Mooney M20J Glide Ratio and Distance

[PT20J]

27 minutes ago, Shadrach said:

unless you’re actually shutting down the engine in the air, the prop is providing very little drag and is in fact likely providing some thrust. It is certainly not robbing energy from the glide. If your are not making book numbers at idle, you’d likely do worse with the prop stopped.

A windmilling prop with an idling engine creates drag. It takes a small amount of throttle -- usually around 12" MAP -- to get zero thrust. This setting is what we use in multi-engine training to simulate a feathered prop at low altitudes (where we might want to bring the engine back on line quickly so that a simulated emergency doesn't become a real one). 

[carusoam]

10 minutes ago, M016576 said:

Indeed- I studied from the best!!!

I have memory challenges, but this conversation was a good memorable one a while back... it is fun having memories like these return. 

Best regards,

-a-

[M016576]

5 hours ago, aviatoreb said:

 

You said the most insightful thing in your previous post.  Best glide is essentially the same as Vx angle of attack.  So knowing where Vx marks on our own individual AoA, no matter what might be the actual speed, we might be able to repeat that in the glide state.  But for accuracy, we are still in the same boat because knowing the true Vx for a given weight, we must either read that out of our POH, or somehow do our own engineering test flights.  In any case, if we trust we are flying Vx then we can just observe whatever our AoA is reading in its own funking "fractions of" deviation from stall AoA units.

Sadly, I did make a mistake when I initially posted that best glide occurs at Vx... and I’ve since corrected my post to say “L/D max.”

in Jets, where your thrust available is relatively constant, Vx does coincide with L/D max... and that coincides with best glide.

 In our prop aircraft- thrust available is higher at lower speeds- and Vx occurs closer to stall.

[Shadrach]

21 minutes ago, PT20J said:

A windmilling prop with an idling engine creates drag. It takes a small amount of throttle -- usually around 12" MAP -- to get zero thrust. This setting is what we use in multi-engine training to simulate a feathered prop at low altitudes (where we might want to bring the engine back on line quickly so that a simulated emergency doesn't become a real one). 

Where did you derive your data?  Lots of people have made the same statement to me over the years. That does not make it correct. An idling engine creates thrust. Any fool can tell that by standing behaving an idling aircraft. In flight, any drag created by the prop on an idling engine is equal to (under certain circumstances) or less than that of the stopped prop (most circumstances).

Edited January 2, 2019 by Shadrach

[Rene]

Wow.  I have to admit that I had no idea I would get so many responses to my post.  I'm really impressed by how active and knowledgeable this forum is, and I have certainly learned a lot.  So thanks everyone for pitching in their thoughts and experiences. 

[Rene]

Hi Shadrach,

I agree that the idling engine makes thrust when airspeed is 0 knots.  But when gliding at 90 knots, I suspect that the idling engine/prop is creating drag.  Less drag than a windmilling prop on a dead engine, but some drag nonetheless.

 

[PT20J]

2 minutes ago, Shadrach said:

Where did you derive your data?  Lots of people have made the same statement to me over the years. That does not make it correct. An idling engine creates thrust. Any fool can tell that by standing behaving an idling aircraft. In flight, any drag created by the prop on an idling engine is equal to or less than that of the stopped prop.

 

8 minutes ago, Shadrach said:

Where did you derive your data?  Lots of people have made the same statement to me over the years. That does not make it correct. An idling engine creates thrust. Any fool can tell that by standing behaving an idling aircraft. In flight, any drag created by the prop on an idling engine is equal to or less than that of the stopped prop.

A propeller attached to an idling engine creates thrust at zero airspeed. As the airspeed increases, the thrust decreases and goes through zero and then becomes negative when the prop starts driving the engine ("windmilling"). You can see this effect clearly with a fixed pitch prop -- as you increase the airspeed in a dive, the rpm increases. With a constant speed prop, the governor changes the blade angle and masks the rpm change. But the prop is still driving the engine at flying airspeeds. 

The Beechcraft Duchess POH states:

ZERO THRUST (Simulated Feather)

Use the following power setting to establish zero thrust.

1. Throttle Lever - SET 8.0 in. Hg MANIFOLD PRESSURE

2. Propeller Lever - RETARD TO FEATHER DETENT

(The feather detent is the highest pitch position before feather)

In the DC-3, we use 15" and 1500 rpm. There's a good discussion of this in: https://www.avweb.com/news/pelican/186778-1.html

Skip

 

[Shadrach]

2 minutes ago, Rene said:

Hi Shadrach,

I agree that the idling engine makes thrust when airspeed is 0 knots.  But when gliding at 90 knots, I suspect that the idling engine/prop is creating drag.  Less drag than a windmilling prop on a dead engine, but some drag nonetheless.

 

Think about it purely as a simple energy equation. 

A Mooney with siezed engine at 10,000ftAGL at a weight of 2740lbs has potential energy of X.

A Mooney with an idling engine at 10,000ft at a weight of 2740lbs using chemical energy to Idle producing mechanical energy of Y.

Those arguing that an idling prop is a drag are suggesting that X is greater than X+Y.

Another way to think of it:

Your prop has three blades that create drag when stopped. How is it that they create more drag when being driven by the engine (three little wings producing lift).

 

[DonMuncy]

Bob Kromer (former president and former production test pilot for Mooney) says they had a torque measuring device installed between the engine and the prop, and found that below some manifold pressure (14 if I'm remembering correctly), the prop would drive the engine, rather than the engine furnishing thrust.

[Shadrach]

20 minutes ago, PT20J said:

 

A propeller attached to an idling engine creates thrust at zero airspeed. As the airspeed increases, the thrust decreases and goes through zero and then becomes negative when the prop starts driving the engine ("windmilling"). You can see this effect clearly with a fixed pitch prop -- as you increase the airspeed in a dive, the rpm increases. With a constant speed prop, the governor changes the blade angle and masks the rpm change. But the prop is still driving the engine at flying airspeeds. 

The Beechcraft Duchess POH states:

ZERO THRUST (Simulated Feather)

Use the following power setting to establish zero thrust.

1. Throttle Lever - SET 8.0 in. Hg MANIFOLD PRESSURE

2. Propeller Lever - RETARD TO FEATHER DETENT

(The feather detent is the highest pitch position before feather)

In the DC-3, we use 15" and 1500 rpm. There's a good discussion of this in: https://www.avweb.com/news/pelican/186778-1.html

Skip

 

See my previous post. There are a lot of half truths in old POHs. Where does the energy from the idling engine go Skip?  If you know John personally, please ask him. I’m a great admirer of his work.

Edited January 2, 2019 by Shadrach

[Rene]

I think we are all agreeing that a Mooney gliding with an idling engine will go farther than a Mooney with a windmilling engine (no power).  The idling prop has less net drag because of the "thrust" component of the idling engine.  Overall, the windmilling configuration has more drag.

[PT20J]

15 minutes ago, Shadrach said:

See my previous post. There are a lot of half truths in old POHs. Where does the energy from the idling engine go Skip? 

The energy goes into turning the prop at 600 rpm or so (ground idle rpm). Windmilling at higher than ground idle rpm requires additional energy and this is the source of the drag. 

[Shadrach]

3 minutes ago, Rene said:

I think we are all agreeing that a Mooney gliding with an idling engine will go farther than a Mooney with a windmilling engine (no power).  The idling prop has less net drag because of the "thrust" component of the idling engine.  Overall, the windmilling configuration has more drag.

Yes,  I should hope we can all agree on that. 

By the way, I been in two airplanes with stopped props. I would not recommend it and I will not discuss in detail on an Internet forum.  Let me just say that one occasion was in the dead of winter. The little continental C90 cold soaked so quickly that the prop would not turn the engine over even at 120ias. That particular aircraft had no electrical system. We made the airport and dead sticked just fine, but there was some pucker for a few minutes

[Shadrach]

32 minutes ago, PT20J said:

The energy goes into turning the prop at 600 rpm or so (ground idle rpm). Windmilling at higher than ground idle rpm requires additional energy and this is the source of the drag. 

So you’re suggesting that I’d see a slight increase in CHTs then?  

Yes, it does require some additional energy to spin the prop faster than idle speed. Where we disagree is the notion that it’s enough energy to offset the energy being added by the idling engine.   

I think where folks are getting hung up is on “0 thrust”. That is not the same condition as gliding with the prop stopped. 

Edited January 2, 2019 by Shadrach

[N201MKTurbo]

You can increase your glide some if your engine quits by aggressively getting to best glide speed. Pull up sharply to get there, you will gain 500 feet or so that would be lost if you just maintained altitude until the plane decelerates to best glide speed. This will minimize the time spent at a speed in excess of best glide.

[PT20J]

39 minutes ago, Shadrach said:

Yes, it does require some additional energy to spin the prop faster than idle speed. Where we disagree is the notion that it’s enough energy to offset the energy being added by the idling engine.   

I think where folks are getting hung up is on “0 thrust”. That is not the same condition as gliding with the prop stopped. 

It takes energy (which creates drag) to windmill the prop on a dead engine, and it takes energy (drag), but slightly less energy, to windmill the prop on an idling engine. That's really all I've been saying and I think we agree on that point. The point about zero thrust was simply to help everyone understand that a prop attached to an idling engine on a gliding airplane produces drag and not thrust.

The more interesting question is whether a stopped prop has more drag than a windmilling prop on an idling engine.  This has been long studied and here is a link to a NACA report from 1933: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091538.pdf

The results were, drag in pounds, at 100 mph airspeed:

Stopped: 94.4

Windmilling, dead engine: 101.0

Windmilling, idling engine: 100

Skip

[JohnB]

13 hours ago, Rene said:

 

The other interesting thing is that glide distance does not depend on weight, as shown on the glide distance chart.  Heavier weights require higher glide speeds, but end up at the same distance.  I always thought that this was a fascinating and very counter intuitive fact.

Next chance I get, I am going to try the glide with the prop control all the way out, to see how much better I can do with my 3 blade prop.

 

That would be a very interesting experiment to repeat with your prop all of the way out, i.e. go to 10kfeet, power idle, airspeed 90 knots, watch stabilized descent rate with or without prop fully back.  I have noted a VERY large difference in glide distance with the prop fully out in mine, which I think is very likely due to the drag created by the prop, particularly prominent in our three bladed prop system. I would be very curious though to find out if your actual descent rate at the same airspeed is reduced by pulling your prop all of the way back. My guess is that it would be, but before I start theorizing on why, if you do it, let me know?

 

JB

 

 

[Rene]

OK Skip, that study about prop drag is really good news because it implies that a windmilling prop with engine idling is "almost" the worst case, so it's good for creating personal estimates for gliding distances.  I didn't read the NACA report, but I am assuming they studied a constant pitch prop?  Has there been a study on the difference in drag between the prop control all the way in and all the way out?

By the way, John B, I plan to do the experiment with prop in and prop out and will let you know the results.

Rene

 

[carusoam]

A few things that come to mind, while slowing in the traffic pattern....

 

What happens...

• Throttle out...
• pushing the prop control in from descent rpm to full in...
• the braking effect is similar to selecting a lower gear / down-shifting in the FireBird.... excellent braking effect...
• You can feel this effort when you pull the prop through the compression stroke by hand. (Keep safety in mind)

Why it happens...

• The compression stroke uses up a lot of energy... compressing all that air in the cylinders...
• Turning the engine at 2700 rpm has a lot of compression strokes...
• two blades or three, are not much different, when the compression is using up so much energy...

Things to consider... some for practice, some for real...

• pull the prop all the way back to minimize the number of energy sapping compression strokes...
• If stopping the prop... pull the prop back to lessen the driving force it has to start turning again...
• In a real engine out situation, the turning engine will be delivering fuel all the way to the exhaust system...
• mixture out or fuel selector off, or both...  when committing to that off-field landing
• Full throttle, no gas (mixture out), would be compressing more air, than closed throttle...

 

It may be important to know your thermodynamics almost as much as your aerodynamics... 

 

PP thoughts only, not a CFI....

Best regards,

-a-

[PT20J]

37 minutes ago, Rene said:

 I didn't read the NACA report, but I am assuming they studied a constant pitch prop?  Has there been a study on the difference in drag between the prop control all the way in and all the way out?

They tested a variable pitch prop at different blade angles. There's more data in the report -- I just pulled out the part that seemed most apropos to the current discussion.

[Shadrach]

I just came across this.  Certainly not the most intellectual discussion of the problem, but three different pilots all performing a stopped vs idle comparison on the same day and deriving data from GPS track.  Don't mind the click-bait title or the banter in the beginning.  6:10 is where the explanation and flight portion of the video begins.

 

[PT20J]

If you look at the 1933 data, you'll notice that the drag difference between prop stopped and engine at idle is pretty small. The researchers also noted that what's behind the prop has an influence. So it is possible that some airplanes may have more drag with the prop stopped and for some the opposite may be true depending on the shape of the fuselage and the specifics of the prop, but the difference will be relatively small. So, to summarize:

1) It doesn't make much difference in glide ratio whether the prop is stopped, the dead engine is turning, or the engine is turning at idle.

2) In all cases, the prop is creating drag, not thrust. 

Skip

[ArtVandelay]

I would think the position the prop stops at would effect drag, and horizontal would be optimal.


Tom

[Shadrach]

3 minutes ago, PT20J said:

If you look at the 1933 data, you'll notice that the drag difference between prop stopped and engine at idle is pretty small. The researchers also noted that what's behind the prop has an influence. So it is possible that some airplanes may have more drag with the prop stopped and for some the opposite may be true depending on the shape of the fuselage and the specifics of the prop, but the difference will be relatively small. So, to summarize:

1) It doesn't make much difference in glide ratio whether the prop is stopped, the dead engine is turning, or the engine is turning at idle.

2) In all cases, the prop is creating drag, not thrust. 

Skip

I did look skim the 1933 data, but I've not had time for in depth analysis.  Lots of variables there. It appeared the point of the study was to find ways the prop and engine could be used to reduce the terminal velocity of an attack aircraft in a dive.  

As to Trent's experiment.  His descent decreased by %13 when the throttle was left at idle as opposed to stopping the prop.  That's nearly 700 ft of forward motion for every 1000ft of descent at the 60mph used. While not a huge delta, it does not strike me as a trivial amount. That's more than a mile if multiplied over 8000ft of descent in no wind. 

[aviatoreb]

15 hours ago, M016576 said:

Sadly, I did make a mistake when I initially posted that best glide occurs at Vx... and I’ve since corrected my post to say “L/D max.”

in Jets, where your thrust available is relatively constant, Vx does coincide with L/D max... and that coincides with best glide.

 In our prop aircraft- thrust available is higher at lower speeds- and Vx occurs closer to stall.

Ah right - thanks for the correction.  

Book says my airplane has Vx at gross at 84 but max glide at gross at 90.  Close but not the same.