Mooney M20J Glide Ratio and Distance

[Marauder]

You bastard!  You had me going there!

Just imagine what it would be like to be subjected to “Jerry Jokes” for 20 or so hours in a cockpit. I’ve done it and lived to talk afterwards. He is one big green font jokester.


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[flyer338]

I am new to this forum and it has been a great place to relearn the things I used to know, and has helped greatly with informing me about the idiosyncrasies of different Mooney models during my search for another Mooney after a eight year hiatus from exercising my pilot certificate. 

My previous Mooney was a1965 C model. I modified it by adding nearly all of the available mods for reducing drag. It went from a cruise speed at 7500 feet (wot and 2350 rpm) of 140 knots to 152 knots, as verified by a 4-way gps box.

i wanted to find out what the best glide speed was with the cleaner air frame. I decided to conduct a test and find out. I posted the results on the Mooney list at aviating.com, but it seems that list no longer exists. What I post here relates to glide ratio and my attempt to stop the prop. I cannot find my original notes so I cannot post my glide ratio and best glide speed result. 

I conducted the test starting at 12500. (Note this was about 5 miles from an airport with a 4000 ft. runway.) I reduced power to idle, mixture to idle cut-off, and ignition off. Then with the Propeller full back, throttle wide open (for maximum cylinder pressure), I reduced my airspeed until I had a stall buffet (flaps and gear both up). I could not get the propeller to stop (mid-time engine with high 70s compression). 

Then I conducted glide ratio tests at different air speeds in 5mph increments with the propeller back and throttle closed. I started the engine again at 5000 and landed normally. I recall the glide ratio as better than book and at a slower air speed. 

The engine was off for about eight minutes. I would not do this today, but I did learn some things I considered valuable at the time.

[Shadrach]

2 hours ago, flyer898 said:

I am new to this forum and it has been a great place to relearn the things I used to know, and has helped greatly with informing me about the idiosyncrasies of different Mooney models during my search for another Mooney after a eight year hiatus from exercising my pilot certificate. 

My previous Mooney was a1965 C model. I modified it by adding nearly all of the available mods for reducing drag. It went from a cruise speed at 7500 feet (wot and 2350 rpm) of 140 knots to 152 knots, as verified by a 4-way gps box.

i wanted to find out what the best glide speed was with the cleaner air frame. I decided to conduct a test and find out. I posted the results on the Mooney list at aviating.com, but it seems that list no longer exists. What I post here relates to glide ratio and my attempt to stop the prop. I cannot find my original notes so I cannot post my glide ratio and best glide speed result. 

I conducted the test starting at 12500. (Note this was about 5 miles from an airport with a 4000 ft. runway.) I reduced power to idle, mixture to idle cut-off, and ignition off. Then with the Propeller full back, throttle wide open (for maximum cylinder pressure), I reduced my airspeed until I had a stall buffet (flaps and gear both up). I could not get the propeller to stop (mid-time engine with high 70s compression). 

Then I conducted glide ratio tests at different air speeds in 5mph increments with the propeller back and throttle closed. I started the engine again at 5000 and landed normally. I recall the glide ratio as better than book and at a slower air speed. 

The engine was off for about eight minutes. I would not do this today, but I did learn some things I considered valuable at the time.

Interesting read! Thank you. I think your “throttle open/maximum cylinder pressure” theory is what prevented you from stopping the prop. In the absence of fuel, an engine is an air pump being driven by the prop. The word throttle literally means to “choke”. The slight increase in the cylinders volumetric efficiency from opening the throttle pales in comparison to the work required to draw air through a “throttled” intake. Too bad you did not try pulling all three knobs back. I think your chances of success would have been much greater. Thanks again for the interesting post.

Edited July 24, 2019 by Shadrach

[PT20J]

Best glide occurs at the angle of attack for L/D max. In unaccelerated flight, L = weight, so best glide occurs at minimum drag which is the point where induced drag = parasite drag. If you reduce the parasite drag, the minimum drag speed should increase.

Also, since lift = weight and the angle of attack for best L/D is a constant, the airspeed must increase as the square root of the weight increase to maintain constant lift. Thus, the best glide speed increases with weight.

We hashed out prop stopped vs. prop not stopped a while back. The results from an old NACA study and some Youtube flight test videos showed that prop driving engine at flat pitch has most drag, prop stopped almost as bad (due to flat plate area of stopped blades), prop in high pitch better and, of course, feathered is best.

Skip

[jaylw314]

19 minutes ago, PT20J said:

Best glide occurs at the angle of attack for L/D max. In unaccelerated flight, L = weight, so best glide occurs at minimum drag which is the point where induced drag = parasite drag. If you reduce the parasite drag, the minimum drag speed should increase.

Also, since lift = weight and the angle of attack for best L/D is a constant, the airspeed must increase as the square root of the weight increase to maintain constant lift. Thus, the best glide speed increases with weight.

We hashed out prop stopped vs. prop not stopped a while back. The results from an old NACA study and some Youtube flight test videos showed that prop driving engine at flat pitch has most drag, prop stopped almost as bad (due to flat plate area of stopped blades), prop in high pitch better and, of course, feathered is best.

Skip

Does max L/D coincide with minimum angle of descent (best glide) or minimum rate of descent?  I imagine it would be the latter, right?  I imagine the best glide speed is a little higher (or the AOA is a little lower) than max L/D...

IIRC, max L/D at full power coincides with best rate of climb, not best angle of climb...

Edited July 24, 2019 by jaylw314

[PT20J]

1 hour ago, jaylw314 said:

Does max L/D coincide with minimum angle of descent (best glide) or minimum rate of descent?  I imagine it would be the latter, right?  I imagine the best glide speed is a little higher (or the AOA is a little lower) than max L/D...

IIRC, max L/D at full power coincides with best rate of climb, not best angle of climb...

Max L/D speed is the speed for minimum descent angle, (i.e., best glide).

Minimum sink occurs at C_L^3/2/C_D max. which works out to about 3/4 of best glide speed.

Best ROC occurs where there is the greatest spread between thrust power available and power required. 

Best angle of climb occurs at the speed where there is the greatest spread between thrust available and thrust required.

So, best climb speeds are dependent on aerodynamics as well as engine power and propeller efficiency.

Good references for aircraft performance:

Dole, Charles E., Flight Theory for Pilots

Anderson, John D., Aircraft Performance and Design

Skip

[flyer338]

4 hours ago, Shadrach said:

Interesting read! Thank you. I think your “throttle open/maximum cylinder pressure” theory is what prevented you from stopping the prop. In the absence of fuel, an engine is an air pump being driven by the prop. The word throttle literally means to “choke”. The slight increase in the cylinders volumetric efficiency from opening the throttle pales in comparison to the work required to draw air through a “throttled” intake. Too bad you did not try pulling all three knobs back. I think your chances of success would have been much greater. Thanks again for the interesting post.

The empirical evidence suggests otherwise: when I opened the throttle with the propeller in max pitch the propeller rpm slowed, and slowed further as I reduced air speed, until it was barely turning over. My explanation is that with the throttle open the cylinders were taking in and compressing more air, and therefore, each piston had greater resistance because it was doing more work. 

[jaylw314]

1 hour ago, flyer898 said:

The empirical evidence suggests otherwise: when I opened the throttle with the propeller in max pitch the propeller rpm slowed, and slowed further as I reduced air speed, until it was barely turning over. My explanation is that with the throttle open the cylinders were taking in and compressing more air, and therefore, each piston had greater resistance because it was doing more work. 

I suspect that is partly accurate--without fuel, the piston is not compressing air--it's squeezing it and letting it rebound, so in theory the "exhaust" pressure is pretty much the same as the intake pressure, so no work is done.

In real life, of course, there's some adiabatic heating and cooling since air is not an ideal gas.  I'm not sure if that would account for the difference, though, because I'm not a physicist in real life  

[Shadrach]

On 7/24/2019 at 4:22 PM, flyer898 said:

The empirical evidence suggests otherwise: when I opened the throttle with the propeller in max pitch the propeller rpm slowed, and slowed further as I reduced air speed, until it was barely turning over. My explanation is that with the throttle open the cylinders were taking in and compressing more air, and therefore, each piston had greater resistance because it was doing more work. 

 

On 7/24/2019 at 5:34 PM, jaylw314 said:

I suspect that is partly accurate--without fuel, the piston is not compressing air--it's squeezing it and letting it rebound, so in theory the "exhaust" pressure is pretty much the same as the intake pressure, so no work is done.

In real life, of course, there's some adiabatic heating and cooling since air is not an ideal gas.  I'm not sure if that would account for the difference, though, because I'm not a physicist in real life  

 

If this were true, the Jake Brake needn't ever been invented...

The name is derived from the manufacturer, Jacobs (of drill chuck fame), and was patented 1962–1965 by Clessie Cummins.^[1] When the driver releases the accelerator on a moving vehicle powered by an internal combustion engine, the vehicle's forward momentum continues to turn the engine's crankshaft. Most diesels by design do not have a throttle, so regardless of throttle setting a full charge of air is always drawn into the cylinders (excluding the valve fitted to certain diesels, such as fire appliances and generators on oil and gas platforms, to prevent diesel engine runaway). Compressed air generated during the compression stroke acts as an air spring to push back against the rotating piston. As such, even with fuel supply cut off and no power strokes taking place, virtually 100% of the energy absorbed by the compression stroke within each cylinder is returned to the crankshaft. This results in very little engine braking being applied to the vehicle.“

Edited November 11, 2019 by Shadrach

[PT20J]

1 hour ago, Shadrach said:

 

 

If this were true, the Jake Brake needn't ever been invented...

The name is derived from the manufacturer, Jacobs (of drill chuck fame), and was patented 1962–1965 by Clessie Cummins.^[1] When the driver releases the accelerator on a moving vehicle powered by an internal combustion engine, the vehicle's forward momentumcontinues to turn the engine's crankshaft. Most diesels by design do not have a throttle, so regardless of throttle setting a full charge of air is always drawn into the cylinders (excluding the valve fitted to certain diesels, such as fire appliances and generators on oil and gas platforms, to prevent diesel engine runaway). Compressed air generated during the compression stroke acts as an air spring to push back against the rotating piston. As such, even with fuel supply cut off and no power strokes taking place, virtually 100% of the energy absorbed by the compression stroke within each cylinder is returned to the crankshaft. This results in very little engine braking being applied to the vehicle.“

Interesting. Hadn't thought of that.

[carusoam]

Air enters and air exits... but let’s not say work isn’t being performed...

Each piston is forced over a distance... defining the work being done FxD...

Plenty of heat is generated during the compression of air...

Expect some pretty heavy thermodynamics to be involved...

When pressing the prop control all the way in... a braking feeling is pretty easy to detect... drop the firebird into second gear prior to entering a corner at 35mph... Same braking effect...

 

When really simplified... the same weight of fuel (gas and oxygen) enters the engine equals the weight of exhaust that leaves the engine...... so no conversion takes place...???

 

PP thoughts on deep mechanical engineering topics... beware of over simplification... the valves actually allow things to happen that are a challenge to account for in conversation... one of those valves is the throttle plate in our gas engine....

so... Also consider what happens when that throttle plate closes and the cylinder tries to draw air in... is it working against a vacuum...? 

Speaking with Chuck N this week reminded me of the work he did measuring glide distances...in his Mooney... @cnoe

Best regards,

-a-

[Hank]

23 hours ago, carusoam said:

Air enters and air exits... but let’s not say work isn’t being performed...

Each piston is forced over a distance... defining the work being done FxD...

. . . .

Speaking with Chuck N this week reminded me of the work he did measuring glide distances...in his Mooney... 

Best regards,

-a-

The first part is correct, a windmilling engine is doing work, it's just not burning fuel--the driving energy is airflow through the propellor. There is drag associated with this, just like when you stick your hand out your car window. Frictional losses from the crankshaft rotating, pistons going up and down and valves opening and closing should be minor, and is often ignored in calculations. The drag varies A LOT with propellor angle (flat angle [hjgh RPM] is high drag), easily verified and felt during simulated engine out work, but I've never shut down my engine in flight (on purpose!) to test the effect of throttle position. But I would expect it to be much, much less than what can be done using fhe prop lever,  if it can be felt at all. 

Also, Anthony, I'm impressed that you had a conversation wuth Chuck Norris the other day!  

[mike_elliott]

On 1/6/2019 at 5:01 PM, Jerry 5TJ said:

This was in my C model, I think.  WOT and about 6 GPH. It took a while to climb up there but the tailwind was nice.    

Or maybe it was in my Ovation.  I dunno:  Memory, it’s the second thing to go. 

Jerry, if you can remember what the heck you were flying, You might want to submit this pic to https://groundspeedrecords.com/

 

[flyer338]

I recall climbing my C model to 17,500’ and getting 140 ktas. I thought about heading east and planning a trip to see how far I might get at 6gph. Now that I am retired, I might try something like this in my 201.

[Hank]

4 minutes ago, flyer338 said:

I recall climbing my C model to 17,500’ and getting 140 ktas. I thought about heading east and planning a trip to see how far I might get at 6gph. Now that I am retired, I might try something like this in my 201.

Why not? Jonathan Paul went San Diego to Savannah non-stop in an E model. Doing it in a C would be more impressive!

[flyer338]

On 7/27/2019 at 5:52 AM, Hank said:

The first part is correct, a windmilling engine is doing work, it's just not burning fuel--the driving energy is airflow through the propellor. There is drag associated with this, just like when you stick your hand out your car window. Frictional losses from the crankshaft rotating, pistons going up and down and valves opening and closing should be minor, and is often ignored in calculations. The drag varies A LOT with propellor angle (flat angle [hjgh RPM] is high drag), easily verified and felt during simulated engine out work, but I've never shut down my engine in flight (on purpose!) to test the effect of throttle position. But I would expect it to be much, much less than what can be done using fhe prop lever,  if it can be felt at all. 

Also, Anthony, I'm impressed that you had a conversation wuth Chuck Norris the other day!  

I have intentionally shutdown my engine during flight, and recorded the effects of changing the propeller pitch and throttle position. My conclusion is minimum drag and maximum glide ratio is at maximum pitch and closed throttle. 

 

[carusoam]

Interesting points Brian.

I have never intentionally pulled the throttle all the way back while trying to extend the glide...

Or pushed it all the way forward with the engine shutdown for maximum energy dissipation...

Do you have any data to share regarding how affective opening and closing the throttle is for either case?

Best regards,

-a-

[PT20J]

17 hours ago, flyer338 said:

I have intentionally shutdown my engine during flight, and recorded the effects of changing the propeller pitch and throttle position. My conclusion is minimum drag and maximum glide ratio is at maximum pitch and closed throttle. 

 

That is what I would expect. Glide ratio is numerically equal to L/D. Lift is fixed (it has to offset weight), so the only thing you can do is decrease drag and rotating the dead engine causes drag. The faster you rotate it, the more drag, so max pitch (low rpm) reduces drag. Rotating the engine requires power from the prop (which creates drag) in order to do the work of compressing air in the cylinders. Closed throttle = less air = less work done = less drag.

Skip

[aaronk25]

Not sure how reliable the poh is for glide when discussing a specific hull. My 77j when the blue lever is pulled all the way back still wins mills at 1600rpms or so (it’s been a while). A rented 82j I did the same test in went down to 500rpms, same conditions. I observed 750fpm on the 77’ and 550fpm on the 82. No idea why such a big difference.


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[carusoam]

Will the engine actually be running at 500rpm?

As mine idles at 700rpm or so... and doesn’t seem to want to run any slower...

PP thinking out loud, not a mechanic...

Best regards,

-a-

[aaronk25]

Will the engine actually be running at 500rpm?
As mine idles at 700rpm or so... and doesn’t seem to want to run any slower...
PP thinking out loud, not a mechanic...
Best regards,
-a-

On both the fuel was cut to idle cut off, just happened that the respective rpms mentioned is what the fan kept spinning at. With the lower rpm having a better glide, I’m assuming that means it went to a steeper pitch than the higher rpm one. Unsure, of why it would’ve different? Maybe prop cable throw is greater on the push pull than it is on my throttle quadrant machine.


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[Tim Jodice]

With all the time I have to do nothing constructive at work and having been a week since my last flight I decided to find out first hand what it is like gliding in a J Mooney.

This was with, myself, 40 gallons of gas and about 80 pounds miscellaneous stuff. 

Bright sunny days with calm wind.

The most important thing to me was to see the difference between a stopped prop and windmilling one. Starting at 12,500 I pulled all of the knobs back and slow down enough to stop the prop. 

Prop Stopped

700 fpm at 90ias
600 fpm at 70ias
400 fpm at 65ias

Windmilling prop knob all the way out.

500fpm at 65ias
700fpm at 70ias
800fpm at 90ias
 

50 rpm more with the throttle open

I didn't bother getting high rpm numbers because there is no debate that it creates more drag.

What did I get from all of this?

I know that the flat plate drag of a prop is 100fpm less than a spinning one.

It takes about 1500 feet and 120 ias to air start the engine.

The difference between going from 130 to 90 by doing a zoom climb as opposed to just letting it bleed of while hold altitude is not much different. Unless you are planning on it like I was it would be at least a few seconds before you would take action. I think I would spend the short time slowing down to best glide by going through the memory items emergency list.

I also learned I would never stop the prop during a real engine failure. Why?

1st reason is I found it hard to do, it took me 3 tries to stop it. The first 2 times I slowed down to fast and stalled before the prop stopped. The third time I slowed down slow enough that I was just about to stall held it there and given enough time it finally slowed down and stopped.

2nd it was ideal conditions, I was at about 11,500 by the time I got it stopped, over an airport, day, VFR and it was the only task at hand. Add IFR, night or passengers and all the other stuff going on during a normal non training flight.

Maybe if I was in a turbo mooney with 20K+ feet to loose and it might mean I make land or glide past a mountain but for me at my usual 7-8K feet it is not worth it in my opinion. 

I know there are all kinds of mathematicians on here so have fun!

https://flightaware.com/live/flight/N5812T

P.S during all of this I heard about 4 or 5 other aircraft. I am so used to hearing a lot of talking on the radio I checked in with Boston make sure my radio is ok. Her response "you are my only customer right now".

 

 

[ArtVandelay]

You should have tried a 180, with the prop windmilling to simulate an engine out after takeoff...I’m guessing you’d lose 1000’ given a half standard rate turn is 1 min.

[Tim Jodice]

Just now, ArtVandelay said:

You should have tried a 180, with the prop windmilling to simulate an engine out after takeoff...I’m guessing you’d lose 1000’ given a half standard rate turn is 1 min.

That is an awesome idea! An hour and a half of playing around was enough for one day. That is for next time.

 

[Hank]

Next time you're out playing, @airtim, figure out not just vertical speed and IAS, but also figure out the distance that you move per 1000' descended. And also try it with the prop windmilling at 2500, 2400 (both popular cruise settings) and All the Way Out.