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Help with stalling

Old 05-08-2010, 11:14 PM
  #1  
rms59
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Default Help with stalling

I've searched the threads here but just can't find what I'm looking for. I know I've posted this in other threads but I think this section of the forum is where I should have posted in the first place. I hope this will give me the answers.

I have this plane...

http://www.bananahobby.com/1813.html

I added some dihedral so I can fly it 3 channel. It flys beautifully but during the landing flair, it stalls and rolls to the right every time. I've adjusted the CG for a nose down attitude which doesn't help.

Flaps make it worse.

I also have an Ultrafly Cessna with flaps that looks like a scaled down version of the bannana hobby plane. I can land that one with full up elevator at the last few seconds and it never has any tendancy to roll. It just sinks faster the longer I flair but the wings remain rock level.

Now, years and years ago, I remembered someone adding a strip of 1/4 inch balsa to the top of the leading edge of a high wing plane to change the flying characteristics. I also remember him adding a bent down piece of light aluminum on each wingtip.

Any ideas would be helpful no matter what they are. I just want to try something.

Thanks,

Dick
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Old 05-09-2010, 08:11 PM
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Sparky Paul
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Stalls when landing are due to flying too slowly!
Keep more airspeed on the airplane!
This is done by -not- flaring until the plane really is ready to touch down.
The flaps won't help, they will let the plane fly slower, but only if you keep the nose down.
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Old 05-09-2010, 08:35 PM
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Rolling Thunder
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also landing deadstick will slow you down too much.Just giving it a little power to keep the prop spining prior to touchdown is usually enough to keep from stalling
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Old 05-09-2010, 11:03 PM
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Rolling to one side when stalling is called a tip stall. One wingtip loses lift before the other one, so that wing drops down. This is avoided by making the wing so that the roots of the wing stall first, and the wingtips last.
The balsa and aluminum mods you mention both try to make sure the wingtips are the last part of the wing to stall.
I suggest you google "fix wing tip stall" or similar, I'm sure you'll find an idea which works.

Edit: look at "washout" too, it's a common way to fix this problem.
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Old 05-10-2010, 12:59 AM
  #5  
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You should hunt through the wattflyer forums for Firebird Freedom and Tip Stalls.
For supposedly being a Z1 plane (i.e. A trainer) it is notorious for tip stalling.
I added some pics to that thread of some stall strips I added to the leading edge at the wing root.
It helps greatly, but speed is still your friend.

Flaps will help lower the stall speed, but they also add a lot of drag and slow you down too. So if you are already on the edge of a stall, dropping flaps will slow you down below the curve and make you stall worse.
Best to add a little power to get away from the stall, then drop flaps and jockey the throttle. Just remember to come in a bit hotter than you have been (i.e. Don't deadstick it in)
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Old 05-10-2010, 03:19 AM
  #6  
rms59
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Originally Posted by Felklaw View Post
You should hunt through the wattflyer forums for Firebird Freedom and Tip Stalls.
That was an awesome and helpful thread. I read up to page 20 and gave you a thanks for that. I especially liked cbatters determination to fix his problem and I'm going to do the same. He also said "Instead of living with the harsh tip-stall I bet we can fix it. All planes stall but they don't have to roll and flip over." which is exactly how I feel.

Tommorrow I will make and install a pair of stall strips. I will report back when the weather gets better which can be up to a week. Bummer.

Originally Posted by Felklaw View Post
Just remember to come in a bit hotter than you have been (i.e. Don't deadstick it in)
I have been coming in hotter but it wants even more. Like I previously said, I have an Ultrafly that simply sinks fast if I come in too slow but never rolls to either side. I'm going to get this cessna to do the same thing.

Thanks to all....for all the advice. I will check back.

Dick
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Old 05-10-2010, 08:27 AM
  #7  
JetPlaneFlyer
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If you are coming in 'hot' and it's still stalling then the model is overweight.

If you want to land slow without stalling then there is really only one effective way to do it and that's to make the wing loading lower by reducing weight or adding more wing area... A higher lift airfoil may also help a bit.

The wing taper is most likely what is causing the tip stall, washout is far and away the most effective cure it.. but you would still have to come in 'hot' even with washout. Washout is not a cure for an overweight model.

Stall strips can work on full size planes but I dont think they will do what you want on the model. In fact on models at low Reynolds numbers stall strips could act like turbulators which would have the opposite of the desired effect. In any case all stall strips do, even if they do work as intended, is make the wing stall earlier, so you will have to come in even 'hotter' than before

I'm not sure how best to add washout to a foam wong (i dont do foam). On a built up balsa wing I'd twist it by hand and apply heat... the same trick may well work on foam.

Steve
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Old 05-10-2010, 10:54 AM
  #8  
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I agree with Steve. Put the plane on a diet if at all possible.

Also, you mentioned 2 other things that caught my eye.

1)You said you moved the CG for a nose nose down attitude? Does that mean you moved the CG forward? Moving the CG forward will increase the stall speed, not lower it.

2)If it always falls to one side, you need to check for a warp or twist in the wing or ailerons or one side of the wing having more washout than the other. The other thing is to check the lateral ballance. See if the model balances side to side. You may have one side heavier than the other and thats why it falls off the same way each time.

Good luck!
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Old 05-10-2010, 11:39 AM
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pd1
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I agree the best way is to lighten the plane.
I also agree that stall strips don't add lift, so your approach speed won't be any better.
One more quick fix is drooped leading edges.

I have a Stuka that liked to drop a wing when stalled. I wanted more lift, not less.
I made the droops out of a 1/4 x 1/4 strip of balsa.
Adding the strips to the outboard section of the wing and reducing the elevator travel turned this plane into a docile flyer.

This isn't the only fix, just one of many.

Paul
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Old 05-10-2010, 12:47 PM
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JetPlaneFlyer
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Paul,
droop leading edges (fitted to the outboard half of the wings) is a great idea.. They effectivly add washout plus they increase airfoil camber thus adding lift (or at least not reducing lift as washout does)

Slats is another idea that may work: http://en.wikipedia.org/wiki/Leading_edge_slats

But none of these 'tricks' is a substitute for light weight

Also as Larry says; checking for and correcting lateral imballance or undesired warps must be done before you consider any modifications at all...


Steve
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Old 05-10-2010, 03:52 PM
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rms59
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Thanks for the extra replies.

I don't know what I can do about the weight. I even left off the wheel pants and landing gear fairings right from the start just to save 2.5oz, and my hitec radio equipment is pretty light. I've added nothing more to the plane and the flying weight is 42Oz. The material is EPO.

As far as wing warp and washout... I hope I did this right. I took off the flap linkages so I can lay each wing panel down on a flat table. Only the first half (thicker part of the wing) near the flap area of each panel will lay flat since about half way out the wing tapers upward. At the tip of the wing panel I measured 2cm from the leading edge to the table and 1cm from the trailing edge to the table. The other wing panel measured the same. So, I see the leading edge at the tip being 1cm higher than the trailing edge. I also can't see any noticeable lateral balance problem. It balances on a 1/4 inch square balsa strip.

I get confused when people tell me to move the CG but I know what they mean. I simply added nose weight so the nose is pointed down a few degrees when using the manufacturers CG point but the landings seem to be worse this way.

Now, I like the droop idea wich made me think of the ailerons. Since I fly this 3 channel with the ailerons solidly pinned in place, is there something I can do with them during the landing phase. What would happen if I moved both of them down a few degrees? Wouldn't that increase lift at the tips?

Again, thanks for all these tips. I'm still grounded by the high winds and I'm going to wait to hear from you guys before I do any mods...

Dick

EDIT: I also calculated 192 Sq in. of wing area for a 55" wing. (not counting area over fuselage)

Last edited by rms59; 05-10-2010 at 04:11 PM.
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Old 05-10-2010, 06:38 PM
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Sounds like your wing tips are twisted the wrong way. The leading edge needs to be the same or preferably a very small bit closer to the table.

They will stall before the center section making for a much nastier stall.

I would also remove that extra weight until the plane sits level at the recommended CG. I would be tempted to move the cg back a little from the recommended location as well. Keep in mind the plane will become more sensitive to pitch inputs as the CG moves back.

How large a battery pack are you running? Could you get by with a smaller pack and still balance at the CG?
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Old 05-10-2010, 06:41 PM
  #13  
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I just did the math, your at 42 ounces and 192 sq inches of wing?

That works out to 31.5 ounces per sq foot.

Thats a very hi wing loading. That thing is never going to land slow with that hi a wing loading.
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Old 05-10-2010, 06:44 PM
  #14  
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Yeah, you are about 1/3 heavier than the claimed all up weight.

Even if you were at the claimed weight, your wing loading would be fairly hi but at 42 ounces its really hi.

Are you running a larger pack or motor or did you add that much weight in the nose?
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Old 05-10-2010, 07:20 PM
  #15  
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A few issues...

If you really do have a 42oz model flying on a 192 sq in wing area then your extremely overweight for sure.. that's an impossibly high wing loading.

I suspect that you have miss-calculated your wing area. I'd estimate you have twice that wing area, about 400 sq in .. If I'm right then the model still wont be a 'floater' but it should fly like a typical mid size sport model. You would need to keep flying speed up during landing.

Your weight is a lot higher than the Bannahobby spec indicates.. they say 32oz.. you say 42. Have you maybe used a larger battery and motor than specified? The one on the bannanahobby video appears to land ok, no hint of wing dropping even when the nose was pulled up quite high on the landing approach.

Washout is when the wing tip has less incidence angle that the wing root.. So you would expect to see the trailing edge at the tip to be higher than the tip leading edge. on the face of it your description sounds like washin, which is VERY bad... but that could be because of the angle the wing root airfoil is sitting at on the table.. probably the leading edge at the wing root is raised also... correct?

To measure washout properly you would have to pack up the wing so that the LE and TE at the root were equal distances from the table surface... than look at the tip. If the TE is higher than the LE then you have some washout.

Washout cant really hurt unless flying upside down is a priority, so if you can twist some in then go for it, just make sure you add the same washout to both wings.

Alternatively if you cant get any twist in the wing you could put droop leading edge on the outer 50% of the span.. either by adding an extension (somewhat ugly) or cutting out a section of leading edge (maybe 1" wide), bevelling, and re-attaching at a slightly 'drooped' angle.

Moving the ailerons down would be a very bad thing to do.. This would do exactly as you predict and increase the lift at the wing tips meaning they would stall first, just what you don’t want... Lifting the ailerons may give a slight advantage.

Steve
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Old 05-10-2010, 08:06 PM
  #16  
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Originally Posted by rms59 View Post
EDIT: I also calculated 192 Sq in. of wing area for a 55" wing. (not counting area over fuselage)
1. You should count the area over the fuselage
2. Please try the calculation again ?....192 sq in means that 55" wing has an average chord of 3.5 inches...and I don't believe that.

Steve
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Old 05-10-2010, 08:31 PM
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rms59
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Ok fellas, I made a math mistake. That figure was for one panel. I would appreciate if someone could check my method of calculating..... This is what I did.

Figuring for one wing panel, the section of wing from the fuselage to the point that it tapers in the rear is 8.5 in. wide X 10 in. long which gives that section a wing area of 85 sq inches. (8.5x10)

Then figuring for the remaining 15 inches of wing panel that tapers toward the tip I had to calculate an average chord. The chord is again 8.5 but the wing tip is 5.75 in. wide. So for this section I took 8.5 in + 5.75 in divided by 2 = 7.125 average wing chord. Now 7.125 X the length of 15 = 106.8 sq inches for that section.

Adding the two together 106.8 + 85 = 192 aprox sq in per panel. So, 192 x 2 = 384 sq inches of total exposed wing area not counting 5.5 inches of fuselage in between.


Everything in this plane is stock except for the servos and receiver which are probably lighter than what was in there.

Fuselage, motor, esc, radio gear weighs 26.6
The wing weighs 9.5
The 2000mah 11.1v 3s Lipo is 5.6

There is no way bananna hobby is correct with that weight unless they changed motors but it came with the motor and esc.

Dick
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Old 05-10-2010, 08:42 PM
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Your calculation is correct.. near enough the 400" that I guessed at. Looks like Bannanahobby are telling big lies about the model's weight.

My previous comments apply....

Steve
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Old 05-10-2010, 08:57 PM
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rms59
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I doubled checked the washout and took a couple of photos. I used some blocks to make the root of the wing equal on my table as shown.

Then measured the tips in relation to each other and you can see that the trailing edge at the tip is lower than the leading edge. It measures 3/16 of an inch difference.

Sorry for the mixup and I really appreciate the help.

Dick
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Old 05-10-2010, 09:00 PM
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Good catch on the wing area guys.

Also, recheck your measurements on the leading edge/trailing edge difference on the outer wing. I notice it has drooped tips so you will need to move in a couple of inches to get away from that droop.

Edit: never mind. I see you did that
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Old 05-10-2010, 09:23 PM
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looks like you do have a bit of washin.. that's very bad!

see if you can get some twist into the wing so that at the tip (with the wing root level as per your photo) the TE is at least about 1/4" - 3/8" higher than the LE

If you can make the twist stick than you should have no more tip stall problems... Some heat from a iron while holding in as much twist as you dare may do it, be carefull not to damage the foam though.

If you cant make the twist 'stick' then some sort of droop leading edge as mentioned previously is probably the best option.

Steve
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Old 05-10-2010, 09:43 PM
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Yeah, you need to be careful applying heat to the foam. Too little and nothing happens. But just a tad too much and it melts rather than bending like you want. Best to take your time and go slooooooow with little heat.

Ive had better luck with this technique.

Block up the center section so it sits level and is held down well but a little above the bench.

Tape a long stick of some type - ruler or cut down yard stick - to the outer tip of the wing. Prop UP the low side of the wing and add a small weight to the side you want to drop down. Adjust so you have maybe twice as much twist as you want to end up with.

Be carefull your only twisting the wing and NOT also bending it.

Go away and let it sit like over night before you check it again.

If that isnt working after a day or two, then begin to apply heat but do it as gently as possible and spread out the heat as much as possible.

Ive had poor luck with heat guns. Maybe a hair dryer but keep it moving around.

Ive had better luck positioning a small light bulb above and below the wing - close enough to get the surface pretty warm to the touch but not close enough to melt the foam.

Then check it every so often.

Good luck!
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Old 05-10-2010, 09:51 PM
  #23  
rms59
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I checked the other wing panel on my table and it's identical. Both panels have wash-in and exactly the same amount. Since I'm a little nervous heating and twisting the wing, and trying to get them both the same, I'm going to try pinning a droop strip to each panel first.

If it works, then I will think about the twist idea or just cut the foam out and reinstall it after making a mod like JetPlaneFlyer suggested. I kind of like that idea because it doesn't involve heat.

I have 30 mph wind here right now and the weather looks bad all week so it will be a few days before I can try it out anyway.

Thanks to all for finding the problem and giving me ideas for a remedy. I'll report back. BTW, this company shows the same plane at a higher weight of 39 oz.

http://www.hobby-wing.com/famous-ces...-airplane.html

Dick
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Old 05-17-2010, 05:27 PM
  #24  
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Reporting back...

Finally, after a week of either rain or just too windy, I finally got a good day to try this wash-in fix. I added the droop edge to the leading edge like it was suggested and VOILA..... It flew perfectly.

To recap, without the droop strips, the plane would stall at the flair and drop the right wing hard as if it wanted to roll over. Luckily no damage since the right landing gear hit first bending it backward some. It would do this every time. It was then we found out it had wash-in.

Now, with the droop strips, and holding the flair out longer than normal to see what it will do, it just drops quicker without any wing drop. Awesome..... Normal landings are beautiful at 45 degree flaps. I intend to cut out about a small section of the leading edge, as was suggested, and droop it so I can get rid of the balsa strips.

This is an awesome plane to fly now thanks to you guys.

Dick
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Old 05-17-2010, 06:02 PM
  #25  
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Congrats on getting it fixed!

You did a nice job installing the balsa strips too
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