What happens to an airfoil at “stall”? And how are “lift” and “drag” defined at high angles of attack?
Imagine a light aircraft. Large wing, low weight, wing strong enough to carry the weight of the plane and safety margin. Assume no thrust – no engines installed or engines shut down.
Absence of thrust means that in steady state, the plane is descending.
The plane might fly at “best glide” AoA. It would then have appreciable forward airspeed, modest rate of descent and good L/D ratio.
If the AoA is any lower, the total airspeed would be higher, L/D would be smaller and rate of descent would be higher.
Now, let´s increase the angle of attack. The forward speed would decrease, and so would L/D. But what happens to RoD?
On the other hand, imagine if the plane is not flying at all: imagine that it has AoA of 90 degrees!
A plane with zero forward airspeed still cannot drop out of the sky at any high speed. After all, as presumed above, it has low weight and large wing. Fast RoD at 90 degrees AoA would mean huge drag. The plane has to reach a steady state at a modest rate of sink and no forward speed. It would be “parachuting” vertically down.
How does the RoD of a parachuting airfoil compare with RoD of a stalling airfoil?
What happens if an airfoil is held at AoA of 80 or 70 or 60 degrees? It should still have a modest RoD – but it should also have a small but nonzero forward speed.
Can someone explain what really changes about the airfoil behaviour if you compare non-vertical parachuting (in “stalled” AoA) with flying “at the back of the power curve”, at AoA slightly below “stall”?
By: rbcondor - 6th January 2007 at 21:06
A buffet always wakes me up – I’ve the belly to prove it ! and if on finals well I just go round again
By: wessex boy - 5th January 2007 at 19:46
Jump, John, Jump! 😀
We always laughed at the training film, but when the Captain says in a shaky voice ‘Watch the altimeter, if we go below 3000′ get out, don’t wait for me…’ it suddenly wasn’t so funny:eek:
By: wulf190a - 5th January 2007 at 18:54
I always thought that the buffet was self service snacks!! how little I know.
By: low'n'slow - 5th January 2007 at 18:11
Jump, John, Jump! 😀
By: wessex boy - 5th January 2007 at 12:56
An aircraft with perfectly normal stall and spin behaviour whose handbook recommends deploying the chute for spin ‘recovery’ rather than using conventional stick and rudder actions.
Moggy
Funny, as the last time I spun A Chipmunk I was told it may not recover and the use of parachutes was mandatory if it went below 3000′
Although good for the pilots the ‘chutes deployed did not do too much for the potential re-use of the airframe
By: Moggy C - 4th January 2007 at 13:28
Ah.. The Cirrus.
An aircraft with perfectly normal stall and spin behaviour whose handbook recommends deploying the chute for spin ‘recovery’ rather than using conventional stick and rudder actions.
Moggy
By: dean f - 3rd January 2007 at 19:02
“ACCUTE SKYHOOK FALIURE” is a common occurence in pre-2009 planes, that have not been fitted with the new improved, mark 47.6 GLASS HAMMER.
These are normally activated by a low fitted lever named a “LEG PULLER”, it is quite distinguisable as it is painted with M.O.D issue TARTAN PAINT.
Or so i have always been led to believe.;)
Dean:D
By: SimonH - 3rd January 2007 at 10:51
It has an advanced stall generator with a fully automated recovery option. The generator stalls the plane, waits to see how the pilot copes with the situation and then deploys the chute if required. The chute is only normally used if the ‘skyhooks’ fail though:D
By: cadaha - 3rd January 2007 at 02:23
That’s what the buffet is for
Ahhhh so the buffet is designed to wake up sleeping pilots if the buzzer doesn’t wake him/her up – great idea that 😀
Question though if the Cirrus SR22 has a parachute why does it need the stall generator :confused:
Carl
By: SimonH - 2nd January 2007 at 21:49
Ok, I shouldn’t really tell you this as I will be banned from every flying club in the land for giving away trade secrets. Aircraft stalls are caused by a highly classified part called a “stall generator”. This is fitted to most aircraft and is designed to make the plane stall if the pilot attempts to make the plane do something it doesn’t like. It is also programmed to activate if the pilot does not pay attention or falls asleep at the controls. Its usually hidden from sight and only the chosen few know where to find it.
Its function is to make flying more difficult for pilots. If people knew how easy flying was, they’d all want to be pilots and the skies would be too crowded for the rest of us. It also gives us something to talk about in the bar after a flight and makes aerobatics more fun. Great things “stall generators” but dont go telling anyone about them;)
For more info see www.stallingfordummies.com
By: chornedsnorkack - 2nd January 2007 at 16:06
This website will answer all your questions:
For example:
http://www.auf.asn.au/groundschool/umodule4.html#aerofoils
Look at the section 4.8, the lift-drag ratio diagram.
It is obviously absurd, isn´ t it?
Look at the baseline. By the divisions marked, you can see that the baseline means L/D ratio of 0. Not 2, not 1, not 0,5, not 0,1. Exactly 0.
And the baseline is reached at AoA about 17 degrees.
How is it possible?
By: SimonH - 2nd January 2007 at 13:51
This website will answer all your questions:
http://www.auf.asn.au/groundschool/contents.html
If you want to know more than that, contact a Test Pilots School;)
By: Deano - 2nd January 2007 at 13:46
chordedsnorkack
I somewhat agree with BlueRobin, you know full well, you are trying to set us up to catch us out, I have seen you at work on PPRuNe as well 🙂
By: BlueRobin - 2nd January 2007 at 13:10
Sorry not going to bite this time.
By: chornedsnorkack - 2nd January 2007 at 11:29
No, not quite. There is some difference between spinning and parachuting!
If the airplane is spinning, the wingtips have a substantial horizontal speed, and therefore their AoA is different from 90 degrees.
But an airplane might also parachute vertically down, without spinning.
In neither case is the plane in “free fall” – the air would resist the wings, and keep the rate of descent modest.
By: BlueRobin - 2nd January 2007 at 10:22
You know full well 😉
You’ve done something similar before
http://forum.keypublishing.co.uk/showthread.php?t=63470
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January 2, 2007 at 9:28 am