Daedalius

I've got a Sureflite Mark II Spitfire (~1/9 scale) that I've built - flown - and crashed. I modified the rudder to look like a MKVIII (the pointy one, for the RAAF trim scheme I wanted to use).

Just before take-offs I noticed the plane wanted to veer left pretty harsly; I theorized the culprit to be an overly springy, slightly off-center tail wheel. To roll straight it took a bit of rudder, I presumed that when the tail came off the ground the extra rudder caused the leftward turn.

That was until it crashed. Taking off downhill into a steady N wind, the plane rolled over some dirt clods and bounced into the air ... without enough airspeed. I tried to level out, but it was too late, the wind had already gotten underneath it and it stalled.

It fell off violently to the left, and turned almost 180 deg. toward the ground, in about as altitude as the wingspan; removing the nose from the rear of the cowling and the rudder as it cartwheeled across the field.

I've since repaired the damage. I shortened, straightened and stiffened the tailwheel, and added some wing fillets. But I fear this is not enough to prevent take-off tip stalls. I'm looking for ideas on how to fix this aerodynamics problem without destroying the scale appearance of this pretty little bird.

Cadet

Airspeed! Airspeed! and more airspeed! They only thing that might work better than keeping the speed up is not flying!
Cheers,
Grant

P.S. A small amount of flap and really smooth flying could help.

Daedalius

It flys like its on rails, once it's in the air. The field conditions here are really rough, (between a cotton field and an interstate, unfinished surface) and it's hard to get up much ground speed with out the plane bouncing out of control.

What I was thinking of doing was adding some inconspicuos stall strips to the leading edge of the inboard wing sections. This would force the stall here instead of at the tips, but I am not sure whether the wash coming off the prop would effect the flow over the stall strips or if they're even effective at these low reynold's numbers.

budcop

My Feedback: (23)

Are tip stall's common with warbird's ? I will be starting a 1/5 scale P-40 warhawk in the next couple of weeks and since I bought the kit all I have heard at the club is those WW11 birds are bad for tip stalling..........Input Please!! , Bud..

Daedalius

Any plane with an elliptical wing can have pretty nasty tip stall. Many WW2 birds, ie. Spitfire, P39, P40, and others, have elliptical wings, and are therefore more prone to tip stalling than other warbirds such as the P51. Another thing to watch out for while flying tail-dragging warbirds is flipping over the nose on landings. This can be resolved by slow landings, but then you are in peril of tip-stalling if you get too slow.

What you might could do, if you have a built up wing structure, is sneak a little extra downwash into the tips when you cover it. This reduces the angle of attack at the tips relative to the wing roots. This forces the inboard portions of the wing to stall before the tip does, thus preventing the plane from falling off violently to one side when it stalls.

You could also preform the above "fix" if after flying your model you discover it does in fact have a particularly nasty tip-stall. I could not do this with my Spitfire because it has a foam core wing.

Rodney

I hope that by "downwash" Daedalius means washout; i.e. the wing tips have a lesser angle of attack than the root does. If you have ailerons only at the outboard ends of the wings, you can simulate this by haveing a little up in both ailerons when the controls are at neutral. Also be sure and use differential on the ailerons, more up than down.

Ollie

At full scale sizes and speeds an elliptical planform with a uniform airfoil and no wash out gives an elliptical lift distribution in which the induced drag is minimized and the whole wing stalls simultaneously.

At model sizes and speeds a similar wing tip stalls because the low chord at the tips results in flow that has a thicker boundry layer that seperates (stalls) earlier. To get a better stall characteristic at model sizes and speeds it is necessary to transition to a thinner, symmetrical airfoil near the tips and possibly add washout also to avoid tip stall with the scale planform.

Cdallas2

Be careful of putting stall strips on such a small airframe. You'll reduce the tendency to tip stall but you may reduce the overall lift too much. By doing that you'll have to keep your speed up at all times - especially landings.

Using washout is a much better idea - it's safer and is known to work.

Rotaryphile

I am constantly amazed at the number of people who build scale models, but neglect to utilize measures taken with the full-scale aircraft to improve handling, such as wing washout and aileron differential. The Spitfire is a tad shy of vertical tail area, and thus prone to stall/snap roll misbehavior. I would use lots of washout - as much as 3%, aileron differential (considerably more up travel than down), and, as always, fly the damn thing off the ground, don't ever even think about yanking it into the air, or you will be sorry. I have noticed that a lot of modelers fly with excessive elevator travel. I like to set elevator travel so that it is enough to do the job. My test for this is to get lots of height and speed, and apply full up elevator. If the elevator travel is just right, in my book, the model will fly just about one loop before yawing badly and partially stalling. If it stalls and yaw/rolls wildly after less than one complete loop, elevator travel is too much. If it keep on looping and looping with no problem, it probably could use more elevator travel to get maximum lift out of the wing. Excessive elevator travel can be very dangerous, for example, in a low level pullout from a steep dive, excessive elevator travel can stall and snap roll you into the ground before you have any chance of recovery. Better to be safe than sorry, and trim your model so that a minimum of skill is required to fly it.

niccolai_m

The left yaw that you describe could be normal for the clockwise rotation prop (as seen from the cockpit) and would be due to precession cause by rotating prior to take off. i.e. as you lift the tail wheel the the prop is pitched forward, the effect is a force on the starboard side of the prop in the forward direction (opposite on the port side), giving you a left yaw. The more sudden the pitch the more vicious the yaw.

Of course if you still suspect tip stall, flaps will help since they increase the incidence of the wing nearer the root.

gubbs3

My Feedback: (1)

Yes the left yaw is from prop torque. The best way to minimize this is to advance the throttle slowly. Prop torque affects all taildraggers but scale models have more of a problem with it because they have smaller, more scale like rudders.

Washout sounds like a very good idea to help the snap stall problem.

The next time you try taking off. Advance the throttle slowly so you can control the torque. Then wait for the tail to come up, all the while advancing the throttle slowly. Once you get to about 3/4 throttle, or more, just give it a touch of up elevator and hold it. It should only take the tiniest bit to get it off the ground at this point. Continue your climb out untill you get about one mistake high before you try rolling the plane to either side.

Hossfly

[QUOTE]Originally posted by Daedalius
[B]I've got a Sureflite Mark II Spitfire (~1/9 scale) that I've built -

SNIP

Just before take-offs I noticed the plane wanted to veer left pretty harsly; I theorized the culprit to be an overly springy, slightly off-center tail wheel. To roll straight it took a bit of rudder, I presumed that when the tail came off the ground the extra rudder caused the leftward turn.

SNIP

The model will turn left during Take-off go-arounds etc. due to p-factor/torque, whatever. I let the purists determine all that, as a pilot I simply need to know to use right rudder and keep it straight. This may well be until well into the air and going along for a time, especially during the acceleration after take-off.

I had a small Spit. of 410 sq. ins., MK 5, clipped wings, for warbird racing, worked with it for over a year to get reliable take-offs. The model was built with scale wing incidence -- a bad decision -- and wanted to jump off the ground early. It took too much nose down on the TO roll for me to handle well.
As mentioned in another reply here, the FIX finally came when I set the ailerons almost 5/16" UP at the inside wing position.
Looked terrible but took off and flew like a champion.
Of course the story is as all stories go, at the very next race, as I passed over a Mustang, he pulled up and cut the fuselage off right behind the wing. Those raised ailerons could not help there.

Broke me little heart, it did.

DIPSY

I saw a very similar thing happen to a guy last year, and it had little to do with tipstall. The comment that you modified the rudder/fin makes me suspect that you didn't have enough fin area to control the model about the vertical axis. Spitfire models are notorious for tip stalling, but it's well known that the fin size is critical on them. All you have to do is look at the real thing and think about what size it would be scaled down......pretty darn small is the answer.
If you are worried about tip stalling, then build some washout into the wing next time.
Remember not to get the Spit too slow when landing.