Engine failure in Vy climbout
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A stationary propeller produces less drag than a windmilling one
Can anyone present data which will support a position on this either way, and if so, how much different? I agree that any flight manual I have seen has either said "windmilling" or "feathered". So, perhaps there is a difference, which manufacturer's know, but don't want to say....
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Sorry, no data, but the "Windmilling Drag" or backwards thrust must surely depend on how fast the blades are turning, which must in turn depend on the friction provided by the broken engine.
Which might vary a lot, depending on what broke?
I suspect that zero-friction is the worst case, where the propeller does it best to resemble an auto-rotating helicopter.
Which might vary a lot, depending on what broke?
I suspect that zero-friction is the worst case, where the propeller does it best to resemble an auto-rotating helicopter.
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If the prop is windmilling in a steady state at some RPM, it must be absorbing the right amount of energy from the airstream to balance what is needed to turn the engine over at that RPM against the internal friction, compression, pumping etc. If it were true zero friction, it would absorb no power.. if it can't overcome the friction, it will stop.
If the prop is stopped, you're looking at whatever is pretty close to the drag figure for a flat plate of the same projected area at that speed - that should be pretty straightforward to figure. My gut feel says less, in the same way that a stalled wing can lift less than an unstalled one (the prop is just a wing..), but honestly I don't know.
Having actually performed a glide approach with the engine shut down, I can't say the transition from idle power to windmilling, to stopped was noticable - I wasn't looking at the VSI etc., but in terms of seat of pants perception of glide angle, it wasn't evident. It did take a lot of effort to actually stop the prop however (well into stall buffet). I did find it a little disconcerting that the prop suddenly stops being transparent, and in this case (decathlon) with centreline seating, it chose to stop dead vertical, right in front of my nose, no missing it..
If the prop is stopped, you're looking at whatever is pretty close to the drag figure for a flat plate of the same projected area at that speed - that should be pretty straightforward to figure. My gut feel says less, in the same way that a stalled wing can lift less than an unstalled one (the prop is just a wing..), but honestly I don't know.
Having actually performed a glide approach with the engine shut down, I can't say the transition from idle power to windmilling, to stopped was noticable - I wasn't looking at the VSI etc., but in terms of seat of pants perception of glide angle, it wasn't evident. It did take a lot of effort to actually stop the prop however (well into stall buffet). I did find it a little disconcerting that the prop suddenly stops being transparent, and in this case (decathlon) with centreline seating, it chose to stop dead vertical, right in front of my nose, no missing it..
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Wow, lots of responses and discussion here - nice to see, thanks everyone.
I meant to try Vx too but for various reasons didn't. Next time. In my 182 though Vx (at gross) is 78, Vs1 is 51 - closer to 30 knots than 10.
Conventional wisdom is that an idling prop is actually generating net negative thrust (obvious really since RPM at idle in flight is higher than on the ground). I suspect the effect on the overall situation is small.
I'm all in favour of a bit of experimental flying, but I don't harbour a death wish, so no thanks. But STOPPING the prop according to most conventional wisdom actually REDUCES drag. Otoh unless the engine suddenly and immediately seizes (unlikely but of course possible), it's not something you're going to have time for in an EFATO. I don't see why a stopped prop would have any impact on trim/attitude but maybe you can explain.
I know. I started it, or at least a segment of it. But at some point the question of what happens immediately following the engine failure led to a discussion and I said I'd try it. Just delivering on my commitments...
@BPF - Big difference between Vy and Vx as Vx is typically less than 10 knots above power off stall.
@UV - What did you do during your simulation? I suspect you reduced the power to idle... By throttling back to idle you still have a little residual power (put simply somewhat like feathering) which reduces drag. It can be nothing like the real thing!
@UV - Try stopping the engine at altitude with the mixture control, then switch off both mags. You may be quite surprised at the result, and thats probably without the prop actually stopped!
@maxred - n5296s - the discussion you refer to revolved around TURN BACK
Vx (best angle of climb) is 54 kts
Vy (best rate of climb) is 78 kts
Stall speed at flaps 20 is 47 knots
There is only a 7 kt spread between Vx and flap 20 stall speed
Hey Mark,
At least you have the starter motor available to make the prop horizontal - if that's what you prefer....gets it out of your view, and eliminates the prospect of a ground strike....
I saw the results of a Cessna 310 with twin bladed props many years ago, who had a gear problem, so he arranged to get over the smooth grassy area, cut both engines, then use the starter motor to make the props horizontal before gliding it gently onto the grass at SY (AUS).
Minimum damage was the result.
Although I do believe the current thinking is to put it onto the bitumin instead...but that's another thread.
At least you have the starter motor available to make the prop horizontal - if that's what you prefer....gets it out of your view, and eliminates the prospect of a ground strike....
I saw the results of a Cessna 310 with twin bladed props many years ago, who had a gear problem, so he arranged to get over the smooth grassy area, cut both engines, then use the starter motor to make the props horizontal before gliding it gently onto the grass at SY (AUS).
Minimum damage was the result.
Although I do believe the current thinking is to put it onto the bitumin instead...but that's another thread.
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Not really! The 'stopped' prop would 'creep' over the compression of the engine every few minutes, and complete a half revolution to stop precisely vertical again.. I'm thinking that with a 4 pot, 4 cycle engine the compression 'stops' are going to be 180degrees apart, so with a 2 blade prop it is a simple matter of where someone bolted it on. Much more scope with > 2 blades and/or > 4 cyls I guess.
n5296s, just on the idling prop; While an idling prop has net negative thrust, it has *less* net negative thrust than an idling prop without the engine running - the chemical energy from the fuel is helping to turn the engine over. Whether that has a significant effect upon glide profile I am not sure.
P.S. No deathwish was involved, engine was stopped at 5000agl directly overhead a large, but quiet airfield at the instructor's suggestion. I found it a worthwhile and interesting exercise, though I'm sure the practise is a little controversial.
n5296s, just on the idling prop; While an idling prop has net negative thrust, it has *less* net negative thrust than an idling prop without the engine running - the chemical energy from the fuel is helping to turn the engine over. Whether that has a significant effect upon glide profile I am not sure.
P.S. No deathwish was involved, engine was stopped at 5000agl directly overhead a large, but quiet airfield at the instructor's suggestion. I found it a worthwhile and interesting exercise, though I'm sure the practise is a little controversial.
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P.S. No deathwish was involved, engine was stopped at 5000agl directly overhead a large, but quiet airfield at the instructor's suggestion. I found it a worthwhile and interesting exercise, though I'm sure the practise is a little controversial.
During my EFATO tests I tried the difference in effect between an idling prop (throttle closed) and a windmilling prop (mixture closed). The difference was in the order of 100-200 fpm. I have not (yet) established the V/S with a stopped prop.
But as others said, with a fixed pitch prop, actually getting the prop to stop is very hard in itself. So unless the engine has completely seized solid, you will have to do your forced landing with a windmilling prop. Maneuvering to get a windmilling prop to stop (which required a half G pushover at well below Vs in the R2160 when I tried it) is not something you are going to experiment with in a for-real situation.
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On the other hand, if you do get the prop stopped in flight, it's going to be really hard to get it turning again without a starter. I used to fly a J-3 on skis, whose engine had no accelerator pump, and did not idle worth a darn. The result was that sloppy use of the throttle would make it quit, an I would have to dead stick onto the forzen lake (which was my intended landing site anyway). There, I would hand prop it.
I have windmill started my 150, having stopped the prop in flight. It's not easy, you've got to be going really fast.
I have windmill started my 150, having stopped the prop in flight. It's not easy, you've got to be going really fast.
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@BPF - For the C 182 Q POH...
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Vx=75 KIAS/KCAS at sea level, Vs1=54 KCAS at sea level (39 KIAS). So a broader spread than the fixed-gear version.
That said, you are trying to keep Vx slower, so you don't make too much speed across the ground, to bring down your apparent angle. When you actually start doing the math, it's a real numbers game, and not always what it would appear!
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Good question. I'm at work now and don't have the book in front of me, but it seems pretty obvious that it should be gear up. I mean, here you are desperately trying to get enough altitude to get over the cliff face that is inconveniently just in front of the runway... you're not going to keep the gear down.
Interesting. The figures I gave weren't quite right (shouldn't enter them without the book in front of me) but they were close... from the 1980 TR182 POH (which I DO have in front of me now) Vx=75 KIAS/KCAS at sea level, Vs1=54 KCAS at sea level (39 KIAS). So a broader spread than the fixed-gear version.
I did note however that the TR182 POH sprcifies that for a short field takeoff a speed of only 59 KIAS (60 KCAS), or 6 knots above stall speed, is recommended until all obstacles are cleared. I wonder if the Vx in the later models was raised to provide a larger margin above stall. In any case I would not want to be 50 to 100 feet off the ground with a pitch attiude neccessary to maintain 59 kts, and have the engine fail
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That's odd, I agree. It led to quite a conversation between me and my instructor when I was doing my CPL. Aerodynamically, Vx is Vx - there can't be one Vx at takeoff and another at altitude (once you're out of ground effect anyway).
One possible explanation is that the PoH also says that a short field takeoff should be flown gear and flaps down until clear of the obstacle. This makes absolutely no sense to me, though of course I did it for the CPL. If I've got a tree looming up, the last thing I need is the huge extra drag of the gear. But maybe 58 knots is Vx gear/flaps down, which would make some sense.
One possible explanation is that the PoH also says that a short field takeoff should be flown gear and flaps down until clear of the obstacle. This makes absolutely no sense to me, though of course I did it for the CPL. If I've got a tree looming up, the last thing I need is the huge extra drag of the gear. But maybe 58 knots is Vx gear/flaps down, which would make some sense.
Actually not retracting the gear until clear of obstacles does marks sense as when the gear retracts it turns sideways significantly increasing its drag over the profile it has when extended. Since the gear retraction is a somewhat leisurely processes the effect will persist for awhile. This extra drag is even more noticeable on the older style high wing retractable Cessna's fitted with main gear doors.
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Makes sense for the FAA-approved 50 foot obstacle, I guess, since you'd be clear of it before the gear had finished retracting anyway. Makes less sense for something significantly higher. I agree that there is more drag for a few seconds as the gear retracts, but no idea how much and hence how to figure out the obstacle height where retracting first works in your favour.
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I'm curious why, in the normal course if events, anyone would want to climb at Vx? Obviously if there are obstacles to clear. But otherwise? Surely better and much safer to climb at higher speed, giving more time to react if the donk stops. And better cooling for the engine, too.
