I have Great Planes Extra 300S 60, with an OS SurpassII 120 I built from a kit. I am having a bad problem with tip stalling that at times is unrecoverable. This last time it was on a landing approach. Now that I have to rebuild it again, I am looking for ways to help tame the tip stalling. I know it will always tend to tip stall because of the wing design, but I have thought about twisting in some washout with the covering, will this help? The wing is a flat tip design, and I read in an article not too long ago, that this creates more of a vortex at the tip, and in turn gives less lift at the tips. Is this why it tends to tip stall so bad? Would it help if I built rounded tips that will give about 3/4 of an inch to each wing tip instead of the flat tips that it has? Would that help give the air a cleaner exit off the wing?
Please give me your thoughts on this problem.

Thanks for your help.

Should I mention I fly at 7000ft above sea level?

Jim,
I have the 60 size GP Extra with a OS 91FS as well and the 33%. In my opinion, the most common cause of poor flying characteristics is excess weight. You have a large engine, and possibly some firewall reinforcement, a big fuel tank, and oversize battery, etc. We live in a world of bigger is better. I'm not discouraging big motors, but you will pay a price in handling. Experienced pilots usually don't have trouble, but it could be part of your problem.

This plane is capable of flying nicely as shown on the plans. I wouldn't try warping the wing unless you are seriously interested in experimentation. Try landing a little faster, that should eliminate the tip stall. Landing in grass will help you slow it down but if you do this, get some larger wheels to avoid snapping off the gear.

Tom

It is what the airplane was built to do. More nose radius to the wing and 3 plus degrees of washout will help.
Darryl Usher

At your altitude the air density will be lower, (Reynolds numbers will be lower) and stall will happen at a higher airspeed.

If you manage to build in washout I guess inverted you will have "washin", which will make it very snappy indeed during inverted flight.

Washout is going to be hard to add to an already built aircraft - you have to ensure that both wings have the same twist. This will be difficult.

You can prevent tip stall by moving to a planform with less taper - but as Darryl says, snappiness is a design feature of this type of airplane.

If you reduce the weight of the plane stall will happen at a lower speed, which could mean you can land it OK without messing with the aerodynamics.

I don't know if moving from a 120 to a 91 will be enough to avoid the problem.


If you really hate the tip stall either avoid stalling altogether or try and get the stall to happen somewhere else first: You could try (as an experiment) stall strips as mentioned elsewhere in this forum at the moment. These provoke a stall which originates at the root. The great thing as I see it is that you don't have to rebuild your wing to add them on, and you could easily keep modifying the strip to see what works best - just make sure you conduct your stall experiments at a decent altitude!!

If you want the plane to fly well for aerobatics, don't add any washout.

See if you can save weight. Check the wheels & landing gear to see if they can be lightened. Wheels can be a big source of unnecessary weight. CF gear could help.

Make all efforts to not use any lead for balance. Of course, the plane should be balanced well, but move components around, even if that means adding a new servo mount or reloctaing the battery. Maybe even mount a lighter battery pack.

For landing anyway, you can add reflex to the ailerons. Set up a switch for landing mode that moves both ailerons up 1/2 inch and give a little down elevator mix. This will require some flight testing at high altitude to get the setting right. You want the plane trimmed so it starts a shallow descent from slow level flight when the switch is flipped.

If you really want to be aggessive, rebuild the wings with a different airfoil.

You could also add a notch or spade to the leading edge about 2/3 out to the tip. Sometimes these gizmos can affect the airflow enough to change the stall.

Tip plates will make the ailerons more effective at low airspeeds, and keep flow on the wing where it belongs. They make the wing behave like it has more span. A leading edge strip near the wing root will cause stall to begin inboard of the tips first.

As John says above, raising the ailerons a bit works for me. Acts like washout. Also makes inverted maneuvers a bit more exciting.

This is great information.

The plane flys wonderfully when doing maneuvers, I have the most problem when landing.
I never thought about weight. I think that could be part of the problem. I haven't weighed the plane, but I'm sure it is on the heavy side. What if I drilled some lightening hole in the stock aluminum gear? Would that weaken the gear too much? The battery is already moved back and I moved the firewall back an inch also, so the fuse is an inch shorter. I did that so not to have to add too much lead to the tail.

I do have the flaperons programmed in, I will try and set up a switch.

Tip plates? I don't know if I want to do that, they would take away from the scale appearance.

I was thinking on adding some wing tip caps and shaping them to the airfoil to smooth out the exit of the air, and in turn probably
add 1 3/4" to 2" on the span. Would this help?

Thanks all, Keep the ideas coming.

Set up a dual rate on the elevator that doesn't allow the stalling angle of attack to be reached. Use it for landing. Too much elevator = snap

I doubt very much if adding tip blocks of that sort will help. And as pointed out washout on an aerobatic model is also a no-no.

Adding a stall trip to the inboard sections will help. Another option to help delay the stall is to reshape the leading edge so that the point is a bit more rounded towards the tip. A sharper shape will stall sooner than a more blunt shape. Don't make it look like a round section. It should still be a parabolic shape but just a softer shape. Of course you'll need to re-cover the leading edge after.

Another option to try is to add some turbulators to the outer 1/2 of the span. For this size model these strips should probably be 2 thicknesses of auto trim striping tape about 1/8 wide. For a trial you can use masking tape but since it's thin use 3 layers. The strips should be located at about the 10% chord point back from the leading edge. Try it on the upper surface first and if it works you can add it to the lower as well so the model acts the same upright and inverted.

But in the end this design has a very tapered wing planform. A fellow in our club suffered from much the same problem as you are having. And that was at sea level.

Lightening the design is always a good thing. I'd be cautious about drilling the gear. It's highly stressed in bad landings and needs the strength. Going to a CF type would be better. Wheels are often heavy. Get light ones. As for other tricks it's sort of too late now since the model is built. But in the future it would pay to judge the wood in the kit carefully and replace heavier parts with lighter stock.

I have a feeling that you're wanting to fix a problem that you don't have.
One of the very first things you (should) do during the first flight of a new aircraft is to take it up to altitude and pull back the power slowly while maintaining altitude until you're into slow flight. Then either continue to pull back the power or pull up the nose until it stalls.
Naturally, you'll immediately recover from that and keep flying.

After you've done that a few times, you'll learn how slow you can go before stalling, and also the characteristics of the stall.
It sounds to me like you slowed it for landing to a speed you "thought" was proper but maybe you didn't yet figure out what actual landing speed should have been?

Highflight