Washout and stall strips

[Sparky Paul]

Out at the local airpatch for lunch... trusty Aiptek 1.3M SD handy..
This is the way Cessna does washout.

[Sparky Paul]

And the stall strip on the NA Navion. Relatively large..

[JRuggiero]

Sparky Paul,

I also saw stall strips on a Piper twin of ancient vintage. I think the strips were outboard of the engine nacelles.

Would such stall strips be useful on model airplanes?

Jim

[Flight Engineer]

I've tried them on a speed 400 plane and they had an effect. I put one on one wing only and it always dropped that wing first.

And I put 2 on the root of a plane that always dropped a wing in the stall. It didn't totally fix it but I was able to get it to stall staight ahead with a bit of help.

We call them toblerone after the chocolate bar on this side of the pond.

I'm going to try "T" section trailing edges next.

[gouch]

I was under the impression it was part of an instrument to let you know that the wing had in fact stalled? is this right? if not what do they do?

[Peter D Rieden]

No - the stall warning sensor is elsewhere. Most people completely missunderstand the purpose of "stall strips" on full size aircraft. They aren't there to promote an early stall at the root (to do that they'd be better placed at the 35% chord position, not the LE). They are there to provide a small region of turbulent flow at high angles of attack which the pilot feels as a buffet. In some cases it's a pure wing-thing where the pilot cue is as much audio as tactile, but usually they are positioned to cause the turbulent flow to impinge on the elevators so that the pilot feels a small amount of buffet through the stick/yoke.

From this it can be seen that there is almost no value in fitting them to R/C models at all. If you're worried about asymentric stalls then simply blunten the LE section towards the tip, increase the washout or fly more cautiously!

Peter D Rieden

[Flight Engineer]

http://www.setp.org/Handling%20Qualities%20Development.htm

http://adg.stanford.edu/aa241/highlift/highliftapproach.html

http://www.aae.uiuc.edu/m-selig/classes/aae316/pics/stallStrips.pdf

http://www.aaib.dft.gov.uk/bulletin/sep00/gmkpu.htm

http://www.seqair.com/ReadingRoom/MartinJ/Martin.html

For smooth surface conditions the modified airfoil showed an increase in lift-drag ratio before stall at a=8° from 67 to 72. Furthermore, the maximum lift increased from 1.33 to 1.37 while the minimum drag was maintained. Double stall was observed on the NACA 63-415 airfoil, but not on the modified airfoil. This was reflected in the standard deviation of both lift and drag in stall which was significantly lower for the modified airfoil indicating smooth and stable stall conditions. No significant differences were observed for dynamic stall. Test on both airfoil sections with zigzag tape at the leading edge towards the pressure side showed that the insensitivity to roughness was improved significantly for the modified airfoil. However, if zigzag tape was mounted at the leading edge towards the suction side less improvement was observed. Mounting of stall strips at and near the leading edge showed that only if they were mounted at the very vicinity of the leading edge the airfoil characteristics were affected significantly. If the stall strips were mounted on the pressure side downstream of approximately 1 % chord length only little influence was seen for positive angles of attack

http://www.mindspring.com/~cramskill/tamstall.htm


http://www.djaerotech.com/dj_askjd/dj_questions/tipstalls.html

http://www.navygouge.com/api/basics/aero1-4.html

Just some examples of the use of stall strips.



The burble associated with boundry layer seperation is a good indicator of impending stall , this is a secondary effect.

increasing the LE radius at the tip is the same as decreasing it at the root. It's a lot less hassle to use a stall strip stuck on.


"Most people completely missunderstand the purpose of "stall strips" on full size aircraft."

Yes they do.

[Sparky Paul]

Originally posted by Flight Engineer
...For smooth surface conditions the modified airfoil showed an increase in lift-drag ratio before stall at a=8° from 67 to 72.
.
A 5 count change, or 7%.

Furthermore, the maximum lift increased from 1.33 to 1.37 while the minimum drag was maintained.

.
a 3% change.


Both changes would be indetectable on the typical model airplane
and be of little value full-scale.

[gouch]

Thanks guys, I appreciate the explanations.
I have only ever seen 2 full size aircraft at touch distance,
A piper seneca and a twin otter! so I struggle a lot at the technicalities:)
Cheers
Paul

P.s Sparky, I love your web page, esp the airflow visuality pages. What do/did you do in your profession?