course by the time the p-51 was on the scene, dogfighting was less important than a surplus of speed for the newer strafe and then outclimb tactics. A 300 mph Zero's excellent dogfighting abilties became irrelevant if they could'nt stay on the tail of a p-51. The p-51's could swoop into and out of dogfight almost at will.

It is interesting that all these different scale kits do indeed seem to have the same handling problem.

On mine though, you could easily envision it being a very stable gun platform for those strafing runs. Flown scale like even in higher winds, mine just locks into a groove and swoops though smooth as can be.

BTW, I've built JK aerotechs Zero, mustang and FW ta-152.
While it did'nt glide worth beans, the zero handled the best. The ta 152 is almost as good, but does glide, and because of it is the easiest to land.

Dean in Milwaukee

[Brad Trent]

Very informative thread! I was talking to a clubmate on the phone just now about his problems with a small Cap 232, which he described as "jerky" loops, and snaps out of high angle turns, and told him I'd look in this forum for any similar problems. Voila, here we are.:D I quoted most of the above responses to him, and he now feels that he may have indeed had the throws too large, and CG too far back. How's that for "real time" help from the 'net?:)

It's a little too late for his Cap; he very thoroughly rekitted it last week, but the info should keep him from making the same mistakes with it's replacement, the Scorpio Texan that he ordered yesterday, which will utilize the same power and radio gear as the late, lamented Cap. Thanks for being here for him.

Brad.

[Gordon]

Years ago I scratch-built a 52" span CAP 21 that was initially a tip-stalling nightmare. But because it was intended as an aerobatic plane, needing to be as happy inverted as erect, I didn't want to use washout. Span 52", power Enya .40 4-stroke - the open valve one so that dates the plane - weight about 3lb.

I played around with CG and control throws as has been suggested above, but the plane just retained the same snap-happy characteristic.

It transpired that the problem wasn't anything to do with CG or control throws.

I had the strip ailerons working as flaperons on the plane (drooped for landing, or mixed with elevator for aeros - I was using a JR-8 Unlimited, a lovely pre-computer era outfit I still own) and when I lowered the flaperons the plane dived. The flaps induced so much down-trim that if you rolled the plane inverted, and then popped the flaps, it nosed up and over in a figure 9 style loop.

After some thought I reasoned that the tailplane lacked effectiveness since when the flaps were drooped, increased downwash on the tail should have kept the plane flying straight and level or thereabouts.

So I doubled the tail area by pinning and taping planks of balsa round its periphery, and the tip-stalling was tamed. Not only that, popping the flaps then caused the plane to go nose-up. So I trimmed away excess balsa round the tail until it stayed about level when the flaps were popped. Then I made a new tail to that size.

The plane turned out to be a beauty, and could be snap-rolled from knife-edge to knife-edge with no problems - 1/2, 1 and 1-1/2 snaps.

So if moving the CG and reducing control throws don't tame a tip-stall, try increasing tail area. You could do worse.

And BTW, I've been using strip ailerons as flaperons as an aerobatic and landing aid for the past 20 years on aerobatic ships, so please don't tell me they cause tip-stalling, as I haven't crashed any of the 8 or 9 planes I've used this setup on due to tip-stalling, or even had any of the planes depart from controlled flight with the flaperons drooped.

Actually, the thing I blame for most tip-stalls is guys having too much control movement. I always set up my planes for the minimum movement that will let me perform the aerobatics I want, including autorotations.

I've tried but couldn't get used to expo and I never seem to need it, but that's probably 'cos I'm just an old stick-in-the-mud. I don't have a problem with others using expo if they find that it improves their plane's response, just as long as they don't expect me to believe it will correct bad aeroplane characteristics that need an aerodynamic cure.

My experience of increasing tail area is consistent with the statements regarding a plane stalling at a particular angle of attack. Increasing tail area dampens, ie slows, the plane's pitch sensitivity, so it won't be as easy to force the wing up to that stalling angle.

Gordon

[Gordon]

Years ago I scratch-built a 52" span CAP 21 that was initially a tip-stalling nightmare. But because it was intended as an aerobatic plane, needing to be as happy inverted as erect, I didn't want to use washout. Power was an Enya .40 4-stroke (so that tells you how long ago this happened) and weight about 3lb so it wasn’t a lead sled.

It transpired that the problem wasn't due to excessive control throws or wrong CG. In fact I only found the root cause of the problem because I had the strip ailerons working as flaperons on the plane (drooped for landing, or mixed with elevator for aeros - I was using a JR-8 Unlimited, a lovely pre-computer era outfit I still have).

When mixed with the elevators the flaperons seemed strangely ineffective, and needed a lot of “up” elevator trim when drooped for landing ie when I lowered the flaperons the plane dived. Lowering the flaperons induced so much down-trim that if you rolled the plane inverted, and then popped the flaps, it nosed up and over in a figure 9 style loop.

After several flights where I tried moving the CG around and playing with control throws as guys have discussed above, I guessed that the tailplane lacked effectiveness since when the flaps were drooped, increased downwash on the tail should have kept the plane flying straight and level.

So I doubled the tail area by pinning and taping planks of balsa round its periphery, and the tip-stalling was tamed. Not only that, popping the flaps then caused the plane to go nose-up. So I trimmed away excess balsa round the tail until it stayed about level when the flaps were popped. Then I made a new tail to that size.

The plane turned out to be a beauty, and could be snap-rolled from knife-edge to knife-edge with no problems - 1/2, 1 and 1-1/2 snaps.

So if changing control response and moving the CG forward fail to cure a plane’s snap-roll problem, try increasing tail area. You could do worse.

And BTW, I've been using strip ailerons as flaperons as an aerobatic and landing aid for the past 20 years on aerobatic ships, so please don't tell me they cause tip-stalling, as I haven't crashed any of the 8 or 9 planes I've used this setup on due to tip-stalling, or even had any of the planes depart from controlled flight with the flaperons drooped. My current flaperon-equipped plane is a 4 year old Ky CAP 232 weiging 7-3/4lb with geared Maxcim and 20 cells.

Actually, the thing I blame for most tip-stalls is guys having too much control movement. I always set up my planes for the minimum movement that will let me perform the aerobatics I want, including autorotations. I’ve never got used to expo BTW, so don’t use it. I realise that others do find expo useful for improving the feel of a plane, but there’s no way that expo will cure designed-in bad snap-roll behaviour; only an aerodynamic cure will do that.

Gordon

[Rotozuk]

Tip stalls.. I have a plane that does a wonderful tip stall right now. It is about due for a recover job, and I'll probably put in a bit of wash out into the tips since it doesn't fly worth beans inverted.

I know it isn't an issue with CG or tail area as it is a flying wing, and I have played with CG and throws. True, big throws make it worse, but I can not fly it with smaller throws and still get around the sky.

Some other causes for tip stall is wing loading, speed, air flow, etc.

The Nasa method for removing tip stall is to use a different air foil on the outer section of the wing. Not pretty, but it works.

I try to avoid wing plan forms with a lot of rear sweep in them. Heck a nice forward sweep should never tip stall on you. :cool:

-Wayne

[raptor22]

Originally posted by Rotozuk
Tip stalls.. I have a plane that does a wonderful tip stall right now. It is about due for a recover job, and I'll probably put in a bit of wash out into the tips since it doesn't fly worth beans inverted.

I know it isn't an issue with CG or tail area as it is a flying wing, and I have played with CG and throws. True, big throws make it worse, but I can not fly it with smaller throws and still get around the sky.

Some other causes for tip stall is wing loading, speed, air flow, etc.

The Nasa method for removing tip stall is to use a different air foil on the outer section of the wing. Not pretty, but it works.

I try to avoid wing plan forms with a lot of rear sweep in them. Heck a nice forward sweep should never tip stall on you. :cool:

-Wayne

Thats the way to do it. Irfoil transitions are great if you are trying for aerobatics instead of and elliptical lift distribution. Like t6he funtana .40.

--Alex

[MartinChou]

A tip stall is a tip stall, high speed or not, forward cg or back, stripe flap or not, over control or otherwise, when stall happens at the wing tip first, you have a tip stall. Worse yet, and normally, you have it stalled first only on one wing tip, and hence a snap! :(

To remedy tip stall, you stall the wing root first. (Very intuitive, nah! :) ) As Wayne suggested, an aerodynamic wash-out (transit to different airfoil) will help to reduce the tendence of tip stall. There are, of course, many other ways to reduce tip stall tendency, or induce root stall first. Such as geometric washout, proper wing tip design, (it is 3D flow!), leading edge radius, or some vortex devices. Be warned that our models are small and Reynolds numbers are quite small and therefore, not all big airplanes' suggestions can be straightly applied. (e.g. vertex devices. turbulant/laminar flow control, etc.)

IMHO, LE radius and wing tip design can be easily altered on our models to fight this problem. Two cases to discuss. One is GWS Me-109 (http://www.rcgroups.com/forums/showthread.php?threadid=186337) , which have numerous reports on its high speed tip stall tendency. It happened to mine, too. I almost lost mine once on a split-S due to this. Another case is my own design, PinkStar (http://www.rcgroups.com/forums/showthread.php?threadid=184991) , a pattern/F3A design. I designed it with a slant wing tip, mainly to smooth out the wing tip vortex, and, contradict to someone's claim above, with big rear sweep wing platform. The slant wing tip plus the rear sweep effectively lengthens the chord length at the tip, therefore, reduces AOA and hence delays the stall. No tip-stall obsvered at all, high speed or low speed. And, most importantly for a F3A design, it has a very solid (controlled) snap roll behavior. :)

As for LE radius, that involves low Reynolds number effect/behavior. Other knowlegable people should know better.

Hope this helps in some cases. :cool:

Martin
:)

[MartinChou]

A little bit further discussion on my PinkStar design at this post (http://www.rcgroups.com/forums/showthread.php?postid=1924535#post1924535) . The effect is shown in this video clip (http://rcgroups.com/gallery/showphoto.php?photo=5621) (the second flyby, starts at 00:30). :)

Martin