Mooney M20J Glide Ratio and Distance

[carusoam]

Trent hit the hot points...

Just the presentation needed a summary.

1) least drag... engine still idling. Compression stroke drag is removed by the amount of fuel being burned.... not really useable for practice in actual engine out conditions...

2) middle amount of drag... prop stopped.  No compression stroke drag...

3) more drag... prop turning at lowest rpm. Some good amount of compression stroke drag...

4) most drag... prop windmilling at 2700 rpm, throttle left full in.  Maximum amount of compression stroke drag...

5) Also not mentioned... Rocket and Missile get one advantage... the full feathering prop is easy to stop and will stay that way...

 

Difficulty in using this sage advice... practice to see if you can keep the blades from turning at you best glide speed around 80-90 indicated...

As sage as it is... it only adds 10% more glide... but if 10% gets you to a field past the trees... you now know how to get there...

Now you made the field... you’re coming in hot... let the prop spin, drop the gear, deploy the speed brakes and flaps, get slowed..!

Best regards,

-a-

[Shadrach]

12 minutes ago, carusoam said:

Trent hit the hot points...

Just the presentation needed a better summary.

1) least drag... engine still idling. Compression stroke drag is removed by the amount of fuel being burned....

2) middle amount of drag... prop stopped.  No compression stroke drag...

3) more drag... prop turning at lowest rpm. Some good amount of compression stroke drag...

4) most drag... prop windmilling at 2700 rpm.  Maximum amount of compression stroke drag...

 

Difficulty in using this sage advice... practice to see if you can keep the blades from turning at you best glide speed around 80-90 indicated...

As sage as it is... it only adds 10% more glide... but if 10% gets you to a field past the trees... you now know how to get there...

Best regards,

-a-

Nice summary. I would add the caveat that we don't know how our airframes would respond. My guess is that the the results would be the same but the differences even greater. I am a little gun shy regarding prop stoppages in the air given my previous experiences. Perhaps I'll do some testing when the weather warms.  I miss having Norman (@testwest) around, his input would have been most welcomed in this discussion.

[aviatoreb]

As some of you may remember my post - I did have the excitement of an engine out during a cross country last April and then the excitement of a dead stick landing.  I think the training was invaluable, helpful, but.... somehow in the "flight of the moment" I am not sure I had the calm focus to distinguish between 90 knots my true Vg and say 88 a computed actual Vg.  I am happy I remembered what Vg is as a concept and ... sort of did a good enough job of it and actually found a runway.  SO I did watch that video above but I wouldn't think it realistic for anyone except for Chuck Jaeger and his likes to fly 88 vs 90 or 92 to sharpen the true Vg - not that it shouldn't be a goal.   ...and so following that event lessen taken away...I am now a proud early adopter of airbag seat belts.  And do practice but know we will be less perfect when it is the true event rather than a practice day.

[FloridaMan]

My M20F at max gross was around 700fpm. It was also really difficult to get down when my engine quit on takeoff. I had to hold level, throw the gear down and hold full rudder to get the speed less than 100mph before diving for the ground. 

[Rene]

On 1/2/2019 at 2:56 AM, JohnB said:

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

 

 

OK I tried the glide with my 3 blade prop control pulled all the way back, and it did make a huge difference.  My glide with the prop control all the way forward was -1000 ft/min, and with the control all the way back it was -600 ft/min.  So this means that at 90 knots, I would get 2.5 nm for each 1000 feet AGL, rather than only 1.5 nm.  The Mooney "book" value is 1.9 nm per 1000 ft AGL.  The only caveat with using the more optimistic number is that in the case of a real engine emergency you may not have oil pressure to increase the prop pitch, so the prop control may be inop.  For practice with simulated engine outs, it's probably best to do it with the prop at high RPM because that is more like the worst case glide.

[jaylw314]

1 hour ago, Rene said:

OK I tried the glide with my 3 blade prop control pulled all the way back, and it did make a huge difference.  My glide with the prop control all the way forward was -1000 ft/min, and with the control all the way back it was -600 ft/min.  So this means that at 90 knots, I would get 2.5 nm for each 1000 feet AGL, rather than only 1.5 nm.  The Mooney "book" value is 1.9 nm per 1000 ft AGL.  The only caveat with using the more optimistic number is that in the case of a real engine emergency you may not have oil pressure to increase the prop pitch, so the prop control may be inop.  For practice with simulated engine outs, it's probably best to do it with the prop at high RPM because that is more like the worst case glide.

If your engine stops because of no oil pressure, you'd be running it until it seized anyway.  It's hard to envision a single engine failure mode that would also keep the prop windmilling where you would lose prop control also.  

[Shadrach]

3 hours ago, Rene said:

OK I tried the glide with my 3 blade prop control pulled all the way back, and it did make a huge difference.  My glide with the prop control all the way forward was -1000 ft/min, and with the control all the way back it was -600 ft/min.  So this means that at 90 knots, I would get 2.5 nm for each 1000 feet AGL, rather than only 1.5 nm.  The Mooney "book" value is 1.9 nm per 1000 ft AGL.  The only caveat with using the more optimistic number is that in the case of a real engine emergency you may not have oil pressure to increase the prop pitch, so the prop control may be inop.  For practice with simulated engine outs, it's probably best to do it with the prop at high RPM because that is more like the worst case glide.

To what are you attributing your increased performance? I don’t believe that the drag reduction was from altering the drag profile of the blade. I believe that by pulling the prop back you brought the blades off of the fine pitch stop and increased their AOA. The prop is more efficient at lower RPM and higher AOA. I’d like to do some stopped prop glide tests to verify. I’m not trying to pee in your Cheerios. I simply don’t believe that there’s any way to duplicate the stopped prop glide performance of the aircraft with the engine running. 

Edited January 5, 2019 by Shadrach

[PT20J]

Here’s a way to think about it. Assume the engine is producing no power and the plane is gliding and the prop control is at the highest rpm position. Spinning the engine at this rpm requires a certain amount of power which creates drag. As you pull the prop control back the power required to rotate the engine at the lower rpm decreases and so does the drag. If you could pull  back all the way to feather, the prop would stop and drag would be minimum. 

For a stopped prop, the first order approximation of the drag is the flat plate area of the blades which obviously depends on the blade size, shape and number.

Skip

[FloridaMan]

According to book numbers (90kts, 475fpm at 24,000), I can glide 87 miles from altitude. 

 

 

 

[aviatoreb]

4 hours ago, Antares said:

According to book numbers (90kts, 475fpm at 24,000), I can glide 87 miles from altitude. 

 

 

 

Do just turn off the engine w 85 miles to go and save some fuel!

[FloridaMan]

If I wanted to save fuel I’d fly something slow like an m20j or a 252

[JohnB]

20 hours ago, Rene said:

OK I tried the glide with my 3 blade prop control pulled all the way back, and it did make a huge difference.  My glide with the prop control all the way forward was -1000 ft/min, and with the control all the way back it was -600 ft/min.  So this means that at 90 knots, I would get 2.5 nm for each 1000 feet AGL, rather than only 1.5 nm.  The Mooney "book" value is 1.9 nm per 1000 ft AGL.  The only caveat with using the more optimistic number is that in the case of a real engine emergency you may not have oil pressure to increase the prop pitch, so the prop control may be inop.  For practice with simulated engine outs, it's probably best to do it with the prop at high RPM because that is more like the worst case glide.

Very helpful sir! Thanks for sharing! Makes perfect sense. I knew prop full back increases your forward glide distance, But had to guess about what it does to descent rate. Well executed experiment and easily verifiable! I didn’t think it would be that big but That is a HUGE difference where you gained 1 extra NM per 1,000 feet by simply pulling out your prop and might help save someone reading this to remember if they do have engine out actually, pull the prop control all the way back. The theory I was thinking of which is confirmed by your test run is if your engine not producing forward thrust, the wind that hits your prop is what’s turning your propeller which is slowing your airplane down. So to keep the same glidespeed as you did, you’d have to pitch down which would increase your rate of descent. Pulling back your prop makes much less wind hit your prop blades therefore less drag.

I would also hazard to guess that the more blades you have, the more pronounced this effect on descent rate will be, so if you have 2 blades there is probably less of a difference and if you have 4 blades, you would have a massive difference in descent rate if you forget to pull your prop all of the way back.

 

Thanks for doing that Rene!

[aviatoreb]

2 hours ago, Antares said:

If I wanted to save fuel I’d fly something slow like an m20j or a 252

Or drive a Vespa scooter.

[PT20J]

6 hours ago, aviatoreb said:

Do just turn off the engine w 85 miles to go and save some fuel!

OK, you win! 

[FloridaMan]

On 1/4/2019 at 5:54 PM, Rene said:

OK I tried the glide with my 3 blade prop control pulled all the way back, and it did make a huge difference.  My glide with the prop control all the way forward was -1000 ft/min, and with the control all the way back it was -600 ft/min.  So this means that at 90 knots, I would get 2.5 nm for each 1000 feet AGL, rather than only 1.5 nm.  The Mooney "book" value is 1.9 nm per 1000 ft AGL.  The only caveat with using the more optimistic number is that in the case of a real engine emergency you may not have oil pressure to increase the prop pitch, so the prop control may be inop.  For practice with simulated engine outs, it's probably best to do it with the prop at high RPM because that is more like the worst case glide.

As a note, with my M20F, full fuel and probably 450lbs of people on board, a 2-blade Hartzell Prop set for 2500 RPM, I was seeing 700fpm at 100mph IAS. 

[gsxrpilot]

On 1/5/2019 at 1:05 PM, Antares said:

If I wanted to save fuel I’d fly something slow like an m20j or a 252

Yep, no more efficient certified four seaters, then the J or the 252.

156 kt TAS - 9.5 gph - 17.7 mpg 

This was yesterday.

[Jerry 5TJ]

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. 

[gacoon]

4 hours ago, 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. 

Looks more like your P46T, what J or Ovatiion can true 171kt @ 23K?

[Jerry 5TJ]

I should have used the green font.  

And it shows 250 true, BTW. 

[Hank]

1 hour ago, Jerry 5TJ said:

I should have used the green font.  

????

[ArtVandelay]

????

Beechtalk uses green font to denote sarcasm.


Tom

[Hank]

1 hour ago, ArtVandelay said:

Beechtalk uses green font to denote sarcasm.

We use emojis     

But only the ones above, not the hundreds available on your phone.  

[FloridaMan]

On 1/5/2019 at 8:56 AM, FloridaMan said:

According to book numbers (90kts, 475fpm at 24,000), I can glide 87 miles from altitude. 

Shoot. I wasn't thinking. That's 90kias not 90ktas; hell, that might get more than 100 miles considering 90kias at 24,000 is closer to 140ktas and 110ktas at 12,000. 

[jaylw314]

19 hours ago, 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. 

You bastard!  You had me going there!

[jaylw314]

12 minutes ago, FloridaMan said:

Shoot. I wasn't thinking. That's 90kias not 90ktas; hell, that might get more than 100 miles considering 90kias at 24,000 is closer to 140ktas and 110ktas at 12,000. 

But you're descending faster too, so your glide angle and distance (per 1000')  would be the same as at low altitude