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P-51 conversion has tip stall problem.

Old 08-13-2015, 05:47 AM
  #1  
Marc
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Default P-51 conversion has tip stall problem.

I converted to electric a Sig P-51 kit that is now discontinued. I believe it was intended for about a .15 glow engine. It is now powered by a 35-42, 1250kV brushless running on a 3-cell, 3000mAh lipo. I don't have a decent scale, but the all-up weight must be around 2.5 lbs. It has a 42" span and a tapered chord that varies from 6" at the tip to 9" at the root - about 315 square inches. I built in 5" of total dihedral with one wing tip flat on the table as called for in the plans - the most I have ever put into a model.

Hand launching is exciting because I have to throw it hard with the right wing tipped down to compensate for the motor torque. It is a pleasure to fly, and it does most all the sport maneuvers including an almost dead-in-the-air flat spin that scares the crap out of me.

The real problem is the tendency to abruptly tip stall on the landing approach. I have to grease it in pretty fast at my pasture flying field - hence I have a fairly durable prop for that purpose. Is the problem the high wing load of around 18 oz./sq. ft., the amount of dihedral, the tapered wing design, or something else?

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Old 08-13-2015, 06:06 AM
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fhhuber
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Crank the ailerons up SLIGHTLY. One turn on each clevice might be plenty. It might be hard to see how much you change their angle.

The issue is a little bit of everything you listed, except dihedral. the aileron adjustment will effectively give the plane a LITTLE washout due to the tapered wing and constant chord ailerons. That will delay the tip stall.

another trick is to make the LE of the wing sharper near the root than at the tip. That can cause the stall to initiate earlier at the root. For a model that is already built you tape on triangle stock with the largest flat sanded to fit the LE. This is easily reversable. Apply this to the inboard appx 1/2 span. If it works you can cut covering and glue it on then recover the leading edge.

You could also try warping the ailerons to lift the tip ends but that can be tricky.

Finally 2 layers of 1/4 inch pinstripe tape appx 1 inch back from the leading edge (aiming for appx 10% to 15% chord, being exact isn't critical) on the upper surface can help
This is an old sailplane trick to purposefully "turbulate" the airfoil..

Mustangs tend to be a bit snappy near stall. sometimes its better to just learn to deal with it. All of the above corrections can adversely affect performance in other ways. You might see the adverse effect and really hate it... you may not notice it at all.

Every choice in aircraft design and setup is a compromise. Some compromises work better than others.
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Old 08-13-2015, 08:46 AM
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Washout is the most sure fire cure for tip stall. If the wing is wood and if it's covered in heat-shrink film you can often twist in some washout by hand-twisting the wing, re shrinking the covering to set the twist then leaving the wing with packing and weights to hold in the washout for several days to get the wood to set into the twisted position. Just make sure you get identical twist in both wing panels, typically two or three degrees of twist will do the trick but any amount of twist you can get in will help.

Note.. washout twist is when the trailing edge at the wingtip is raised.
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Old 08-13-2015, 08:53 AM
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fhhuber
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I think this would be an older kit that is discontinued, but If I remember it, the wing has D tube sheeting which could make post-construction warping difficult.
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Old 08-13-2015, 10:19 AM
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That last bit about Flat spin .......

It is a pleasure to fly, and it does most all the sport maneuvers including an almost dead-in-the-air flat spin that scares the crap out of me.
Says to me CoG .... a P51 like most other warbirds and sport jobs should not go into a flat spin without serious work on part of pilot. It is the arena of aerobatic machines ... not fighters.

How is your CoG ? To get an easy to enter flat spin ... a rearward CoG is needed. If you have dialled in Expo .. DR ... etc. - you may be masking the rearward CoG ... it would also contribute to the 'bite' she has when landing.

My 2c's worth.

I agree that washout etc. is great tool ... but this is an established model that as I recall flew well as a glow job. The fact here is now we have an overpowered model possibly ... with possibly rear CoG ...

Nigel

Last edited by solentlife; 08-18-2015 at 07:32 PM.
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Old 08-13-2015, 02:54 PM
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Glacier Girl
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The Mustang was a tip stall beast in real life, and so too in r/c. Wash out will help, ditto correct CG. If you look at Horizon's Hangar 9 version, it came with removable NACA droops for the wings to help with this. Along with working flaps.

Maybe look at finding a pair of those droops to add to your bird.
http://www.e-fliterc.com/Products/Su...ProdID=HAN2835
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Old 08-13-2015, 06:08 PM
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Another option ( a bit of a pain to do...) is to split the ailerons into 60% aileron, 40% flaps. roll rate won't go down much. 10 deg of flaps would go a LONG way to making the landing approach more stable.
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Old 08-13-2015, 10:31 PM
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solentlife
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Only real scale models had bad stall tendencies because they needed to be flown. But most model designs were not scale ... only sport scale with modified wing form to sweeten the characteristics.
My Royal P51 with HB50 up front was a doll ... till the factory OEM Rx battery failed !
The Rojair Mustang ... Cotswold Mustang .... Pilot .... the list goes on - all flew sweet ... because none were true scale.

Nigel
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Old 08-15-2015, 10:47 PM
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Marc
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Thanks to all of you for your suggestions. I haven't flown the P-51 recently because of a brisk east wind. When it swings back around to the south I will be able to have a long landing approach into the wind.

After reading quite a bit about tip stalling, I believe the most certain solution would be to washout the wing tips. Unfortunately, the P-51 wing has a sturdy D-tube construction. The wing is built very straight and the CoG is perfect. What I am going to try is to lift the ailerons by a turn or two on the clevises. I have also given the motor a bit more right thrust to compensate for torque roll on launch.

When I get back in the air with this one, I will simulate several high altitude approaches to see if tip stall is less of a problem and to see if there is a pattern to falling of the the left or right. I will post the results in a few days.
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Old 08-16-2015, 07:59 AM
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Lifting the ailerons is unlikely to help significantly, if any all because the ailerons are full length strip type.
You need to reduce the angle of attack at the tip only, lifting strip ailerons just reduced the incidence of the whole wing. It doesn't impart any significant 'twist' to the wing. Lifting ailerons can have some limited effect if they are part span type fitted outboard, but in my experience it doesn't help at all with strip ailerons, all it does is increase stall speed (like flaps that move the wrong way) which means you have to come in even hotter.

Even with a D-box it is often possible to get a bit of twist into the wing. You need to put the wing on a flat surface with a tapered strip or packing under the trailing edge to 'lift' the tip. Apply at least double the amount of twist that you need because it will spring part way back. Then put heavy weights or clamps on the wing to force in a washout twist. Apply heat to the covering to re-shrink in the twisted position and leave for at least a week (the longer the better).

If that doesn't work stripping the covering and steaming in washout will do it.
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Old 08-16-2015, 08:15 AM
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If aileron chord is constant and wing panel tapers by 50% chord then raising the ailerons to effectively reduce AOA 1 deg at the root will effectively reduce AOA 2 deg at the tip, which is in effect washout of 1 deg.

I've been using this trick since the 1970's and it has had good results more often than not.
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Old 08-16-2015, 09:59 AM
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Yes, but it would be very unusual to have a tip 50% of root, so the numbers don't stack up in this case. On the OP's semi scale P-51 the tip chord is about 80% (ignoring the root fillet)

To get 1 degree AoA change on a 200mm root chord would mean deflecting the aileron up by 3.5mm which is a lot on a narrow strip aileron. More typically and following your 'SLIGHTLY' advice it would be half of that. So in the real world taking the OP's plane as an example the theoretical twist inferred by deflecting the aileron (say) 2mm is about 0.15 degrees. Even if you did wind the aileron the full 3.5mm up you still only get 0.2 degrees of twist.

That's just not enough to make a noticeable difference IMHO... Add to that the upward deflection increases stall speed, so you need to come in a little hotter anyway, and it's not a great solution.

Then you have the question of if moving the ailerons up is aerodynamically the same as twisting the whole wing chord. I'd say no, and wind tunnel simulations tend to back me up (I can run some if you like). It has some limited effect in delaying stall but not as much as twisting the whole wing chord.

Like i said, i have tried it and for me it didn't work but the OP is free to give it a go if he wants, it does have the advantage of being easy to do and easy to reverse if it doesn't work, so no harm in trying.
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Old 08-16-2015, 10:42 AM
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Actually, "stall speed" is weight vs wing area and the incidence change just changes the aircraft attitude at minimum airspeed to maintain aerodynamic flight. You could deflect the ailerons up 45 deg and have virtually no change in the speed where it stalls. (It does change the "shadow" of the wing slightly due to bringing the TE closer to the LE.)

Its actually a neat trick to mix in spoilerons and deflect the ailerons up 10 to 20 deg, forcing you to point the nose up to get the same effective AOA and thus presenting the bottom of the aircraft as one big airbrake.

Simple flaps (hinged similar to ailerons at the back edge of the fixed surface as typical in many models... for simplicity) don't add wing area. the do change the effective AOA of the center section of the wing effectively inducing significant washout, forcing the center to stall before the tips (since stall occurs essentially due to AOA, not airspeed... It seems tied to airspeed because as you slow down you need more AOA to maintain the same lift.)

Sometimes a tiny change in effective washout is all you need to force the center of the wing to stall before the tips. And that is the "game" we are playing attempting to cure a bad tendency to snap in a stall.
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Old 08-16-2015, 12:23 PM
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Glacier Girl
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I know not very realistic, but you really wouldn't see em in the air. Wing fences.
Make em out of clear plastic and they'd be nearly invisible.

I installed em on my Squall, a bird built to fly well over 100 mph, but was a tip stall beast when slowed down. One of those things, make something to fly fast and induce a problem at low speed.
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Old 08-17-2015, 03:17 PM
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The full size Mustang was one of the first ones with the Laminar Flow wings where the thickest part of the wing is closer to the centre of the chord. Part of the reason for being a tipstaller, but also giving it a good top speed.

Gord.
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Old 08-17-2015, 07:54 PM
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Originally Posted by fhhuber View Post
Actually, "stall speed" is weight vs wing area and the incidence change just changes the aircraft attitude at minimum airspeed to maintain aerodynamic flight. You could deflect the ailerons up 45 deg and have virtually no change in the speed where it stalls.
That's not quite correct. Certainly weight and wing area are very important but they aren't the only variables

The stall speed formula is:

V = √( 2 W g / ρ S Clmax )

where:
V = Stall speed M/s
ρ (rho) = air density KG/M^3 (about 1.2 kg/m3)
g = 9.81 m s^-2
S = wing area M^2
Cl_max = Max Coefficient of Lift
W = mass KG

The other important variable here is 'Clmax'. This is the maximum coefficient of lift, i.e. how much lift the wing can make per unit of area before stalling. Clmax is reduced when you deflect the ailerons up because you reduce the camber of the airfoil. The general rule is more camber = higher higher Clmax.

https://en.wikipedia.org/wiki/Camber_(aerodynamics)

That's why 'heavy lifter' and 'slow flyer' RC planes use very heavily undercambered airfoils. It's also why stall speed is reduced when you deploy flaps. Flaps are basically devices to increase airfoil camber, deflecting the ailerons up has the opposite effect..
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Old 08-17-2015, 07:56 PM
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Originally Posted by Glacier Girl View Post
I know not very realistic, but you really wouldn't see em in the air. Wing fences.
Make em out of clear plastic and they'd be nearly invisible.
Yeah, i used that trick on a Zlin Akrobat that had a nasty tip stall. It helped a bit.
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Old 08-17-2015, 07:59 PM
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CLmax change for the minor deflection (or even a substantial deflection) of the ailerons is far smaller than you expect... The base airfoil hasn't changed.

Wing area is a constant. air density can be considered a constant. Gravity is a constant. And the weight is a constant...

We end up with change of stall speed is essentially zero.

Sorry Jetplaneflyer you just keep failing to understand reality.
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Old 08-17-2015, 08:39 PM
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The formula I referenced is REALITY it's derived directly from the lift equation: https://www.grc.nasa.gov/www/K-12/airplane/lifteq.html

I understand how it works pretty well i think.

I agree the effect will be minor for small deflection of ailerons, I never said otherwise. However the 'positive' effect on tip stall will also be tiny, so a minor negative effect in one area and at best a tiny positive in another pretty much cancel each other out.

FWIW, here's a polar plot showing a typical NACA 0013 airfoil with aileron deflected by the abount you suggested. It causes a 6% reduction in Clmax. i'd be the first to agree that it's not a big change but neither is it totally insignificant on a plane that already lands 'hot'.

Argue that this isn't 'reality' if you like but the plot is generated with xfoil software which is about as good a tool as is available for this type of simulation. If you can come up with anything better i'm all ears.
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Old 08-17-2015, 08:56 PM
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Interestingly on a 'semi symmetrical' (NACA 2413) airfoil most likely used on the Mustang, the negative effect of the flap deflection is much greater than on the symmetrical airfoil used in the last graph (see attached). In this case the loss in Clmax is around 12% which really is quite significant.

Also very notable is that deflecting the aileron up on this airfoil makes virtually no difference to the AoA that the airfoil stalls at. In both cases the stall angle is about 12.4 degrees, give or take a tenth. So if you believe the software (and no reason not to) there is no possible beneficial effect on this 'semi' airfoil, the only thing you would achieve is increasing stall speed quite significantly.
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Old 08-17-2015, 09:00 PM
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Reality from actually flying the models disagrees.
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Old 08-17-2015, 09:02 PM
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Originally Posted by fhhuber View Post
Reality from actually flying the models disagrees.
Your 'reality' maybe. I've been doing this for 45 years and my 'reality' agrees with the theory.

End of the day Marc can believe who he chooses or can try it for himself.
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Old 08-17-2015, 09:11 PM
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fhhuber
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Flying RC since 1975..
CL before that
FF before that
Designing my own planes since age 10.

And proving people like you wrong every day.
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Old 08-17-2015, 09:24 PM
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Originally Posted by fhhuber View Post
And proving people like you wrong every day.
I'm still waiting for proof

Maybe Marc will try it and maybe it will work for him, that's fine. Airplanes are funny things and sometimes don't do as we expect them to. All we can do is give advice based on our own experience and maybe back it up by some sound theory. And by the way.. It's ok to disagree, don't get in a wad about it and don't get personal. It's only toy airplanes after all

PS... 10 years old.... so you were a late starter
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Old 08-18-2015, 03:56 AM
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Great stuff!

I'm still rooting for Nigel's solution. CG.
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