Gliding with the prop stopped

[BillieBob]

If an experiment shows that there is no difference, what is the explanation?

The experiment is flawed. If you can stop the prop whilst maintaining the attitude for best glide (e.g. by feathering), there is a benefit in doing so. In practice, however, in an aircraft with a prop that cannot be feathered, any benefit in gliding range from a stationary prop is more than outweighed by the height lost in stopping it aerodynamically.

Bulldog props used to stop in the spin all the time. If it didn't re-start itself in the recovery, a quick burst of starter always did the trick.

[mad_jock]

nah there is two of them

2.5meters 1.25m length of a blade.

Which is what I was thinking at the time, but on you taking the piss I realised i was talking pish. Anyway its like slamming the brakes on in the air. You have to be aware you will get quite bad leans if you do it S&L

[bingofuel]

Me, taking the piss, I wouldn't dare to do such a thing with such an esteemed member of pprune.
Anyway I am old so I use good old feet and inches, rods and furlongs, none of this foreign metric stuff

[mad_jock]

Nope thats not the way I work, please in the future, if you see me talking pish, take the piss.

[bingofuel]

That's assuming I can tell you are talking 'pish' and not just baffling me with bull!!!!.

[mad_jock]

True you do have a point.

[BackPacker]

Those saying that a stopped engine subsequently needs a very high IAS to send it over compression....have you not tried pulling 'G'? Pitching up changes the airflow through the propellor disc and improves the situation.

Interesting. I might just try that next time. Curious to see the aerodynamics explanation for this though. "Changes the airflow" sounds a bit weak to me.

OTOH, if it doesn't work you've just lost a significant amount of height to get to your high IAS anyway, and lost speed due to the induced drag from these high Gs. So a lot of energy is gone and you might need another 1000 feet or so for a next attempt. I know the prop starts windmilling again at 140 knots, so why would I start pulling out of my dive at 120 already?

[ShyTorque]

"Changes the airflow" sounds a bit weak to me.

The theory is as taught by the RAF during the CFS QFI Course. I'll pass it on but I think you've failed the course.....

[mad_jock]

I believe its because of your old prop theory vector diagram.

When the prop is stationary it is always stalled in level flight because you have lost the vector to do with the rotation of the prop. If you hurl the thing around a bit you can get the angle of attack below the stall angle of the prop and then it starts creating lift which then turns it thus lowering the angle of attack etc etc.

For Bingo possible bull!!!! alert

[gasax]

FWIW on boats there have been a number of experiments which generally 'prove' that they is more resistance from a freewheeeling prop than from a stopped or locked prop.

For this (and a couple of gearbox related reasons) it is not unusualy for some yachts to have propshaft brakes fitted.

In an aircraft the same theory must hold good but there are differences - obviously density - although as the prop is sized to absorb the engines power this should not have an effect. The other obvious difference3 is the speed at which the prop rotates. In the boat tests the prop was genuinely freewheeling, in an aircraft it is driving the engine and so not rotating anything like as fast. As a starter motor can pretty much do the same then it is drawing a little (one to two horsepower) power from the aircraft's path.

So the boat case is much more like an autogyro in aircraft terms.

So a stopped aircraft prop creates pure drag, a slowly rotating one probably creates slightly more - but the difference is probably too small to really notice unless conducted under very still and carefully controlled conditions.

Which largely means it is probably not worth the trouble!

[BackPacker]

For Bingo possible bull!!!! alert

Okay, I'll give it a first try. For brevities sake, let's assume the (fixed pitch) prop doesn't have any twist but has a uniform 18 degrees or so AoA. (With AoA in this respect I mean the AoA of the prop blad when turning, with the aircraft stationary. So this AoA will decrease once the aircraft starts speeding up.)

In level flight with the prop stopped this means the AoA is now 90-18=72 degrees, well above the stalled angle. Now you're going to "pull some G's" to change the AoA of the prop by changing the AoA of the whole airframe. But you can only change the AoA of the airframe up to the airframe stall AoA, either positive or negative. So that's roughly again 18 degrees one way or another. Best you can do to the prop AoA is therefore 72-18 = 54 degrees. Still well above the stall AoA.

Now of course the AoA of the prop is not a uniform value throughout its length, due to the twist. Near the root it may be as much as 45 degrees. So at that part of the blade it *may* just work. But that area is also the place where the prop is normally least effective, plus (due to its proximity to the root) the required moment is also the greatest.

And anyway, between the 45 degrees at the root and the 18 degrees at the tip, the prop is still stalled throughout its length in level flight, or in a level dive. And still, in a dive with sufficient airspeed the prop will start windmilling anyway. So I don't think stalling/unstalling has anything to do with this. After all, even a stalled airfoil will create some (impact) lift.

Nevertheless, now that I'm thinking about it, I think the whole idea of changing the AoA of the airframe has some merit, but for a different reason.

Suppose you've got a prop that stops in flight in a more or less horizontal position. And you've got a clockwise-turning engine (as seen from the cockpit). In level flight both blades present a more-or-less flat surface to the airflow and nothing much happens. Until the impact lift against the not-quite-perpendicular blades is so strong that the prop moves the engine through the first compression. But anyway the drag on both sides is the same.

Now you're changing the AoA of the airframe by pulling Gs. This means that the prop blades now have a different AoA too. The blade at the left hand side of the airframe presents more of a leading-edge-on profile to the airflow, so its drag is lowered. While the right hand blade presents more of a perpendicular profile to the airflow, so its drag is increased. It might be this difference in drag that helps push the engine through its first compression.

OTOH, I apparently failed the QFI course (though I always thought you couldn't fail if you didn't try) so I'm standing by for a better explanation...

[mad_jock]

I would have to sit down and do the vector diagrams but I think the additional vector of pulling G and the resultant change in AoA percieved by the prop would be more than you have described. Its more to do with the rate of turn than anything else and there will be a cosec in there somewhere which will be loosely linked to you increased stall speed pulling G.

Because the prop aerofoil is 90 degs out of plane to the wing aero foil, my gut feel is that this would cause it to have a reduced stall speed.

Tis quite interesting mental problem though.

Genghis and India Mike can you helpout please?

[bookworm]

Regardless, I think the effect of stopping a fixed pitch prop is much less than the effect of feathering a variable pitch prop. There was a NACA paper on this -- I'll try to dig it out.

NACA paper.

The table on the final page gives some experimentally measured drag values for a particular prop. I would infer:

* Compared to a feathered prop (which is much less draggy), a stopped prop, freewheeling prop and prop driving a dead engine are much of a muchness.

* A freewheeling prop has less drag than a stopped prop, which has less drag than a prop driving a dead engine

* A prop on a running engine throttled to idle produces thust at low speed, but almost as much drag as a prop driving a dead engine at high speeds.

[Agaricus bisporus]

A prop turning an engine must be absorbing quite a lot of power, so it must create much more drag than one stopped. How many horsepower do you suppose it requires to turn, say, a 200hp engine at 2000rpm? Lots.

A Freewheeling prop creates vastly more drag than the same prop stopped (unfeathered). A stopped prop only has form drag. A rotating one adds aerodynamic (rotational) drag. All sailing boat owners know this.

[bookworm]

A Freewheeling prop creates vastly more drag than the same prop stopped (unfeathered). A stopped prop only has form drag. A rotating one adds aerodynamic (rotational) drag. All sailing boat owners know this.

Perhaps that's why boats never get off the surface.

A stopped prop is still an aerofoil being dragged through the air or water. It creates lift, and therefore some associated induced drag, as well as form drag. So does the windmilling prop. I can see no reason why in principle one should be draggier than the other. Here's another paper that makes it clear that the trade-offs are more complex than you would like.

[IO540]

How many horsepower do you suppose it requires to turn, say, a 200hp engine at 2000rpm? Lots.

If you extend the best-economy plot in this diagram (a 250HP engine) all the way down to zero fuel, the intercept on the power output axis is somewhere around minus 60HP which I think represents the friction losses at ~ 2000rpm.

I hope I got that right.

It is about 25% of max rated power.

[Piper.Classique]

Yeah, right. All good stuff no doubt. So, did you pick your field yet?

[24Carrot]

Talking of complexities, it isn't just the prop that produces drag on an aircraft.

Compared to the free-wheeling prop, the stopped prop will impart more of a twist to the airflow over the fuselage and tail-fin/rudder. After correcting with rudder, you will have more induced drag at the tail. If you end up with any extra yaw, the whole aircraft's profile drag will go up too.

I have no idea how big this effect is!

Edited to add:
A stopped prop probably needs some aileron deflection too.

[ShyTorque]

A windmilling engine is overcoming frictional drag from its own components, churning engine oil and compressing air, to some extent.

This requires energy, which has to come from somewhere (i.e. from potential energy).

[mary meagher]

All very interesting. But experience has shown that if the propellor has stopped turning, for whatever reason, unless you have plenty of altitude, the field will select you. Good luck.