Please Help........

I have a Lanier F86 Sabre propjet with a tip stalling problem. The CG is already about 3/8"-1/2" forward. The plane does fly a little nose heavy. I do not want to move the CG forward anymore. The plane flys well but you have to land it very fast (I expected this, but it still wants to tip stall even at a pretty good speed). I have read somewhere about reflexing the ailerons up to induce washout. Would this help? If so, how much do I reflex them? Any help would be appreciated......Thanks, Steve

I don't think moving the CG forward will help alleviate your tip stalling problem because it's a wing/wingtip design issue. Even if you move the CG more than 1" forward, the plane will still tip stall. Moving the CG forward will make the plane more stable but will increase landing speed. Deflecting both ailerons up about 1/16" will create somewhat of a wing "wash out" and will decrease the tendency to tip stall, provided that you have barn door ailerons and not full length strip ailerons.

Thanks for your help........What exactly do you mean by "Barn Door" versus "Strip" ailerons. I think they are strip ailerons. The ailerons are 1 1/4" wide by 16" long. The ailerons stop 2" short of the wingtip. Does this mean that reflexing them up would do nothing?

You have almost strip alerons since they are pretty long, so deflecting them up won't help a whole lot. You can try it to see what happens. Deflecting both ailerons up to reduce tip stall works best if your ailerons are short and closer to the wingtips. This makes sense since you want to create washout only at the wingtips and allow the wing center section to stall first.

You may have but worth checking with you to see if you have done a lateral balance?

What wing incidence do you have? What elevator trim do you have?

Is it possible that you CG is TOO forward!

Thanks for all your help guys....I don't believe the CG is too far forward. Wing and stab incidence are within 1/2 degree of each other (around 0 degrees). The plane flies a little nose heavy (5 clicks of up trim ...about 1/16" on the elevator) but feels good. I have the CG about 3/8 of an inch forward of the instructions and I like it here. I know lateral balance is not the issue (it is very close). I will try to reflex the ailerons up about 1/16". Any other suggestions....................Thanks Steve

To me, 3/8" forward is pretty far forward. Have you ever flown the plane with the CG at the factory recommended location?

Swept wings tipstall easier than straight wings.
The "cure" is a LOT of reflex, going up incrementally along the span of the surface.
Reflexing the entire surface will help some..

Since it is pretty hard to find places to lose weight with a model like this, it might be time to build a bigger and possibly lighter wing. It might be interesting to see what happens if you could come up with some tip extensions for the wing you have now? My contention is, if this plane was light enough, it would fly just fine. What ever trim gimmicks you come up with to enhance low speed flight will compromise the planes' other flight characteristics.

If the tip stall you are experancing is in the form of an aleron reversal how dose the airplane respond to the rudder. Tip stall is coused by leading and laging wing. With a swept wing as a wing leads it appears to the air to be longer and provides more lift and as it lags it apears to be shorter and provides less lift. There is a condition kown as adverse yaw when you deflect an aleron doun you increse drag and couse the wing to lag and you can actuly have less lift. The cure if you are not good enouph with your left thum might be to cuple the rudder and alerons so they work to gether If you have a computer radio you might program it to a rate switch so you can turn the cupling on and off. I think the C.G has little to do with your problem. Joe

Just my experiences to add to the information bucket!!

On my Edge 540 I use reflex aileron. No more than 5 degrees and I need to mix a little down elevator trim with it. It adds a little to the landing speed but not so you would notice. What I found is that it does not cure tip stalling but provides for plenty of warning. Prior to adding the reflex, the tip stall would come on without warning and progress rapidly. After adding reflex, the airplane gets mushy near the stall, just the way I expect it to feel (and then it tip stalls). I set the reflex as a fixed amount in the "approach" flight mode. I use this mode for takeoff also and the reflex makes no difference to the takeoff - the airplane is "skull dragged" into the air by a big YS 4 stroke.

Low speed fin effectiveness plays a part in this too. Yaw stability, or lack of it at low speed, contributes to the wing drop associated with tip stalling. The fin could be too small, or blanketed by the fuselage at high alpha (this happens on scale airliners), or blanketed by wing downwash, also at high alpha.

Another trick, which works with models because of their low Re, is to put a turbulator (trip strip) on the outper wing. This can cause early boundary layer separation and, after re-attachment a more energetic boundary layer which remains attached over more of the wing at high alpha. Whatever!!! It works on gliders so it's worth a try. The positioning of the trip strip is critical so some flight testing is needed. A strip of fine monofilament (6lb fishing line) is all that is required along, say, the outer 1/4 of the wing panel. Start off with it at 30% and move it back 5% at a time until upper air testing shows it works.

For your CG. Try not to consider CG location in terms of a percentage of the chord or other such criteria. Determine the mean aerodynamic chord and aerodynamic centre of the airplane and position your CG according to this. There are plenty of web sites explaining how to do this. For your Sabre, I would suggest 30% MAC as a good starting point.

jfitter wrote: "... Another trick, which works with models because of their low Re, is to put a turbulator (trip strip) on the outper wing. This can cause early boundary layer separation and, after re-attachment a more energetic boundary layer which remains attached over more of the wing at high alpha. Whatever!!! It works on gliders so it's worth a try. The positioning of the trip strip is critical so some flight testing is needed. A strip of fine monofilament (6lb fishing line) is all that is required along, say, the outer 1/4 of the wing panel. Start off with it at 30% and move it back 5% at a time until upper air testing shows it works...."

I was going to suggest the same thing when I read this. I have used two layers of 1/16" map tape (Letraset, Chartpak or whatever) to create a turbulator on the outer 1/3 of the wing on several airplanes and it seems to have helped. I placed these closer to the front edge however (at about 10% chord) because I didn't want the turbulator to create the seperation too far back, where the flow might not be able to reattach itself. The situation here is somewhat different than on a sailplane, where there is probably a lot written on turbulators.
Allan

I had a similar problem with an early ducted fan called the Skyburner.

I solved the problem by installing counter-rotating vortex generators and a wing fence. Seemed to work very well. The wing fence keeps the span wise flow from traveling to the tip. The vortex gens act much like the turbulator strips.

vortex gens are used on B-737, A-4, Learjet and many others. Fences on the learjet, mig etc

I just made up 3-sets for each wing, forward of the aileron, about 6" total length, set at about 30^ off the cord line. / \ / \ / \

here's a picture of the wing. The clear vortex gens are in the checkerboard.

Jack

Move the CG back to the recommended place; I can't recall a recommended CG that wasn't a little forward anyway. Jim

swept wings = fences , QED

[:'(]I HAD THE SAME F-86..PUT A .60 OS IN IT. TIP STALLED ALSO.. NO MATTER WHERE THE CG WAS!! THE WING DOES NOT HAVE ENOUGH CAMBER.. REFLEX IS NOT THE ANSWER!!!! REFLEX MAKES YOUR PLANE THINK IT IS NOSE HEAVY..THATS WHY ,UP TRIM ,IS NEEDED AFTER THIS MOD.. WING FENCES ARE VERY GOOD.. BETTER YET, IS TO MAKE INBOAD WING LEADING EDGE MORE BLUNT...IT WILL STALL FIRST ,,STALL WILL BE STRAIGHT AHEAD. SWEPT WINGS ARE NOT AN EVIL THING!!! THEY JUST LOOK SCARY.. RD PS ---MY FLEW LIKE ,,KA-KA..

(how about NOT yelling).
Quite the opposite. If you want the wing to stall early you make the leading edge sharper. Take a trip to the local airport and have a look at the puddle jumpers. Many of them will have short lengths of angle aluminium rivetted onto the wing root leading edges. These are called WEDGES and their purpose is to promote early wing root stall - the pilot then gets to feel the onset of the stall (mushiness, nose down pitch, etc.) while at the same time having full aileron control because the wing tips are still flying.

Thanks for all your help guys..................I put some reflex in the ailerons and moved the CG back about 1/4". This helped some. When I rotated the plane before, it would always drop a wing (this is on a long paved runway with plenty of speed). It doesn't seem to do that anymore. I also noticed it is a little more stable around the roll axis. I think this is as good as it gets. I will have to build up plenty of speed on takeoff and land pretty fast, but it doesn't seem to drop a wingtip as easily as it did. Any more suggestions would be appreciated. Thanks.

jfitter has it backward, blunt wings stall less easily or more correctly less abrubtly. Glad to see the reflex helped Bushong, I've found it has always helped even on strip ailerons.

jfitter has it correctly.. a sharper leading edge close to the root promotes stall buffet early. so the pilot can feel the buffet and know he's getting into "interesting areas" of flight.
On a model, it's a aid, but the pilot still must know the plane's flying characteristics, since he can't feel any buffet, only see the consequences of going too far in the "interesting area".

If you don't mind changing the appearance of the wing slightly, you can try a leading edge cuff at the outboard section of wing. A cuff would approximate a washout at the tip and could be mounted temporarily for experimentation. Look at the cuffs on the Nexstar trainer for an idea of design and mounting considerations.

Good luck!
DT56

Wing fences seem to work well on this model. I do not experience any of the tip stalling issues reported in this thread.

Did you make both changes at the same time before flying it? It would be interesting to know the effect of each change individually. Perhaps only one change would have provided sufficient improvement.

Other have touched on this: An excessively forward CG can cause an earlier main wing stall stall, provided the horizontal tail has enough power to overcome it.

I second this idea. Look at any of the swept wing planes of that era and you will see fences. They are excellent devices in the right situation. I would try this. The worst that can happen is they don't change anything.