Tip-stalling

[onetenor]

Posted by onetenor on 11/10/2015 04:56:15:

Fast flying planes need to fly FAST and that includes taper wings and swept wings et al. When I was learning to fly I was told that some aircraft have to be flown onto the ground (or Carrier decks ---- them in particular Watch planes landing on a carrier.They are flown on fast for just the reason to avoid tip stalling .They are literally dumped on the deck. It is not absolutely the same for land planes but it is pretty close. The undercart on the DH88 is fairly well forward of hte Cof G and I think it was designed that way for that very reason. To stop nosing in during wheeler landings. and with some power still on. Practice touch and go's to get the feel is the best advice I can give. Good luck.

John

Further to the above may I refer you to Andy Sephton's piece in this month's RCME. He makes mention of the Mossie's tip stalling and his mention of wheeler landings John

[Martin Harris - Moderator]

I was given to understand the latter was the main reason as there was little or no impending stall warning through the airframe - the strips create turbulent air over the tail and give a better physical warning - not much use on RC models until further telemetry advances!

I don't know whether there was any significant benefit in preventing tip stall, but some research I did a year or two ago strongly suggested that the Chipmunk also has built in washout.

Edited By Martin Harris on 11/10/2015 21:18:50

[ron evans]

Had to have a look at some Chipmunk pics Colin and that strip extends out to the u/c position. Might be worth a try on that DH88.

Edited By ron evans on 11/10/2015 21:33:16

[Colin Leighfield]

Absolutely, every bit helps Ron. Martin, the Chipmunk has got 2 degrees of wash-out. I've only heard recently the suggestion that the leading edge strips on the Chipmunk were to help stall warning, it may be true, but when I was first shown around one before flying in it with the ATC around 1950 the pilot said that they helped to prevent tip stalling at high angles of attack by disrupting the air flow over the leading edge. It appears to me that they would.

[Martin Harris - Moderator]

I'm almost certain that I read about the stall not giving enough warning through the airframe and the Air Ministry requiring the mod to create pre-stall buffet. I was also told this when I enquired after my first encounter with a Chipmunk when I started Gliding but your reason may be just as valid...

[Colin Leighfield]

With acknowledgements to the excellent SBGB publication "The De Havilland Canada DHC-1 Chipmunk, The Poor Man's Spitfire", you will see here that the stall bars originated from early test flight experience in Canada and not from the UK Air Ministry. Despite having wash-out, the Chipmunk would drop a wing sharply at the stall. The "bars" caused the inner wing to stall first and stopped this happening. There were some negative consequences though and the buffeting around the canopy and over the tail is described as one of these. Clearly if it happened it was a warning, but only as a result of the disruption of the aid-flow over the inner wing that has already prevented the tip stall from happening. The day had already been saved!

[ron evans]

Fascinating stuff Colin and it's got me wondering if all the handling traits of a full size are replicated on a scale model. Highly tapered wings are one obvious example.

I've been looking at some utube footage of a 10th scale F7u-1 flying free in the Langley low speed wind tunnel, from '47, I wonder what weight the model would have to be to make the results meaningful?

[Simon Chaddock]

ron

It all comes down to that wonderful Reynolds number. This dimensionless number (it has no units) that is formed from an equation that takes into account the physical size of the object along with the density and viscosity of the fluid it is travelling through. Viscosity in particular does not scale with size so it has a much greater effect at small scales.

However with full size verification it is possible to extrapolate small scale wind tunnel results quite accurately for different shapes, until 'compressibility' starts to come into play. For purely practical reasons super sonic wind tunnels have to have a small 'throat' so it requires a high degree of extrapolation (and hence possible error) to predict full size performance.

Now of course with all the background test data available it is possible to 'electronically' predict full size performance (including super sonic) pretty accurately without going near a wind tunnel but it is not cheap!

However this is not the case for models where there is by comparison no money to fund such expensive research so 'experience' and 'suck it see' still play an important part.

[ron evans]

Simon

I think I understand your explanation, the shape would be fixed in the wind tunnel and the results corrected to a full size value.

With the F7U-1 I've mentioned the model was flying free and controlled in the wind tunnel, with observations made on how it handled near the stall with different C/G positions. I think the weight would have had an effect on the handling so I suppose they arrived at a figure for the model using the Reynolds equation.

Suck it and see fan

Ron

Edited By ron evans on 12/10/2015 16:39:33

Edited By ron evans on 12/10/2015 16:47:16

[Martin Harris - Moderator]

 

Posted by Colin Leighfield on 12/10/2015 09:38:46:

Wiith acknowledgements to the excellent SBGB publication "The De Havilland Canada DHC-1 Chipmunk, The Poor Man's Spitfire", you will see here that the stall bars originated from early test flight experience in Canada and not from the UK Air Ministry. Despite having wash-out, the Chipmunk would drop a wing sharply at the stall. The "bars" caused the inner wing to stall first and stopped this happening. There were some negative consequences though and the buffeting around the canopy and over the tail is described as one of these. Clearly if it happened it was a warning, but only as a result of the disruption of the aid-flow over the inner wing that has already prevented the tip stall from happening. The day had already been saved!

Looks like my memory has failed me or someone else recorded events differently.

Although my Airsail Chipmunk (pretty close to scale in many respects) has never been a "wing dropper" - and I never got round to installing the strips - it does seem to emulate the full size by increasing the spin rate after about 3 turns and adopting a more nose down attitude.

It would be interesting to see the results of Roy adding leading edge strips, though...

Edited By Martin Harris on 12/10/2015 18:09:36

[Colin Leighfield]

I'm learning a lot from all of this chaps, I hope the information about the origin of the stall bars has been helpful. I think the Airsail Chipmunk is probably closer to scale than the Dennis Bryant design, which varies from scale in some respects for no apparent reason. I think the Airsail has wash-out, as per the full-size? The Bryant design doesn't, but I have altered mine to include it and modified the wing root incidence to accommodate. I am trying to get back to that build but other things keep getting in the way.

[Martin Harris - Moderator]

Yes, it has washout, Colin.

[Colin Leighfield]

Thanks Martin, staring from scratch now I would choose the Airsail design , but the Bryant version will have been worth it when I get there, I'm sure.

[Ben Kenobi]

For what it is worth I think the potential exists to provide some sort of 'stall warning' - how detailed depends how far you want to go.

I've done extensive work with 'Pixhawk' type hardware / Arduino etc etc.

I feel that it is possible to provide feedback based on airspeed versus pitch, the telemetry could be enabled via a switch that calls out altitude and airspeed if needed, or triggered some sort of haptic feedback based on pitch and speed. Perhaps some sort of self learn could be built - alternatively a deliberate stall with a switch to confirm the parameters for the flight controller.

The technology does exist but not in the mainstream, feel free to Google Arducopter / Ardupilot, ignore the UAV part and look at the telemetry potential, available sensor tech such as 9 axis gyros / magnetometers, airspeed and pitch can be measured and acted upon. This could be extended to assist the RC pilot by preventing flight envelope excursion aka as per Airbus - even self learning the flight envelope to a degree.

I realise as a noobie I may be out of line or stepping on toes - even missing the point.

[Martin Harris - Moderator]

Airspeed indicators certainly exist with several telemetry based systems. I did do some limited experiments with a glider using airspeed info to try to instigate a "stick pusher" on a glider but with more of an eye to allowing stall-free thermalling at height...

To be quite honest, the key to avoiding tip stalls is maintaining a reasonable airspeed visually (by which I mean maintaining a reasonable pitch attitude).  Although it is impossible to gauge angle of attack accurately by eye, avoiding sudden pull ups on takeoff and building up airspeed before climbing and not allowing the nose to rise unduly on the approach will "tame" most models. As for spinning off a final turn, it shouldn't happen unless you turn too tightly or with the nose high.

...or too much rudder, but I suspect 95% of model pilots won't be guilty of this!

Edited By Martin Harris on 12/10/2015 21:49:25