To Vortex Generate, or Not?

[Space Bat]

That steve is incorrect -

A stalled wing is a stalled wing...and this is seen in many incidence on conventional aircraft...lift is reduced or removed this is irrefutable fact....it is impossible to generate lift.....

As I pointed out that on a RC plane the fact of high area wing and low wing area - the plane can still work due to kite like effects and - with the added thrust the air created by the prop and high thrust will cause the wing to behave better in high alpha than a conventional real plane....

The NACA airfoil results are misleading and sorry incorrectly interpreted - as this is how the wing will behave without mass induced loading on the wing creating downward force - in other words stuck to a plane with an engine..people on board and laden with fuel....wanting to go down at 9.81 m/s2

Having done approximately 100 hours flying (paltry I know ) however my dad being a pilot my brother being a pilot...I can assure you when a wing stalls you go down bloody quick and the recovery procedure is standard on how to get and generate lift....and sadly some incredible pilots have died on a stalled wing where lift was non existent to prevent them impacting earth and dying...

Also a good read is the report on Air France 447 one of the worst case of a stalled crash in history where over 228 people died

 

[Space Bat]

This is an excellent video - originally seen elsewhere

As the guy hits stall speed - the wing drops as it can't generate lift...this will eventually enter a benign slow stall spin.....if the AOA was higher the spin would be worse and sometimes dependant on wing design can enter a death spin which can be unrecoverable - this is what happened I believe to Art Scholl who deliberately entered a flat spin during filming Top Gun in a Pitts..


http://www.youtube.com/watch?v=SXwVyxorvno

 

[hone]

[video=youtube;SXwVyxorvno]http://www.youtube.com/watch?v=SXwVyxorvno[/video]

To make it show up as a video hit the button that looks like film up where the smiley faces are and input the link

 

[Steve_B]

Space, I'm not disputing the fact that planes 'fall out of the sky' when they stall. When the wing stalls the lift decreases to less than the planes weight so the plane will fall, and quickly. But that does not necessarily mean that the wing isn't making any lift at all. I covered this in my previous post. Stalled means the flow detaches, it doesn't mean all lift is lost.

If you don't believe the actual wind tunnel tests (and there are many that show the same thing) You can see the effect of lift from stalled surfaces when you fly harrier but also when your control surfaces deflect by more than 15 degrees or so in normal flight. For instance when you move the elevator 45 degrees it must obviously be stalled yet it still responds vary powerfully. Even in something as simple as sticking your hand out of a car window at speed you can feel the lift even when your hand is at 45 degrees and so 'stalled'.

I'm not making this stuff up.... I've researched on the subject from authoritative sources. The wind tunnel data you say is misleading came from this paper: http://prod.sandia.gov/techlib/access-control.cgi/1980/802114.pdf

And an aerospace engineer who also appears to be interpreting the data in the same way: http://www.aerospaceweb.org/question/airfoils/q0150b.shtml

In the 'normal' airplane world post stall lift is academic because drag is too massive. At 45 degrees for instance the drag is equal to the lift, which means you would be just as well hovering on prop thrust as flying on wing lift, most 'normal' airplanes don't have that sort of thrust.

 

[Space Bat]

I think we are agreeing the same thing Steve..but our method and interpretation a little different...you can't compare a wind turbine to the reality of how a wing will respond on an aeroplane with significant mass wanting it to descend...and a wing on its own can behave differently due to kite effects....

The wing is at the point of stall is at its most efficient for lift...however once beyond this flow is removed and it separates and lift is reduced or removed...now you have to remember that on a real plane any reduction of lift can be quite significant dependant on design and its flying weight loaded..and what happens is not enough lift is generated to offset overall weight the margin for this is closer than on an RC plane thus what ever lift the wing exhibits or capable of producing on it's own is negated as the overall weigh its carrying forces the wing down or the plane down regardless -and the plane falls or enters a spin....at this point the lift begins to remove completely and the plane will descend quicker until it accelerates at g - some buffeting can be exhibited however in most cases and drag can cause a plane to descend at slower than g and this can allow for better ability to recover....and indeed one of the tricks of stall recovery is to use this effect and allow this to work to your advantage in putting in a recovery...

But as you say - a RC plane tricks this completely due to the greater wing area against overall mass of the plane - and the wing loading is incredibly low...added with well over 1:1 thrust to weight...larger props and massive control surfaces the wing will behave quite abnormally to how a real plane will behave...and kite effects also play a factor on how RC planes responds...at abnormal attitudes...

I am not disagreeing with what you are saying as for the most part it's correct - however a stalled wing on a plane can not generate lift as I interpret what you are saying in a real world conventional aircraft.....

 

[Steve_B]

ok we both agree that post stall lift is irrelevant for 'real' full size airplanes (with the possible exception maybe of some jet fighters) because 'real' planes don't have enough thrust to push a massively inefficient high drag stalled wing through the air.

However the subject here is 3D models, which have massive thrust to weight ratios, and they do in fact produce some lift from the wings even when fully stalled. It's also perfectly true that a good proportion of the lift that keeps our planes up in harrier flight comes directly from prop thrust, the wings don't produce all the lift.

BTW.. although that paper was done with wind turbine blade research in mind the wind tunnel data was recorded in perfectly normal wind tunnels and uses symmetrical airfoils very similar if not identical to the type we use on 3D models. Even the Re numbers analysed are in the right ballpark (at least for larger 3D models).. so it's good data IMHO.