engine out glide distance

[JammedStab]

According to the Slingsby POH.....

"It must be remembered that there is a significant difference in the rate of descent between engine idling and engine windmilling."

Have to admit, I have not read that before. Is there really a significant difference? Perhaps some instructors may know.

[B2N2]

If this wasn't covered during your training that's a little depressing.

At idle the engine is still running and developing some power.
Windmilling the airflow keeps the propeller rotating. This means rotating the crankshaft and moving the pistons up and down against the compression stroke.
That takes a lot of energy.

A constant speed prop does better if you pull it all the way back ( coarse) during a glide. You actually feel the airplane accelerating when you do this. Difference is a couple of hundred feet per minute in your rate of descent.

A car with 140hp engine doesn't need 140hp to drive 60mph.
Actually more like 40hp as the car is already in motion and doesn't need to be brought into motion.
Now consider a propeller.
On the ground when you do your preflight it takes quite some effort to turn the propeller even one revolution.
No imagine 1200-1400 rpm windmilling and how much energy that would take.

Technically not 100% correct comparison. Remember the car example? The propeller is already in motion so it does require energy to continue to turn but not as much to get it to turn? Make sense?
Been a while since I've done this but I used to turn off the fuel in C152/C172's as a demonstration.
Engine isn't running but windmilling.
Slow down to just about stallspeed is required to get the propeller to stop.
But the decrease in rate of descent and the increase in glide range is pretty impressive.
Now to get the propeller to turn again you need a lot of energy despite the fact it kept windmilling till about 55-60kts.
You'll need to dive ( in a C172) to about 100-110 kits to get the propeller to turn again before slowing down to best glide speed again.

Now a mechanical failure can lead to engine stoppage as in the propeller stops turning also as fuel or ignition issues keep the engine windmilling.
Especially if you start at higher altitudes it may be a good idea to slow down to get the propeller to stop turning and therefore the engine to increase your glide range.
At low altitude you're better off concentrating on where to put it down.

[B2N2]

On a single engine ( with constant speed prop )6 aircraft loss of oil pressure will cause the prop to move to fine pitch in which case you could slow down till windmilling stops.

[DaveW]

At idle the engine is still running and developing some power.
Windmilling the airflow keeps the propeller rotating. This means rotating the crankshaft and moving the pistons up and down against the compression stroke.
That takes a lot of energy.

That's not what causes the additional drag.

If you draw out the forces for an undriven (windmilling) propeller, you can see that the aerodynamic force that for a driven propeller would be thrust now acts aft of the propeller disc due to the different AoA. This presents as additional drag.

http://www.pilotfriend.com/training/flight_training/fxd_wing/images/56.jpg


The angle of attack of a fixed pitch propeller, and thus its thrust, depends on the forward speed of the aircraft and the rotational velocity. Following a non catastrophic engine failure the pilot tends to lower the nose so that forward airspeed is maintained while at the same time the rotational velocity of the engine/propeller is winding down. As the forward velocity remains more or less unchanged while the rotational velocity is decreasing the angle of attack must be continually decreasing and at some particular rpm the angle of attack will become negative to the point where the lift component becomes negative ([reverses) and the propeller autorotates, driving the engine. This acts as greatly increased aerodynamic drag which seriously affects the aircraft's L/D ratio and thus glide angles. The drag (including the negative lift) is much greater than that of a stationary propeller, also the engine rotation may cause additional mechanical problems if oil supply is affected.

If the forward speed is increased windmilling will increase, if forward speed is decreased windmilling will decrease, thus the windmilling might be stopped by temporarily reducing airspeed, probably to near stall, so that the negative lift is decreased to the point where internal engine friction will stop rotation. This is not something which should be attempted without ample height.

In the diagram the upper figure shows the forces associated with a section of a propeller blade operating normally. The lower figure shows the forces and the negative angle of attack (aoa) associated with the propeller now windmilling at the same forward velocity.
A variable pitch propeller may have a feathering facility which turns the blades to the minimum drag position (i.e. the blades are more or less aligned fore and aft) and halts windmilling when the engine is stopped.

(Source) (http://www.pilotfriend.com/training/flight_training/fxd_wing/props.htm)

Edit: Looking at this thread again, B2N2 will rightly point out that his quote I chose above relates to an engine at idle, not to engine out and a truly windmilling prop. I'll leave this post here, though, since it's not always a point well understood.

[rusty sparrow]

My instructor often used to pull the mixture out to cut off (Cessna 150) for PFLs - dead prop landings were the norm. One he pulled and the cable came out with the control! That, and gliding, were good practice for the real thing.

[noflynomore]

Is "flying a fail course" the same as failing a flying course?
Probably ought to be...

[bookworm]

There's a NACA paper (http://naca.central.cranfield.ac.uk/reports/1934/naca-report-464.pdf) on propeller drag which compares (table on the penultimate page) the drag of:

1. A stopped, fully fine prop
2. A stopped feathered prop
3. A "free-wheeling" fine prop
4. A fine prop driving a dead engine
5. A fine prop in front of a running engine at idle

At high speeds (just 100 mph for the large prop they tested), the feathered prop does best by a huge margin. There is little difference between 1, 4 and 5, and the freewheeling prop 3 offers about half the drag of those if you can't feather, though it is of course not an option in most aircraft.

At lower speeds, the idling engine produces some thrust, but otherwise there isn't much difference from the higher speed case.

[horizon flyer]

I think B2N2 is a little wrong about turning the prop over against compression causes highest drag.

It takes more energy to turn over with a closed throttle, as pulling a vacuum, with it fully open the pistons bounce on the cylinder pressure, which acts like a spring and recovers the compression energy. So fully open and full course is the lowest drag for a spinning prop. Of course full coarse for a stopped prop if achievable closest to feathered.

[piperboy84]

I think B2N2 is a little wrong about turning the prop over against compression causes highest drag.

It takes more energy to turn over with a closed throttle, as pulling a vacuum, with it fully open the pistons bounce on the cylinder pressure, which acts like a spring and recovers the compression energy. So fully open and full course is the lowest drag for a spinning prop. Of course full coarse for a stopped prop if achievable closest to feathered.

But does a windmilling prop provide enough oil pressure and rpm to keep the prop coarse on a fail to fine csp?

[Gertrude the Wombat]

So, is the practical outcome I'm getting from this for a fixed pitch prop that the throttle should be fully open to get a better glide? - don't recall that ever being taught or tested.

[JammedStab]

If this wasn't covered during your training that's a little depressing.

At idle the engine is still running and developing some power.
Windmilling the airflow keeps the propeller rotating. This means rotating the crankshaft and moving the pistons up and down against the compression stroke.
That takes a lot of energy.

A constant speed prop does better if you pull it all the way back ( coarse) during a glide. You actually feel the airplane accelerating when you do this. Difference is a couple of hundred feet per minute in your rate of descent.


In all my various checkouts in different piston engine aircraft, no instructor or material has mentioned that there is a significant difference in glide distance until reading the Slingsby POH.

That being said, there appears to be a difference in opinion on the reasons why between yourself and Dave W. Which is correct?
Then again, according to Bookworm's post, based on a NACA study, there is little difference.

[B2N2]

I think B2N2 is a little wrong about turning the prop over against compression causes highest drag.

It takes more energy to turn over with a closed throttle, as pulling a vacuum, with it fully open the pistons bounce on the cylinder pressure, which acts like a spring and recovers the compression energy. So fully open and full course is the lowest drag for a spinning prop. Of course full coarse for a stopped prop if achievable closest to feathered.

What is this black magic that thou speakest of?

With a four stroke engine there is clearly a compression stroke in the cycle.

[DaveW]

That being said, there appears to be a difference in opinion on the reasons why between yourself and Dave W. Which is correct?

I don't think there is a difference of opinion, hence my edit.

B2N2 was talking about an engine at idle, whilst I (initially) mis-read the comment as being about a failed engine, which was the situation I then went on to describe.

The NACA study that bookworm links to appears really interesting as a comparison between theory and what often happens in practice.

[horizon flyer]

B2N2 Just the laws of physics. The exhaust stroke empties the cylinder of gas the induction stroke increases the volume in the cylinder to be filled by air pressure if the throttle is closed very little incoming air so the bottom of the piston has to push against air pressure as it pulls a vacuum in the cylinder. If the throttle is wide open then the pressure across the piston is almost equal on induction and just the pumping losses in physically moving the air. On an already spinning engine the compression force is supplied by the flywheel and any other piston on its firing stroke in a running engine, note a 4 cylinder 4 stroke has only 2 firing strokes per rev and the prop drives the engine 30% of the time that's why 6 cyl are better. When the piston gets to the top of stroke any stored gas pressure will push the piston down so recovering most of the compression energy, so less pumping losses with the throttle wide open, thats one reason why the best altitude to cruise at is at throttle wide open at cruise power.

This can be confirmed on old cars with key ignition and push button start. Both feet on the floor key off no choke spin the engine, note turnover speed, gently press the accelerator to the floor spin the engine on starter motor once over the first compression it will spin faster due to lower drag. Warn wait before starting as exhaust can be full of fuel and explode if not given a few minutes. I have used this method to move a car with a stalled engine, just put it in first gear floor accelerator slowly turn over with starter and it will move good for 50 feet to get out of danger.

Note in physics there is no such thing as suck.

So no black magic just an understanding of physics.

[horizon flyer]

On a fixed pitch prop the physics would point to wide open throttle for least drag.

But the mixture must be full back, no avgas being pumped into the a hot exhaust and ignition off. I think instructors teach closed throttle just in case the engine has a sudden spurt which could affect your aiming point or tempt you to climb out then fail again. Once you are committed you are committed.

The other interesting thought is an air start from a stopped prop. Fully open or closed ?
Open may take more power to get over the first compression stroke but spin faster once spinning. full closed may take less power but spin slower so they may be nothing in it. The best position could be half throttle a balance between the 2 settings would need an air test to find out.

[dirkdj]

There is an excellent little book 'Engine Out Survival Tactics' by Nate Jaros; you can download it for Kindle; He is an F16 instructor pilot and Bonanza owner;

[mary meagher]

In Texas I was trying to qualify to fly a grob motor glider. I was very current in gliders, and reasonably current in spam cans. But during my lesson the instructor asked me to stop the engine, and then start it again BY DIVING the aircraft! So my engine has stopped at an inconvenient height and I have to DIVE to get it going????? this went against all my instincts. Height is money in the bank for a glider pilot. One does not throw it away, but use it to choose a suitable field in which to park the glider.

So I never did qualify to fly the motor glider in the US, and to tell the truth, never wanted to fly one at home in the UK either. Most early motor gliders seem pretty feeble beasts compared to a 150 Supercub.....which has, even when towing up a glider, plenty of power. ( And if the engine fails the field chooses you!)

[India Four Two]

Mary,

Back in the dark ages, I spent a couple of years instructing on a Motorfalke, which had a modified VW engine. It had an electric starter which was utterly reliable. However, to cater for the failure case, we were required by TC to teach "air starts", which involved diving quite steeply, as you described, in order to accelerate to a speed where the prop will turn over - about 90 kts I seem to recall. It was quite disconcerting for pilots who hadn't done it before.