Diesel engine and Prop.(Long)

Guest Discussion
#1

I posted a reply to an article on May 1st and an interesting point was brought up that left me with a question that hopefully someone can explain.

Is there an airspeed at which a rotating prop produces less drag than a stationary prop?

Explain the relationship if you would between airspeed and drag. I know a prop is a rotating wing. I know that as airspeed increase the drag increases by a larger factor. But I do not understand the relationship between a rotating wing, airspeed, and drag. So someone with a PhD. in astro, quantum, Micro, or any other prefix you can apply to the word aeronautics, help a mechanical based engineer understand. (Use small words if you feel it necessary).

This question does not apply to Multi-engine configs…only to SRxx’s or the alike.

FWIW Mr. Woor brought up a good point with the diesel engine configuration. Just how big is the battery going to have to be to start this thing?

Bob #509

#2

Is there an airspeed at which a rotating prop produces less drag than a stationary prop?

I may be wrong here, but I’ve always thought of this rather simplistically: When the engine is “dead”, a rotating prop has to do work to move the pistons through compression. Since the airflow is providing the energy to do that work, that becomes a major portion of the resulting drag.

The same prop, when stationary, is doing no work at all, and so needs to rob the airflow of much less energy.

I’m sure there’s a more complex explanation out there - perhaps a more correct one - but that’s the way I remember it.

  • Mike.
#3

Is there an airspeed at which a rotating prop produces less drag than a stationary prop?

My understanding from the basic ground school material:

A stationary prop will cause parasite drag as its cross-section is presented to the airflow.

A spinning prop will consume energy to drive the engine against its internal frictions. This friction (and associated drag) increases in a steepening curve with RPM and in any case is much higher than the drag of a stationary prop.

You can expect to glide as much as 20% farther if the prop is stopped. Maybe more with a diesel having higher compression ratio. The advantage of the diesel is that you wouldn’t have to do any slow flight heroics to stop the prop. It should stop at a higher airspeed.

Steve

#4

I posted a reply to an article on May 1st and an interesting point was brought up that left me with a question that hopefully someone can explain.

Is there an airspeed at which a rotating prop produces less drag than a stationary prop?

Explain the relationship if you would between airspeed and drag. I know a prop is a rotating wing. I know that as airspeed increase the drag increases by a larger factor. But I do not understand the relationship between a rotating wing, airspeed, and drag. So someone with a PhD. in astro, quantum, Micro, or any other prefix you can apply to the word aeronautics, help a mechanical based engineer understand. (Use small words if you feel it necessary).

This question does not apply to Multi-engine configs…only to SRxx’s or the alike.

FWIW Mr. Woor brought up a good point with the diesel engine configuration. Just how big is the battery going to have to be to start this thing?

Bob #509

Bob,

I think you answered your own question. No the component of drag for a prop at any given pitch is the same whether the prop is spinning or not.

The only difference is a stationary prop produces no thrust to compensate for the component of drag. So now we come to what I think is the heart of your question–does a windmilling prop produce more or less energy than a locked prop at the same pitch? No–it takes energy for the relative wind to rotate the prop (albiet prop drag remains constant). However, once the prop has momentum the component forces tend to counteract or equalize. But, as the prop windmills we have to add the energy required to rotate the prop and its connected components, thus, we may derive the following simplified expression:

E = T - (do + dp + dr),

where E is available energy, T is thrust, do is other drag, dp is prop drag, and dr the energy required to rotate the prop. Therefore, where T is 0 then E will always be negative, and where dr is 0 then E will be less. Of course this is something of an over simplification.

sps

#5

On a (slightly) related topic, I notice that in this month’s http://www.aopa.org/pilot/features/feature.htmlAOPA Pilot article on the SR22, they make this comment:
If the engine fails, you can pull the throttle back and the prop changes to a coarser pitch to reduce drag.

But yet I don’t remember reading that in the SR20 POH nor hearing it at the Wings Aloft training.

Any ideas whether that really is the case, that moving the throttle to idle does put the prop in a better pitch for best glide?

Thanks

Steve

#6

I think what AOPA meant to say was pulling the propellor control back will result in a higher blade angle (courser pitch) and reduce drag. This is definitely the case. To prove it to yourself, establish a power off glide with the prop at full increase, then pull it to full decrease. The difference is quite pronounced.
As for closing the throttle, that will result in a lower blade angle (finer pitch) as the prop tries to maintain the RPM, and thus more drag.
Joe

On a (slightly) related topic, I notice that in this month’s http://www.aopa.org/pilot/features/feature.htmlAOPA Pilot article on the SR22, they make this comment:

If the engine fails, you can pull the throttle back and the prop changes to a coarser pitch to reduce drag.

But yet I don’t remember reading that in the SR20 POH nor hearing it at the Wings Aloft training.

Any ideas whether that really is the case, that moving the throttle to idle does put the prop in a better pitch for best glide?

Thanks

Steve

#7

If the engine fails, you can pull the throttle back and the prop changes to a coarser pitch to reduce drag.
But yet I don’t remember reading that in the SR20 POH nor hearing it at the Wings Aloft training.
Any ideas whether that really is the case, that moving the throttle to idle does put the prop in a better pitch for best glide?
Thanks
Steve
Steve,
By coincidence, I happened to be looking through some of the Emergency checklists on the ARNAV display yesterday to pass the time on a longish trip. I recalled seeing something about this under “Engine Failure In Flight”. I checked the POH, which has this to say (Section 3 - Emergency Procedures - Page 11):
_* With a seized or failed engine, the distance that the airplane will glide will be more than the distance it would glide with the engine at idle, such as during training.

  • If the propeller is windmilling, some additional glide range may be achieved by moving the Power Lever to idle and increasing airspeed by 5 to 10 knots._
    I have questions about both statments, which hopefully someone more knowledgable than I can answer.
    First, I agree on glide being more with a seized engine than with an idling engine; but a generic failure could include e.g. fuel starvation, in which case, would the glide not be less than with an idling engine?
    Second, “If the propeller is windmilling, some additional glide range may be achieved…” sets off warning bells in my head. What does “may” mean? I suspect it means that it will, provided that the propeller moves to the course pitch position, in which case the advice to increase airspeed a tad is good. But if the prop stays put… then the extra airspeed will shorten the glide, right? If I haven’t gone off the rails yet, then the next question becomes… How does one KNOW if the prop is in course pitch? I believe that for that to happen, oil pressure is required, so one could make an educated guess based on the oil pressure gauge; but is there a way to know?

All this is my own speculation; I’d love to hear this stuff corroborated or refuted by someone who knows.

  • Mike.
#8

If the engine fails, you can pull the throttle back and the prop changes to a coarser pitch to reduce drag.

Thats assuming you still have oil pressure after the engine has failed.

gianni

#9

Of course, I’m not sure how the single-lever system in the Cirri would work in this regard.
Joe

I think what AOPA meant to say was pulling the propellor control back will result in a higher blade angle (courser pitch) and reduce drag. This is definitely the case. To prove it to yourself, establish a power off glide with the prop at full increase, then pull it to full decrease. The difference is quite pronounced.

As for closing the throttle, that will result in a lower blade angle (finer pitch) as the prop tries to maintain the RPM, and thus more drag.

Joe

#10

Of course, I’m not sure how the single-lever system in the Cirri would work in this regard.

Joe

I have suggested (no one liked my idea)
Here is my $.05, in case of an engine failure bring the throttle all the way back, slight right over the notch then further back. (think of it as driving a Mercedez and placing the gear shift on 1 or low (you know the little zig-zag on the gear shift)) By doing this we would disconnect the single lever (MP/Prop) and would provide almost feathered prop for best glide. Not a complecated thing to do but Cirrus did not think would be necessary??? We all have our opinios… Isn’t it great… Have a great Cirrus day.
Woor

I think what AOPA meant to say was pulling the propellor control back will result in a higher blade angle (courser pitch) and reduce drag. This is definitely the case. To prove it to yourself, establish a power off glide with the prop at full increase, then pull it to full decrease. The difference is quite pronounced.

As for closing the throttle, that will result in a lower blade angle (finer pitch) as the prop tries to maintain the RPM, and thus more drag.

Joe