stall warner at high speed during aeros?
 

Hi

Sorry if i shouldnt post here but i am an air cadet who wants to join the services as a pilot and generally loves flying.

The other day i was on an AEF flight in a Tutor doing aeros when the stall warner went off with the airspeed nowhere near the stall speed. Didnt ask at the time...to busy enjoying myself.

Just wondered what caused this?

Thanks

Tiger

Wings stall because their angle of attack (AoA) gets too high not because their speed gets too low.

Stall warners are therefore triggered by devices that are sensitive to AoA not airspeed.

JF

Is this THE John Farley?

Many thanks for the explanation. All makes sense now.

Tiger

Tiger

John Farley's explanation can be illustrated at the other end of the speed range that you experienced - the stall warner won't go off BELOW the stall speed (which will be that for 1g flight) if the g is low. Next time you're in the Tutor look at the airspeed as you go over the top of a loop - if the speed is low, and the stall warner hasn't gone off, then check the g-meter. It'll be (well) below 1g.

You'd be surprised at the number of 4th year MEng Aeronautical Engineering undergraduates that can't grasp this on a flight test course, so you've done well to pick it up. Enjoy!

Tiger,

Yes, your question was answered by Mr. Harrier.

I recommend trying a search of the PPRune archives for John's contributions - always very informative and occasionally quite funny.

You might also look at The Harrier Development Story , his lecture to the RAeS.

Re: stall warner at high speed during aeros?
 

Lots of people who should know better get mixed up with angle of attack and angle to the horizon. I was reading something on the net the other day about an F-14 that climbed vertically at an angle of attack of "almost 90 degrees". The author was talking about the angle to the horizon, in reality the angle of attack was likely around single digits, or even slightly negative.

I reckon most engineering students should have flight test courses and a couple of hours of general handling as it would help greatly in understanding (and would be great fun!). Unfortunately it comes down to cash at the end of the day.

Tiger - there are some great people on this forum and I for one greatly value their time and efforts, even if I don't pop up too often.

Re: stall warner at high speed during aeros?
 

It's interesting that the term "relative wind" was frequently heard in class during my student days, but I don't know why it's out of vogue today. It is a simple way to distinguish (e.g.) between Angle of Incidence and AOA, or pitch angle and AOA.

Re: stall warner at high speed during aeros?
 

Ok thanks guys for all your help. Just to clarify...is there a difference between angle of attack and the pitch angle. So for instance if the pitch angle was 10 degs would the angle of attack be the same?

Cheers

Tiger

Re: stall warner at high speed during aeros?
 

Angle of attack is the angle between the mean chord line and the surrounding airflow (effectively the direction of travel).

Pitch angle (more commonly called incidence) is the angle between the mean chord line and the horizon.

They're clearly connected, but it's best to regard them as totally separate entities.

G

Re: stall warner at high speed during aeros?
 

If VSn is the stall speed at the load factor n,
If VS1 is the stall speed at the load factor 1 (in straight and level flight),

Then VSn=SQRT(n) x VS1

(BTW, in a turn, n=1/cos (bank angle Phi)...)

For example, if n= 2, i.e. at 60° bank angle, then VS2= 1.41VS1

It also works if n=0, in zero-gravity, then you canNOT stall. Sweet !

It also works if PHI=90°, n=infinite, therefore you cannot sustain a turn and you will fall out of the sky, whatever your speed...

So, if n<1, your VSn will be smaller than VS1, and your stall speed indicator won't horn, even if IAS is lower then the stall speed given in the book.

Genghis, am I right ?

Re: stall warner at high speed during aeros?
 

That's my understanding, with a couple of provisos...

- Particularly flexible wings can behave a bit differently, but that's a bit of a specialist case.

- This assumes a balanced turn, and the assumption of still air (or at-least steady horizontal wind).

A classic demonstration of this is that you can go well below the placarded 1g stalling speed at the top of a phugoid - without stalling. (I seem to remember that ETPS' Andover demonstrates this particularly well).

G

Re: stall warner at high speed during aeros?
 

Tiger - With an aircraft pointing and travelling straight up, or 90 degrees to the horizon, the angle of attack will be zero as the angle between the chord of the wing and the direction of flow is zero.

If a fast jet flies along the crowdline at an airshow, level with the ground, but pointing 15 degrees up, then the angle to the horizon and the angle of attack are the same (give or take).

As you can see, the relationship varies and Gheghis's advice above is very good, consider them as different.

However, most aircraft wings are fixed at a certain incidence (angle) to the fuselage, so that when the fuselage is flying level the wing will have a positive angle of attack and generate some lift. In that case, my examples above will not be quite right. The second one for instance may have an angle to the horizon of 15 degrees, but an angle of attack of 17 degrees, if the wing is fixed at 2 degrees to the fuselage.

Re: stall warner at high speed during aeros?
 

As an ex-flying instructor, may I point out that in the stalling exercises, one demo was to be flying along, minding ones own business and suddenly heave hard back on the control column. Due to inertia the aircraft took a little time to respond to this control input, so the angle of attack went through the roof. The student needed to note that this caused the stall warner to sound and that this was a "high speed stall".

P.P.

Re: stall warner at high speed during aeros?
 

What an intersting post! To pick up on a few points, P.P. is correct in stating that an aircraft may well stall if you snatch the control column rearward, but if you do this at an IAS significantly above the basic stall speed you risk overstressing the aeroplane.

As leonard17F implied, the manoeuvring stall speed is a function of the loadfactor (n). Clearly you don't need to turn to increase the loadfactor, you can also pull some 'g' . Fly at 1.41 x Vs1 and you can pull 2g before the ac will stall. Therefore, if you are flying S & L at just twice the basic stall speed the wings will be able to produce 4g before stalling! This may well exceed the 'g' limit of the aircraft.

A demo used when teaching stalling in the Tucano was to pull-up into a gentle wingover. Most stall symptoms were present, ie nose high, low decreasing IAS, sloppy control response, but even with lots of bank, the aircraft would not stall (an eye on the 'pointless' AoA gauge helped to make this a good demo). On the way back down from the wingover the IAS would clearly increase, but at a suitable IAS the stick could be pulled firmly rearward and despite the nose-low, high increasing IAS, firm controls etc, the aircraft could be made to stall well above the basic stall speed. The main point of the demo was to highlight to the student that the 'buffet' that occured just prior to the stall was a major indicator of the possible impending doom of stalling. As pointed out by JF, it is the high AoA that stalled the wing.

Long ago I met a Harrier head-on at low-level. He pulled the stick back sharply in an attempt to avoid a collision, and dully pulled the tailplane off the GR7 (plastic aeroplanes!), however, I digress.

H Peacock