stalling a propeller ??
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stalling a propeller ??
Hello,
just wondering can you stall a propeller ?
If so , under what conditions ?
I was asked this in a mock briefing for the FI rating.
Many thanks.
just wondering can you stall a propeller ?
If so , under what conditions ?
I was asked this in a mock briefing for the FI rating.
Many thanks.
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Depending on the pitch of the prop (e.g constant speed prop in coarse pitch/min. rpm) you could stall it by going to full power on the ground with the brakes on. It would unstall as the aircraft gathered speed and the prop AoA decreased. In the early days of VP props people used to run off the end of runways through trying to take off in coarse pitch. Oops.
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Its not very PC these days but....
If you make sure you over head the runway at about 4-5k pull the mixture then stall the aircraft. Keep it in the stall and really crank it back and don't let it spin. Eventually the prop will wind down and stop turning all together. It is now stalled and everything is very quiet. As soon as it stops pitch for best glide speed
Select fully rich and normal restart in the air drills (aka engine failure restart) and crank it. If it dosn't start don't muck about trying to start it, set up for a glide landing and then try again. if it still dosn't start do a forced landing on the runway and be prepared for a major bollocking off the CFI and airport manager.
They used to do it days gone by supposedly because you have a better glide range but as you generally loose more height and range by stalling the prop than you extend the range after stalling. It has been decided by those that know as dangerous and a bit stupid.
tescoapp
If you make sure you over head the runway at about 4-5k pull the mixture then stall the aircraft. Keep it in the stall and really crank it back and don't let it spin. Eventually the prop will wind down and stop turning all together. It is now stalled and everything is very quiet. As soon as it stops pitch for best glide speed
Select fully rich and normal restart in the air drills (aka engine failure restart) and crank it. If it dosn't start don't muck about trying to start it, set up for a glide landing and then try again. if it still dosn't start do a forced landing on the runway and be prepared for a major bollocking off the CFI and airport manager.
They used to do it days gone by supposedly because you have a better glide range but as you generally loose more height and range by stalling the prop than you extend the range after stalling. It has been decided by those that know as dangerous and a bit stupid.
tescoapp
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Intuitively, I'd say the latter isn't to actually stall the prop, but rather to make the slipstream driving it get so small that the friction from rotating the cranckshaft etc makes the prop stop.
One indicator: It's easier to stop the prop this way when flying something with a big engine (i.e. takes more energy to turn over). As this normally means you fly something with a higher stall speed (=more slipstream), I guess a condition should be that a/c stall speeds are the same.
Anyway, I'd go for DB6's description.
Cheers,
Redbar1
One indicator: It's easier to stop the prop this way when flying something with a big engine (i.e. takes more energy to turn over). As this normally means you fly something with a higher stall speed (=more slipstream), I guess a condition should be that a/c stall speeds are the same.
Anyway, I'd go for DB6's description.
Cheers,
Redbar1
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Tescoapp, you're talking about stopping a prop, not stalling it in the aerodynamic sense. In simple terms a propeller is just few wings stuck onto a hub. In the same way as you stall a wing by taking it beyond the critical angle, you stall a prop by increasing its pitch relative to the airflow. You can do this by either increasing the pitch at a given airspeed or reducing the airspeed (to zero in my example above) at a given pitch.
As an aside, the twist that you see in propeller blades (coarse pitch at the root, fine at the tips) is there because the inner portions are travelling more slowly than the outboard portions, thus requiring a higher blade angle to achieve the angle of attack necessary to generate significant lift (i.e. thrust).
As an aside, the twist that you see in propeller blades (coarse pitch at the root, fine at the tips) is there because the inner portions are travelling more slowly than the outboard portions, thus requiring a higher blade angle to achieve the angle of attack necessary to generate significant lift (i.e. thrust).
Last edited by DB6; 11th Dec 2004 at 08:03.
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But you must admit it is stalled DB6 
Any situation you can think of where the prop angle of attack is great than the critical angle it will be stalled. The situation i described is the most graphic to see. Most other stuations you will need to sit down and do a flow vector diagram to work out what the angles are to find out if it is stalled.
tescapp
Any situation you can think of where the prop angle of attack is great than the critical angle it will be stalled. The situation i described is the most graphic to see. Most other stuations you will need to sit down and do a flow vector diagram to work out what the angles are to find out if it is stalled.
tescapp
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tescoapp,
Had exactly what you describe happen to me once while teaching a spin; you couldnt tell that the engine had stopped (apart from the low volts light that always came on at low RPM) until the prop stopped........
As for stalling the prop - I guess DB6 is strictly speaking correct.
Next question : If a windmilling prop stops; how fast do you need to go to get it windmilling again ????
Answers on a postcard....
Had exactly what you describe happen to me once while teaching a spin; you couldnt tell that the engine had stopped (apart from the low volts light that always came on at low RPM) until the prop stopped........
As for stalling the prop - I guess DB6 is strictly speaking correct.
Next question : If a windmilling prop stops; how fast do you need to go to get it windmilling again ????
Answers on a postcard....
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Seems to me that if you run a coarse pitch prop at high RPM and low airspeed it will stall as the relative airflow will approach the blades from greater than the critical angle and the blades will loose their aerodynamic efficiency. This could certainly be the situation at low speed on take off or in a tailslide. If you stop a prop in flight the relative airflow will be from directly ahead (plus or minus a bit of aircraft angle of attack and or sideslip) so the prop will be stalled aerodynamically in the braking sense ie in reverse! The first case is analagous to an aeroplane stalling in errect flight, the second to an aircraft stalling inverted; both stalls but two very different situations!
DB6, the helical twist on a prop is mostly there to alter the pitch angle of the blade so that the relative airflow approaches the blade element at the same, aerodynamically efficient, angle of attack along the entire length of the blade from root to tip. The change in blade profile (thickness/chord ratio and the use of a different aerofoil section along the blade) also helps. The fact that the blade is going more slowly near the root does not change the fact that it will be at its most aerodynamically efficient at a particular angle of attack to the relative airflow.
Happy landings!!
DB6, the helical twist on a prop is mostly there to alter the pitch angle of the blade so that the relative airflow approaches the blade element at the same, aerodynamically efficient, angle of attack along the entire length of the blade from root to tip. The change in blade profile (thickness/chord ratio and the use of a different aerofoil section along the blade) also helps. The fact that the blade is going more slowly near the root does not change the fact that it will be at its most aerodynamically efficient at a particular angle of attack to the relative airflow.
Happy landings!!
Last edited by 3 Point; 10th Dec 2004 at 18:54.
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Tescoapp, au contraire mon vieux haricot, unless you are talking about in the inverted sense since the airflow in your example is approaching the aerofoil from above 
. Or the American sense where the English language is occasionally something of a challenge and stalled can mean anything .
3 point, entirely correct and I shall write out 100 times 'read what you have written before posting'
. I have edited the above post to reflect what I meant to say.
. Or the American sense where the English language is occasionally something of a challenge and stalled can mean anything .3 point, entirely correct and I shall write out 100 times 'read what you have written before posting'
. I have edited the above post to reflect what I meant to say.
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DB6 for gawds sake sir please don't lump me in with those FAA types when it comes to theory.
Must admit a bit wooly on the old prop vector diagrams. Really miss flying props to be honest.
tescoapp
Must admit a bit wooly on the old prop vector diagrams. Really miss flying props to be honest.
tescoapp
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They used to do it days gone by supposedly because you have a better glide range but as you generally loose more height and range by stalling the prop than you extend the range after stalling. It has been decided by those that know as dangerous and a bit stupid.
His scenario was that the fire wouldn't go out after the mixture was cut, because the engine driven fuel pump (on a PA28) was drawing residual fuel from the gascolator. Once the prop was stopped, attention could then be given to the swiftest descent possible.
Fortunately, I've never had to try it, but I often remember Danny and his pragmatic approach, with great fondness.
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Since the typical stalling (critical) angle of attact (aoa) for an average aerofoil section is "about" 14-16 degrees, now consider the average pitch of a propellor. I'd say its around 7-9 degrees from the plane of rotation(guess). So even if at a standing start, max RPM was suddenly applied, the blades would only reach a max aoa of 7-8 degrees. So to answer your question, to stall a propellor, you must exceed its critical angle of attack.
To do this, wait for a suitable gale (not long..uh winter's still with us) and park you aircraft facing down wind. Now fire up the engine, and with the initial reverse flow(depending on how intense the gale is), the blades should be operating at idle rpm, well above thier critical aoa, and hence they are stalled.
I hope that this has answered your question!??
Mr L
To do this, wait for a suitable gale (not long..uh winter's still with us) and park you aircraft facing down wind. Now fire up the engine, and with the initial reverse flow(depending on how intense the gale is), the blades should be operating at idle rpm, well above thier critical aoa, and hence they are stalled.
I hope that this has answered your question!??
Mr L
