Gliding with the prop stopped
Moderator
Thread Starter
Gliding with the prop stopped
After a half an hour of dancing in the cumulus this afternoon, I had a few thousand feet to loose. After a suitable engine cooldown, I decided to glide down. It had been a long time since I have actually stopped the prop (on a single). While gliding the C-150 (fixed pitch, 48" climb prop, and standard O-200) at idle, I consistantly obtained a VSI rate of descent of 450 FPM trimmed at 60 MPH. I allowed this to settle for more than a minute, and that was the consistant rate.
I have read over the years, of changed gliding descent rates with the prop actually stopped, this is what I would try. With the mixture leaned, and the prop stopped, I glided - oh what a peaceful sound! The observed descent rate was exactly the same, at 450FPM. Again, I allowed it to settle for more than a minute at 60 MPH, and the result was consistant.
To save a really hard cooling, I startered the engine, instead of a forced windmill start. The engine was not eager to run, but did. (my runway was just off just in case).
So what thoughts do posters have about the affect of a stopped fixed pitch prop on the rate of glide descent ? Affect? No affect? Greater? Less?
I have read over the years, of changed gliding descent rates with the prop actually stopped, this is what I would try. With the mixture leaned, and the prop stopped, I glided - oh what a peaceful sound! The observed descent rate was exactly the same, at 450FPM. Again, I allowed it to settle for more than a minute at 60 MPH, and the result was consistant.
To save a really hard cooling, I startered the engine, instead of a forced windmill start. The engine was not eager to run, but did. (my runway was just off just in case).
So what thoughts do posters have about the affect of a stopped fixed pitch prop on the rate of glide descent ? Affect? No affect? Greater? Less?
Join Date: May 2001
Posts: 10,815
Likes: 0
Received 0 Likes
on
0 Posts
Makes sod all difference, like yourself I have done the scientific see what it really does and its just not worth it.
If you were at 10k plus struggling to make "feet dry" it might get you onto dry land but for your normal flight don't bother.
O and BTW this will be sooooo out the comfort zone of most FI's in the UK.
Stopping the engine then pitching up to sub 50knts to get the prop to stall. They would ****e themselves and it would be so far outside there training experence they wouldn't be comfy at all.
If you were at 10k plus struggling to make "feet dry" it might get you onto dry land but for your normal flight don't bother.
O and BTW this will be sooooo out the comfort zone of most FI's in the UK.
Stopping the engine then pitching up to sub 50knts to get the prop to stall. They would ****e themselves and it would be so far outside there training experence they wouldn't be comfy at all.
I concur, I;ve done it in a Condor and there isn't 100ft/min in it.
DAR
I would think for most aircraft the slight potential improvement in glide performance is not worth the gyrations required to get the prop to stop. In the event of an actual engine failure I think your energy should be going towards trying to get the engine running again. Once you are sure it will not produce usable power than the important thing is to concentrate on keeping the aircraft under control and managing the flight path so it touches down at the desired spot.
I tried the engine stopped trick in a C172 when I was a young instructor. To get the prop to stop I had to be in stalled condition and so there was an extra loss of altitude during the process, which IMO probably cancelled any gliding efficency gain. I also tried to get the prop to windmill start. As I recall even quite high airspeeds (130 kts + I think) would not budge the prop, so I restarted with the starter.
In any case the older I get the wider the yellow stripe down my back gets. I personally try to avoid deliberately creating emergency situations as in this case failure of the starter would require an actual (and entirely avoidable)force landing. This was not meant to imply you were reckless just that I have made a personal decision not to do something like this.
However for aircraft fitted with a variable pitch prop, going to full coarse results in a very usefull reduction in the gliding rate of descent
I would think for most aircraft the slight potential improvement in glide performance is not worth the gyrations required to get the prop to stop. In the event of an actual engine failure I think your energy should be going towards trying to get the engine running again. Once you are sure it will not produce usable power than the important thing is to concentrate on keeping the aircraft under control and managing the flight path so it touches down at the desired spot.
I tried the engine stopped trick in a C172 when I was a young instructor. To get the prop to stop I had to be in stalled condition and so there was an extra loss of altitude during the process, which IMO probably cancelled any gliding efficency gain. I also tried to get the prop to windmill start. As I recall even quite high airspeeds (130 kts + I think) would not budge the prop, so I restarted with the starter.
In any case the older I get the wider the yellow stripe down my back gets. I personally try to avoid deliberately creating emergency situations as in this case failure of the starter would require an actual (and entirely avoidable)force landing. This was not meant to imply you were reckless just that I have made a personal decision not to do something like this.
However for aircraft fitted with a variable pitch prop, going to full coarse results in a very usefull reduction in the gliding rate of descent
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,598
Likes: 0
Received 0 Likes
on
0 Posts
I have not compared the VSIs but I concur that stopping a fixed pitch prop requires a speed very close to, or in my case (R2160) even below Vs. Getting it to windmill again required around 140 knots.
In an emergency I would not consider stopping the prop, partly because of the high rate of descent at Vs, or the maneuvering required to get below Vs (basically a ballistic flightpath) and the mental energy that takes. Much better to spend that time and energy finding a suitable landing spot.
But a second consideration is that a windmilling engine *might* spontaneously restart itself. (For instance, just suppose there's a slug of water in the lines or maybe vapour lock. With the engine running it might eventually be sucked through.) But a stopped engine definitely will not.
In an emergency I would not consider stopping the prop, partly because of the high rate of descent at Vs, or the maneuvering required to get below Vs (basically a ballistic flightpath) and the mental energy that takes. Much better to spend that time and energy finding a suitable landing spot.
But a second consideration is that a windmilling engine *might* spontaneously restart itself. (For instance, just suppose there's a slug of water in the lines or maybe vapour lock. With the engine running it might eventually be sucked through.) But a stopped engine definitely will not.
Join Date: May 2001
Posts: 10,815
Likes: 0
Received 0 Likes
on
0 Posts
Or the lump of ice in the carb melts.
Its good for ****s and giggles when you are near your home runway as BPF says. But now I have done it once or twice I really can't see the point of doing it again.
There is no point putting it in FI training.
If the FI has an empty aircraft and wants to do it in the overhead again I don't have an issue with that. I don't actually mind them doing it with a student if they do it sensible. I wouldn't these days though.
My first thought though if I heard that an instructor had done it would be. There is someone that is confident with thier own abilitys.
My next question would be where did they do it.
If it was somewhere stupid they would be defined as a fud.
If is was sensible I would find out if they pattered it. If they didn't they would be an hour building fud.
If they ticked all the boxs it wouldn't be mentioned.
Its good for ****s and giggles when you are near your home runway as BPF says. But now I have done it once or twice I really can't see the point of doing it again.
There is no point putting it in FI training.
If the FI has an empty aircraft and wants to do it in the overhead again I don't have an issue with that. I don't actually mind them doing it with a student if they do it sensible. I wouldn't these days though.
My first thought though if I heard that an instructor had done it would be. There is someone that is confident with thier own abilitys.
My next question would be where did they do it.
If it was somewhere stupid they would be defined as a fud.
If is was sensible I would find out if they pattered it. If they didn't they would be an hour building fud.
If they ticked all the boxs it wouldn't be mentioned.
But a second consideration is that a windmilling engine *might* spontaneously restart itself. (For instance, just suppose there's a slug of water in the lines or maybe vapour lock. With the engine running it might eventually be sucked through.) But a stopped engine definitely will not.
As an aside I was thoughly rubbished in the instructor forum for getting my students to verbalize the EFATO actions while the aircaft is still on the runway, one of which is to retard the throttle to idle. I know of one aircraft that was destroyed after the pilot failed to do exactly that. The aircraft was taking off on a relatively short strip when the engine failed at about 40 knots. The pilot stomped on the brakes and just as the aircraft was starting to slow the engine roared back to life at full throttle, the pilot then released the brakes only to see the engine die again. By this time there was not enough room left to stop and the aircraft broke its back when it tumbled into a ditch at the end of the runway.
Originally Posted by Big Pistons Forever
As an aside I was thoughly rubbished in the instructor forum for getting my students to verbalize the EFATO actions while the aircaft is still on the runway, one of which is to retard the throttle to idle. I know of one aircraft that was destroyed after the pilot failed to do exactly that. The aircraft was taking off on a relatively short strip when the engine failed at about 40 knots. The pilot stomped on the brakes and just as the aircraft was starting to slow the engine roared back to life at full throttle, the pilot then released the brakes only to see the engine die again. By this time there was not enough room left to stop and the aircraft broke its back when it tumbled into a ditch at the end of the runway.
Pilot DAR,
Thanks for the data. I've filed it away in my brain.
-- IFMU
Thanks for the data. I've filed it away in my brain.
-- IFMU
Moderator
Thread Starter
Well, it's not so much "data", rather my informal observations. I did not gather this "data" with the normal flight test methods expected of me, so it's not really formal, but certainly add it to the collective wisdom any way it helps.
By the way, this exercise was done not so as to propose the purposefull stopping of the prop in flight, but instead, to see what one might expect if the engine seized. That did happen to a friend in his C 180, a long time ago. (force landed on lake safely).
By the way, shut down tests I have done in twins in the last while each showed that a windmill start is very hard to achieve. The DA 42 with MT props and Lycoming IO-360's would not windmill start at any speed up to near Vne. Changing pitch of the stopped prop helped a little, but not much. The Twin Comanche I tested with MT props the other week (would not unfeather)) had to be startered - and it was not nice!
In gliding a Caravan last month, I found the descent rate at flight idle was reduced by close to half by feathering the prop (engine running in flight idle). That perhaps has something to do with the rather "disking" characteristigs of the Caravan in fine pitch.
By the way, this exercise was done not so as to propose the purposefull stopping of the prop in flight, but instead, to see what one might expect if the engine seized. That did happen to a friend in his C 180, a long time ago. (force landed on lake safely).
By the way, shut down tests I have done in twins in the last while each showed that a windmill start is very hard to achieve. The DA 42 with MT props and Lycoming IO-360's would not windmill start at any speed up to near Vne. Changing pitch of the stopped prop helped a little, but not much. The Twin Comanche I tested with MT props the other week (would not unfeather)) had to be startered - and it was not nice!
In gliding a Caravan last month, I found the descent rate at flight idle was reduced by close to half by feathering the prop (engine running in flight idle). That perhaps has something to do with the rather "disking" characteristigs of the Caravan in fine pitch.
Join Date: Jun 2003
Location: EuroGA.org
Posts: 13,787
Likes: 0
Received 0 Likes
on
0 Posts
The traditional explanation is that a windmilling engine is robbing the gliding aircraft of energy, because of the power required to drive the engine against its friction and compression.
If an experiment shows that there is no difference, what is the explanation?
I have never tried stopping my engine (and never will) but I was once with an instructor who did stop the engine. It took a severe pitch-up; almost to a stall, to stop it windmilling. This was a C150. I don't recall how he restarted it.
If an experiment shows that there is no difference, what is the explanation?
I have never tried stopping my engine (and never will) but I was once with an instructor who did stop the engine. It took a severe pitch-up; almost to a stall, to stop it windmilling. This was a C150. I don't recall how he restarted it.
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,598
Likes: 0
Received 0 Likes
on
0 Posts
If an experiment shows that there is no difference, what is the explanation?
Of course there will be differences in the exact energy conversion, but as we've seen here, the difference is negligable in a typical spamcan.
While gliding the C-150 (fixed pitch, 48" climb prop, and standard O-200) at idle, I consistantly obtained a VSI rate of descent of 450 FPM trimmed at 60 MPH.
...
With the mixture leaned, and the prop stopped, I glided - oh what a peaceful sound! The observed descent rate was exactly the same, at 450FPM. Again, I allowed it to settle for more than a minute at 60 MPH, and the result was consistant.
...
With the mixture leaned, and the prop stopped, I glided - oh what a peaceful sound! The observed descent rate was exactly the same, at 450FPM. Again, I allowed it to settle for more than a minute at 60 MPH, and the result was consistant.
1) Prop turning, fuel burning, with throttle set to the idle stop
2) Prop turning, no fuel being burnt
3) Prop stopped, no fuel being burnt
From what you write, it sounds like you're comparing 1 and 3. The engine still produces substantial power at idle, so you're trading off the power of the engine against the work of turning the prop.
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.
Join Date: Aug 2010
Location: Australia
Age: 65
Posts: 144
Likes: 0
Received 0 Likes
on
0 Posts
Sphincter twitch
Many moons ago during my aerobatics training in a Super Decathlon the fan used to stop regularly during prolonged spins. Right or wrong, my bum used to take an extra grab of the seat when the fan stopped even though we were at a very safe height in the aero training area. Old Ian the instructor used to say "Use yer finger. Use yer finger." 
He meant on the starter button but I admit to never being entirely comfortable with no noise upfront in this aircraft with what always seemed a tired battery; and all this post CPL.
He meant on the starter button but I admit to never being entirely comfortable with no noise upfront in this aircraft with what always seemed a tired battery; and all this post CPL.
Join Date: Dec 2006
Location: .
Age: 37
Posts: 649
Likes: 0
Received 0 Likes
on
0 Posts
In gliding a Caravan last month, I found the descent rate at flight idle was reduced by close to half by feathering the prop (engine running in flight idle). That perhaps has something to do with the rather "disking" characteristigs of the Caravan in fine pitch.
Smithy
SF 25 or 28 motor gliders, choice of fine, coarse, or feather, and with a prop brake. Not a serious difference windmilling, mag off, in fine or coarse, but with the prop stopped the glide angle is much better. It is actually visible without needing to measure height loss against time. Feathered is obviously better again! We usually only leave it in flight idle long enough to cool the engine before shut down, and normal landings are as a glider, prop stopped, feathered and horizontal. I have to add that the coarse pitch setting is actually quite acceptable for a take-off on a reasonable runway, so not actually that coarse a pitch. One can stop the prop without using the prop brake, but it takes a while at just above the stall.
Join Date: May 2001
Posts: 10,815
Likes: 0
Received 0 Likes
on
0 Posts
The simulated feathure for my TP type is 10% torque which equates to 165shp.
If you take the torque below 10% you are using the prop as a 5 meter disk air brake. You can nearly double the VS using it depending what the flight idle is. Its not unkown to be FL100 on the beginning of the downwind and make a normal circuit and still hit the stabilised approach gate at 500ft at a sea level airfield. One of the joys of having pressurization and not worrying about engine cooling.
There is quite a good article out there by a BAe test pilot for the twin drivers about when to nurse an engine running ruff and when to shut it down. The jist of it is that if the engine is not meeting the simulated feathured power output you shut it down otherwise you won't have enough rudder authority at low speeds.
If you take the torque below 10% you are using the prop as a 5 meter disk air brake. You can nearly double the VS using it depending what the flight idle is. Its not unkown to be FL100 on the beginning of the downwind and make a normal circuit and still hit the stabilised approach gate at 500ft at a sea level airfield. One of the joys of having pressurization and not worrying about engine cooling.
There is quite a good article out there by a BAe test pilot for the twin drivers about when to nurse an engine running ruff and when to shut it down. The jist of it is that if the engine is not meeting the simulated feathured power output you shut it down otherwise you won't have enough rudder authority at low speeds.
Join Date: Sep 2010
Location: Ontario Canada Gulf Coast USA
Posts: 46
Likes: 0
Received 0 Likes
on
0 Posts
Pilot DAR,
Thanks for sharing the info. I would have thought an improvement
in glide would have been seen in a stopped prop vs. idle power or windmilling.
I find I can get about 650 fpm glide in a 152 with drooped tips, 3/4 fuel and little or no wind. I had always hoped for somewhat better glide if an actual engine stoppage occured while using a 152.
I realize there are subtle differences between the 150 & 152 but now I will not anticipate/expect much difference in what I am currently seeing in FPM loss at best glide.
Thanks
Thanks for sharing the info. I would have thought an improvement
in glide would have been seen in a stopped prop vs. idle power or windmilling.
I find I can get about 650 fpm glide in a 152 with drooped tips, 3/4 fuel and little or no wind. I had always hoped for somewhat better glide if an actual engine stoppage occured while using a 152.
I realize there are subtle differences between the 150 & 152 but now I will not anticipate/expect much difference in what I am currently seeing in FPM loss at best glide.
Thanks
Avoid imitations
Join Date: Nov 2000
Location: Wandering the FIR and cyberspace often at highly unsociable times
Posts: 14,578
Received 435 Likes
on
229 Posts
Bookworm has the right input. An idling engine is still driving the propellor so any comparison of ROD with that of a stopped prop is meaningless. Try pulling the mixture to cut-off first.
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.
"My" Bulldog (XX623) went through a period when it occasionally stopped its prop in spins (can't remember which way the prop went now, far too long ago, but it occurred when the airframe tended to catch up with the prop in roll).
It would then need the starter button unless I pulled harder than for a normal "recovery from the descent", when it would go usually go over compression and restart itself.
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.
"My" Bulldog (XX623) went through a period when it occasionally stopped its prop in spins (can't remember which way the prop went now, far too long ago, but it occurred when the airframe tended to catch up with the prop in roll).
It would then need the starter button unless I pulled harder than for a normal "recovery from the descent", when it would go usually go over compression and restart itself.