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?

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.

:ok: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 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.

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.

That is why I teach that a forced approach has 2 parts. The first part is after the aircraft is trimmed for the glide and pointed towards a suitable spot the priority is get the engine going again. Once the cause checks have proven fruitless than the second part commences with the shut down check to secure the engine. This will prevent it from restarting at an awkward moment and minimise the posibilty of a post crash fire. At this point the pilot concentrates on flying the aircraft to a safe landing and doesn't touch any engine controlls.

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.

You describe more of an aborted takeoff rather than an EFATO, and I was taught the first thing to do if you abort is pull the throttle. If you are airborne and the engine comes back after quitting you can always go around or decide to land somewhere. Having the engine back even if its not producing full power gives you more options. I am not an instructor but I would question the wisdom of chopping power in a EFATO because that limits your options rather than expanding them.

Pilot DAR,

Thanks for the data. I've filed it away in my brain.

-- IFMU

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.

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.

My non-scientific guess would be that a stopped propellor blade converts all the energy into turbulence. While a moving blade provides a little less full-on face to the airflow and thus generates less turbulence, but at the same time converts some energy into heat due to the internal friction of the engine.

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.

I can't tell from what you write what exactly you're comparing with. There are three possible states:

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.

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." :hmm: 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.

I thought that this was the case with all VP props, in fine pitch there is much more drag. Is this correct?

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.

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.

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

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.

Hey Mad Jock

What are you flying, a B29 superfortress????
(prop diameter 16ft 7inches)

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.

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

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

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

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

True you do have a point. :p

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?

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..... :)

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

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!

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...:ok:

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?

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.

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.

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.

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.

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

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.

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).

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.