Been thinking ... relationship between back-stick and speed

[NineLine]

Oh trust me... I can imagine, some images cannot be erased from you mind because of google and simple searches :)

 

Add aircraft, wind tunnel and tests... :) You could never imagine what "G-" based searches drag to the beach...

[IvanK]

A handling excercise done in many Air forces (well 30 years ago at least in non FBW aircraft like the MB326 :) in advanced flight training is "Manoeuvring on the Buffet" In this exercise the student is required to keep the aircraft on the the first hint of buffet (also referred to as the "buzz" or "tickle"). The student is required to hold the buzz whilst varying airspeed with nose position using bank and rudder to vary nose position. Airspeed is varied from very slow speed up to say a speed sufficient to get 4-5G and back to slow speed. AOA and stick position (in pitch) remain virtually constant throughout. Some very very very minor changes obviously occur to cater for turbulence though for all intents and purposes stick is fixed in pitch. G and IAS are continuously varying.

Edited July 19, 2013 by IvanK

[Merlin-27]

A handling excercise done in many Air forces (well 30 years ago at least in non FBW aircraft like the MB326 :) in advanced flight training is "Manoeuvring on the Buffet" In this exercise the student is required to keep the aircraft on the the first hint of buffet (also referred to as the "buzz" or "tickle"). The student is required to hold the buzz whilst varying airspeed with nose position using bank and rudder to vary nose position. Airspeed is varied from very slow speed up to say a speed sufficient to get 4-5G and back to slow speed. AOA and stick position (in pitch) remain virtually constant throughout.

 

Very interesting. I'm sure that is not easy to do. That is where the "feel" of the aircraft makes a huge difference.

 

Some very very very minor changes obviously occur to cater for turbulence though for all intensive purposes stick is fixed in pitch. G and IAS are continuously varying.

 

I think you meant "intents and purposes." I'm not trying to be a know-it-all but figured you'd want to be aware. :)

 

LINK

[Exorcet]

THis graph IS NOT OK starting from M=0.2 (for the reason I wrote above). It's a simplification that most of sims or projects (as jcomm wrote) suffered from. And not only sims as well - P-51 G-load diagram in flight manuals used IAS^2 based curve that gives wrong G-load values at high speed. The diagram for A-10, though, are more right, for example, so intially I used it to determine max CL at various M. Later I got a lot of diagrams for A-10 including CL_max vs M and the determined curve was very close to it.

I definitely got it wrong with respect to how much variation there is in cl/AoA_crit when you go into compressible flow, but the mechanism at work behind stall is the same up to transonic. The explanation for stick position I gave should be right for the most part shouldn't it?

[IvanK]

yep thanks Merlin Edit done :)

 

Regarding the Manoeuvring on the buffet exercise its actually pretty straight forward once you get the hang of it.

[Yo-Yo]

I definitely got it wrong with respect to how much variation there is in cl/AoA_crit when you go into compressible flow, but the mechanism at work behind stall is the same up to transonic. The explanation for stick position I gave should be right for the most part shouldn't it?

 

I think you can not understand that LOCAL velocities (and local M) around the wing surfaces differ from the plane speed. If you found the NACA report I mentioned you would find the results of measurings how max CL depends on M EVEN AT MODERATE M NUMBERS.

[Exorcet]

I think you can not understand that LOCAL velocities (and local M) around the wing surfaces differ from the plane speed.

No I get that, that's why I mentioned M .3 in my second post, you go into the incompressible region and things start changing, though I underestimated how much. Thinking about it though, at max cl, local values will differ even more from freestream than at low cl since the wing is working harder, that is something I did not really consider before.

 

If you found the NACA report I mentioned you would find the results of measurings how max CL depends on M EVEN AT MODERATE M NUMBERS.

What I just want to confirm is that conceptually my explanation is correct since that's what was being asked about in the thread originally. Even if the max cl drops, overall you should be pretty far from it at high speed, but approach it at lower speed, which explains why large pulls of the stick would be more dangerous when slow.

[Echo38]

This is one of the reasons I love flight--no matter how many thousands of hours one puts into studying it, there's always a ton left to learn about it!

[Suchacz]

Just some vids to show and describe how the lift is generated and why stall conditions can occur.

 

[Anatoli-Kagari9]

Curiously in this reference:

http://digital.library.unt.edu/ark:/67531/metadc54797/m1/13/sizes/l/

 

(read near the end of the page...)

 

it's interesting to see that for the profile under test, CLmax increased again, very rapidly, past Mach 0.60 ....

 

But now a very basic, probably stupid, question... Given constant q, assuming i.e. straight & level flight, an increase in speed translates into a reduction on CL given that CL is inversely proportional to q and thus to the V^2. I believe this natural variation of the CL will apply to the whole CL range, thus implying a lower CLmax at higher speeds (?) What am I missing here?

Edited July 20, 2013 by jcomm

[Exorcet]

Curiously in this reference:

http://digital.library.unt.edu/ark:/67531/metadc54797/m1/13/sizes/l/

 

(read near the end of the page...)

 

it's interesting to see that for the profile under test, CLmax increased again, very rapidly, past Mach 0.60 ....

 

But now a very basic, probably stupid, question... Given constant q, assuming i.e. straight & level flight, an increase in speed translates into a reduction on CL given that CL is inversely proportional to q and thus to the V^2. I believe this natural variation of the CL will apply to the whole CL range, thus implying a lower CLmax at higher speeds (?) What am I missing here?

 

There is only an inverse relationship if you also hold lift constant. In reality, lift isn't constrained and it will grow with speed while the other variables do what they want.

 

Also holding q constant while varying speed in level flight is a bit of a contradiction. You could hold a constant q with varying speed by climbing to different density. I just want to make sure I know what you're asking.

 

In any case while lift is related to speed, cl isn't. CL depends on Mach and Reynolds number. Up to Mach 1 you go through about 4 regimes:

 

Incompressible laminar (1)

Incompressible turbulent (2)

Compressible (3)

Trans/supersonic (4)

 

Where these come into play varies from wing to wing and as Yo-Yo brought up, what the wing is doing. (1) and (2) have flow structures that don't change Mach number, M is basically meaningless here and the difference between them is down to Re. Laminar flow is more prone to separation so it's expected that the max cl and AoA is lower in (1) than (2). With a large wing, (1) is very small though and might even be below takeoff speed.

 

(3) starts around M.3 and this is where a different Mach number will represent different airflow around the wing. Now cl and such can start changing even if you ignore Re since density is able to change. In the test above, it was noted that the pressure peaks on the wing became less intense and spread out.

 

So basically the deep blue region here started to shrink which would reduce the chances of a stall. The stall comes from a very large blue (low pressure) region pulling the air back.

 

(4) is where the stall mechanics can change. You go from stall induced by suction peaks to stall induced by shockwaves, but it's still an adverse pressure gradient issue, just that now the high pressured shock impede the flow of air over the wing.

[Anatoli-Kagari9]

Yes, I am aware of that, but a little bit confused:

 

- Most books I have access to read:

 

"At a constant AoA of the wings, the lift coef is almost constant up to transonic velocities, where it will vary with the Mach number..."

 

but apparently the reduction in CLmax happens, at least for certain profiles, well bellow transonic regimes...

 

It's usually given the example of recovery from high speed dives, where the increase in wing load and thus lift coef, will require a higher AoA and this in turn increase the local vel over the wing causing supersonic flow speeds over the wing surface at lower flight speeds and thus a reduction in M_crit

[Exorcet]

Well like you said, transonic velocity isn't a definite number. M=1 is really the only definite number, but when and where any part of the plane sees M=1 can vary and doesn't need to match up with velocity that the plane sees overall.

 

Again I don't have specific data, but transonic is usually considered ~M.8. However if you calculate the local M on an airfoil, you can find supersonic flow at much lower numbers. I recall an example I did where I found supersonic flow at a freestream Mach below .65. Couple this with reduction in speed to reach local M and you can hit it unexpectedly fast.

 

I think one important thing to note is that when we're talking about cl max, we're talking about the wing when it is working the hardest, so all the differences between local and freestream values are high. What may be small effects at low cl become amplified and you end up with greater variation in performance when you start changing flow conditions.

[Hadwell]

lol just layman explain it... if the wings front isn't facing the direction of airflow, it acts like a brake... instead of producing lift, it just drags... the P-51s wing can't even remotely get close to sonic speeds since the wing is meant to be efficient at low speeds...

 

stick ur hand out a car window, if u have the flat of ur hand pointed towards the direction the car is moving, ur whole arm goes flying back, hence airbrake... but if u have the side of ur hand pointing the direction the car is moving ur hand floats on the air... the reason u can pull back more on the stick at higher speeds is simply because it takes you more time to slow down to a stall speed at a higher speed... and the stall speed goes up the more/faster you pull back on ur stick.

Edited July 31, 2013 by Hadwell

[AndyJWest]

lol just layman explain it... ....the reason u can pull back more on the stick at higher speeds is simply because it takes you more time to slow down to a stall speed at a higher speed

 

Wings don't stall at a fixed speed. You can stall at any speed - it is the angle of attack that matters.

[Echo38]

I remember reading about one of the SR-71s which had some sort of control failure during high-speed level flight; it pitched up violently, stalled at Mach 3, and broke up. The G-force killed one of the pilots, but the other was somehow thrown clear of the aircraft with only injuries.

 

Do note that I don't recall the source, so this story may or may not be true. But a stall is indeed quite possible at that speed--at any speed, as others have pointed out--if you've got the elevator and enough force to deflect it sufficiently. Good luck surviving, though--when the airfoil suddenly goes from "going really damn fast" to "airbrake mode," it's going to slow the aircraft down much too quickly, even if the "vertical G's" are low (which I'm guessing they won't be, even if the wing has lost lift).

 

Edit: found what looks to be a scan of a newspaper, mentioning the incident: http://www.wvi.com/~sr71webmaster/losses0003a.JPG

Edited July 31, 2013 by Echo38

[Hadwell]

Wings don't stall at a fixed speed. You can stall at any speed - it is the angle of attack that matters.

 

read my whole post before u say that... i said that...

[AndyJWest]

Hadwell, I read what you wrote. You don't need to 'slow down' to stall, end of story.

[Exorcet]

lol just layman explain it... if the wings front isn't facing the direction of airflow, it acts like a brake

And the back surface does the opposite (pressure recovery) though this is the weaker effect. The difference is counted by thrust though.

 

 

... instead of producing lift, it just drags... the P-51s wing can't even remotely get close to sonic speeds since the wing is meant to be efficient at low speeds...

The wing is always going to produce lift except at extreme conditions. And transonic/compressibility flow was a huge issue for WWII fighters, Mustang included. They were getting pretty fast, but high speed flow was poorly accounted for.

 

stick ur hand out a car window, if u have the flat of ur hand pointed towards the direction the car is moving, ur whole arm goes flying back, hence airbrake... but if u have the side of ur hand pointing the direction the car is moving ur hand floats on the air... the reason u can pull back more on the stick at higher speeds is simply because it takes you more time to slow down to a stall speed at a higher speed... and the stall speed goes up the more/faster you pull back on ur stick.

Changing the angle of your hand changes the angle of the lift vector. It's not just drag pushing you back, plus the force of your arm (and the car) keeps your hand from actually losing speed.

 

Like Andy was saying, there is no stall speed really, just angle.