Magni M24 Orion May 30, 2023 - Dauphin Island Alabama

[Tyger]

The amount of drag caused by a stopped prop might depend a lot on the aircraft and, in a pusher, where the prop is in relation to the fuselage.

 

[Mayfield]

I would think the magnitude of propeller drag would be least with a stopped feathered propeller, a bit more with a stopped propeller, and greatest with a windmilling propeller. Windmilling, to me, means the engine is being driven by the propeller.

Aerodynamics for Naval Aviators:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

There's an interesting chart on page 149.

On the previous page:
"At smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high RPM can create such a tremendous amount of drag that the airplane may be uncontrollable. The propeller windmilling at high speed in the low range of blade angles can produce an increase in parasite drag which may be as great as the parasite drag of the basic airplane. An indication of this powerful drag is seen by the helicopter in autorotation. The windmilling rotor is capable of producing autorotation rates of descent which approach that of a parachute canopy with the identical disc area loading. Thus, the propeller windmilling at high speed and small blade angle can produce an effective drag coefficient of the disc area which compares with that of a parachute canopy."

 

[PJB]

Interesting insight to the thinking around PFL's and their teachings [practiced forced landings]. Personally the delta in float for prop stopped/windmilling was the least of my worries, especially since the student once taught very seldom flies the same aircraft. The bigger take away's I wanted for my students were know where the wind is, assess and know the chosen landing spot and then assess and manage energy. This pilot did a great job in terms of not being too badly hurt and seems any of the issues he did have were less about the float and more about the chosen landing spot. My comment around the energy state of the aircraft were that it didn't really seem to this old duffer he had come to that attitude from a great height and if he had I'm surprised he didn't choose a better place to put the wheels.

 

[Vance]

There is much controversy on this.
With an adjustable prop, feathering with an engine out decreases drag, hence the practice to feather the prop in multiengine aircraft engine out scenarios.
With a Rotax with fixed pitch propellor, the propellor stops when the engine stops; intuitively to me this should create more drag than if the prop is spinning with some power.
If one would crank up the idle when the airplane is on the ground the airplane would taxi faster, indicating to me the prop is contributing to moving forward, not retarding it. ???

If we were flying I would demonstrate the increased glide distance of a stopped propeller in a gyroplane.

Since we are not flying and we need to overcome your intuitive sense of things I will give a simple explanation a try.

A propeller makes thrust by accelerating the air.

For it to make thrust the air needs to be coming out the back faster than it goes in the front.

My approach speed in The Predator for an engine out landing is 50kts.

If the air coming in the front is going faster than the air coming out the back there is drag in relation to the area of the disk.

At idle the air coming out the back might be going 10kts.

If the propeller is stopped there is just the drag of the individual blades which have much less frontal area than the propeller arc.

There are lots of things actually affecting how it works.

In my experience in The Predator I glide about 20% farther with a stopped propeller than with an idling propeller.

 

[TyroGyro]

Interesting.

You glide further/farther with a stopped prop, Vance?

At POH best glide speed, I presume?

 

[Vance]

Interesting.

You glide further/farther with a stopped prop, Vance?

At POH best glide speed, I presume?

About 20 percent farther at fifty knots indicated air speed with a stopped propeller compared to an idling propeller.

The direct drive Lycoming idles at around 800 rpm.

I geared engine tends to turn the propeller even slower at idle.

 

[Vance]

The Predator has a lot of drag and I suspect in something like a Calidus that has less drag the difference would be greater.

I don't practice engine stopped simulated emergency landings in a Calidus so I don't have any numbers.

I feel it is important for my learners to know the difference.

I have a friend who overshot his landing zone in an American Ranger because he didn't understand this.

 

[Aerofoam]

I would think the magnitude of propeller drag would be least with a stopped feathered propeller, a bit more with a stopped propeller, and greatest with a windmilling propeller. Windmilling, to me, means the engine is being driven by the propeller.

Aerodynamics for Naval Aviators:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80t-80.pdf

There's an interesting chart on page 149.

On the previous page:
"At smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high RPM can create such a tremendous amount of drag that the airplane may be uncontrollable. The propeller windmilling at high speed in the low range of blade angles can produce an increase in parasite drag which may be as great as the parasite drag of the basic airplane. An indication of this powerful drag is seen by the helicopter in autorotation. The windmilling rotor is capable of producing autorotation rates of descent which approach that of a parachute canopy with the identical disc area loading. Thus, the propeller windmilling at high speed and small blade angle can produce an effective drag coefficient of the disc area which compares with that of a parachute canopy."

I actually had a Boeing engineer arguing with me that a stopped prop makes more drag than a windmilling prop.
I said to him:
This is simple, it takes energy to spin a prop, that energy comes from somewhere, in this case it comes from the moving aircraft, so it has to create drag because the airplane is giving up energy by trading airspeed for prop speed since the prop is now doing work.
He still didn't believe me so I put a 10" airplane prop on a screw driver so it could freewheel against the handle and had him hold it out the window while we drove.
The first test was with the prop stopped, it was easy to hold out the window at 60mph.
He could barely hold it against the wind when he let it spin up, it generated about 10lbs of force and he had to use both hands.
On electric UAVs and RC planes, the speed controller is usually programmed to stop the prop when you throttle off, the difference is huge.
An efficient airplane with a 20+ to 1 glide ratio with power off will lose more than half of the glide ratio if the prop is allowed to windmill.
A feathered prop that is slowly spinning will not produce much drag because it isn't doing much work, it doesn't take much energy to turn a feathered prop a few RPMs. I would guess this could be less drag than a stopped fixed pitch prop.

 

[Andino]

I think some here erroneously inferred that I meant a windmilling prop as having less drag than an engine idling prop.

 

[Aerofoam]

I think some here erroneously inferred that I meant a windmilling prop as having less drag than an engine idling prop.

I didn't notice anyone inferring this, but there is a lot of confusion swirling around the topic, pun intended.....

 

[Andino]

I've practiced many gyro engine-idle "deadstick" landings, but the few gyro engine-outs I've had seemed a much shorter glide range. Proper measured comparisons would be interesting. (I wonder if Vg varies per condition?)

I believe that experimenting all possible scenarios with a good CFI over a long runway would be beneficial to the student.

 

[Andino]

The CFI who gave me my checkride related a story about an engine out he had with a 9-series Rotax in a Magni. He said that the actual engine out was a "much different" experience than merely reducing to idle speed (he landed successfully on a sandbar alongside a creek).
I imagine not too many folks actually turn the engine off to simulate an engine out – for obvious reasons.

In what way did he say it was "much different?"

 

[Andino]

Interesting.

You glide further/farther with a stopped prop, Vance?

At POH best glide speed, I presume?

"farther" regards distance. "further" regards degree.

I guess a theoretical issue is at what rpm does a prop's thrust exceed its (non-feathered) drag. It certainly must vary widely per prop.

 

[TyroGyro]

"farther" regards distance. "further" regards degree.

No doubt Tyger can adjudicate...

 

[Andino]

No doubt Tyger can adjudicate...

Meanwhile, have fun:

“Farther” vs. “Further”—What’s the Difference?

People use both further and farther to mean “more distant.” However, American English…

www.grammarly.com

Is it 'further' or 'farther'?

Helping you navigate the linguistic road ahead

www.merriam-webster.com

 

[TyroGyro]

Meanwhile, have fun:

“Farther” vs. “Further”—What’s the Difference?

People use both further and farther to mean “more distant.” However, American English…

www.grammarly.com

Is it 'further' or 'farther'?

Helping you navigate the linguistic road ahead

www.merriam-webster.com

“England and America are two countries separated by the same language!” - George Bernard Shaw

I speak the King's English, although I suspect the Irish speak it better than either of us...

 

[WaspAir]

There is no such thing as the Queen’s English. The property has gone into the hands of a joint stock company and we own the bulk of the shares.

(Attributed to Mark Twain)

These days, there are probably more English speakers in India and Pakistan than everywhere else combined.

 

[TyroGyro]

There is no such thing as the Queen’s English. The property has gone into the hands of a joint stock company and we own the bulk of the shares.

Highly recommended for all who are (still) learning English...

The Complete Plain Words - Wikipedia

en.wikipedia.org

Amazon Sign In

Among many gems, it recounts the tale of the Scotsman who fell into The Serpentine, Hyde Park in London, and could not understand why no-one came to his rescue, when he cried "I will drown and nobody shall save me !"

Now, that's deep...

 

[Sv.grainne]

Funny: I'm hearing impaired and for years I ve had a problem understanding Pakistanis, Indians, others speaking English. I think my main issue has been with the speed in which some speak . I've had to appologize repeatedly when this happens, "I'm sorry but I just can't understand you"

This started in Saudi Arabia after I was in a bombing, never got any better even with hearing aids. At the time I had a crew of great Paki outside plant guys, they were great and I was always struggling to understand.

Funny, but they all called me "meester Bob"!

 

[WaspAir]

Among many gems, it recounts the tale of the Scotsman who fell into The Serpentine, Hyde Park in London, and could not understand why no-one came to his rescue, when he cried "I will drown and nobody shall save me !"

Here's a lesson on that same theme (wait for the punch line):