Mooney M20J Glide Ratio and Distance

[carusoam]

19 minutes ago, airtim said:

 

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".

 

 

There is a couple things funny in here...

1) The MSer with the most (during A flight), real, high altitude, engine out, experience is @aviatoreb and he is a mathematician too.

So... I invite him to the conversation to see how the ‘ideal’ experiment lines up against the real life surprise...

Fortunately the prof. had a textbook landing...   

2) See if you can add some clarity for me...  

With the knobs all pulled back, then pushing the throttle all the way in...

the engine achieved 50 more rpm...

What was the slowest RPM the engine would turn?

I think there may be something in here of interest...

 

Great details!

Thanks for sharing.

Best regards,

-a-

[Tim Jodice]

Just now, carusoam said:

There is a couple things funny in here...

1) The MSer with the most (during A flight), real, high altitude, engine out, experience is @aviatoreb and he is a mathematician too.

So... I invite him to the conversation to see how the ‘ideal’ experiment lines up against the real life surprise...

Fortunately the prof. had a textbook landing...   

2) See if you can add some clarity for me...  

With the knobs all pulled back, then pushing the throttle all the way in...

the engine achieved 50 more rpm...

What was the slowest RPM the engine would turn?

I think there may be something in here of interest...

 

Great details!

Thanks for sharing.

Best regards,

-a-

When I stopped the prop I had the throttle closed, prop all the way out to the stop(low RPM) and mixture cut off.

The rest of it was done with the throttle open prop out and mixture cut off. 

With the prop windmilling closing the throttle slowed down the engine by 50 RPM confirming as many have said that using the engine as a vaccuum pump take more energy than simply compressing and releasing the air.

Slowest as in windmilling? 

[carusoam]

Yes...

What was the windmilling prop speed?

oddly, I never thought about that...

I have run the engine out of fuel, but not with the throttle closed...

The interesting idea that we might shine a light on is the energy used by compressing air...

It may have value in understanding more about the differences flying LOP vs. deep LOP...

Best regards,

-a-

[Hank]

I'm curious:  fly at Vbg. Record VSI at 2500, 2400 with throttle at WOT & Closed. Then prop all the way out, WOT and Closed.

Which way has the slowest descent at Besr Glide? Sure, ither combinations may loise fewer feet per minute, but at lower airspeed you advance fewer feet per minute, and the actual descent per mile may be greater. So test at Vbg, please, for a real world comparison. 

Also interest would be idle vs. engine shut down . . . . We can come up with lots of reasons for you to fly. I'm stuck in annual with damaged ignition harness (90° bends loosened up by a genius last year    ), nobody has them in stock so a set is being made.  

[Tim Jodice]

8 minutes ago, carusoam said:

Yes...

What was the windmilling prop speed?

oddly, I never thought about that...

Best regards,

-a-

I didn't write it down this is off memory so take it with a grain of salt. It was about 1200 at 90, 900 at 70 and 600 at 65. 

Another thing I found interesting that I forgot to write is with the prop stopped depending on the position (I forget) of the blades I could feel turbulent air in the rudder pedals.

[Tim Jodice]

15 minutes ago, Hank said:

I'm curious:  fly at Vbg. Record VSI at 2500, 2400 with throttle at WOT & Closed. Then prop all the way out, WOT and Closed.

Which way has the slowest descent at Besr Glide? Sure, ither combinations may loise fewer feet per minute, but at lower airspeed you advance fewer feet per minute, and the actual descent per mile may be greater. So test at Vbg, please, for a real world comparison. 

90 (88) being best glide is why that was the fastest speed I wrote the numbers down for. Like spinning the prop faster going down at I higher indicated airspeed has more drag.

BUT if you are going in to a head wind it may be better to be less efficient aerodynamically so that the head wind is smaller component of your ground speed.

Or if you want to get the most out of a tail wind fly at minimum sink rate and ride as long as you can/need. 

[aviatoreb]

1 hour ago, carusoam said:

There is a couple things funny in here...

1) The MSer with the most (during A flight), real, high altitude, engine out, experience is @aviatoreb and he is a mathematician too.

So... I invite him to the conversation to see how the ‘ideal’ experiment lines up against the real life surprise...

Fortunately the prof. had a textbook landing...   

2) See if you can add some clarity for me...  

With the knobs all pulled back, then pushing the throttle all the way in...

the engine achieved 50 more rpm...

What was the slowest RPM the engine would turn?

I think there may be something in here of interest...

 

Great details!

Thanks for sharing.

Best regards,

-a-

Oy - I am not sure that's a day I want to relive.  It was a lovely squeaker landing and hand shakes to the fire engines that showed up following a live emergency dead stick landing.  But it could have gone either way.  That thread is here somewhere - search the words holy yikes or saki - and you will find my report shortly thereafter.

I will say it is quite a thing when the emergency is live and practice is the only thing that helped me to keep doing the right things and push through might have otherwise been panic inaction.  Instead it was shaky voice and self talk to keep pushing on and that's exactly what I did.  Also, best glide alone was not the only problem solving.  Initial best glide while finding an airport and then commercial spiral fast descent since there was smoke in the cockpit, and then briefly best glide after the first turn and then mitigating the result of an otherwise much faster than usual speed over the fences.  All to a good ending.  This time - remembering there is a lot of luck involved in this sort of thing.

Oh and the math - Im a math prof even - pretty good at math.  I'm not sure if that was a prerequisite.  Any of us can, must, do it if our number comes up.  Practice saved the day that day.

[exM20K]

16 minutes ago, aviatoreb said:

there is a lot of luck involved in this sort of thing

Meh.  You made your luck, or at least you made your luck useful by keeping your cool. All the luck in the world is useless if one is upside down, spinning, or otherwise in a LOC state.

 

and practice means you’re not doing it for the first time.  Way better, no?

cheers

-dan

Edited April 15, 2020 by exM20K

[aviatoreb]

26 minutes ago, exM20K said:

Meh.  You made your luck, or at least you made your luck useful by keeping your cool. All the luck in the world is useless if one is upside down, spinning, or otherwise in a LOC state.

 

and practice means you’re not doing it for the first time.  Way better, no?

cheers

-dan

Absolutely - practice saved the day.  I had all sorts of useful self talk in my head that day.  Don't get too slow. Don't get too slow.  Promised myself not to do something dumb like trying to stretch a glide - promised long before that day and remembered it then.  Remembered my glide speeds.  All sorts of the things we practice for, I did.  I could hear my shaky voice on the radio but I kept saying and doing the things.  He even offered me a frequency change and I told him I was overloaded and I would stay with him.  The fellow from ATC called me on the phone a few minutes after touch down to see how I was. He seemed almost as happy to talk to me as I was to him.  I never did listen to that actlive but I bet its there.

Edited April 15, 2020 by aviatoreb

[EricJ]

This discussion reminded me to dig this up:

 

[aviatoreb]

4 hours ago, EricJ said:

This discussion reminded me to dig this up:

 

Great ad.

And fun they put a full feathuring prop on before the rocket engineering did.

But commercial hype. Rocket engineering planes get 18:1 in full feathering.  Don't modern gliders get easily better than 50:1 and some even up to 70:1?  Not even close.  Other than they are bot gliding.

[carusoam]

I think I remember the ad, probably printed in AOPA’s mag...

It helped in my selection of a Mooney... over the other bricks on the list...

But, I didn’t remember the fact that they used a two blade, full feathering prop for the ad...   

Full feathering would be nice, if...

• it were an option by pulling the prop knob back...
• not the result of loss of prop OilP...

Come to think of it... my C’s prop lost its oil pressure once on departure...

That would have been more exciting than I remember...  

Best regards,

-a-

[Ibra]

On 4/15/2020 at 3:04 AM, airtim said:

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

Nice, thanks for the testing especially those figures with prop stopped , so you get better L/D at Vbg = 65ias 1: 13? I would have expected in the M20J at 90ias at 1:10  my guess it depends lot on weight, 1SOB brings Vbg is near 70kts and L/D near 13? while MTOW Vbg is near 90kts and L/D about 1:10?

Also, have you timed how long it takes to burn 1000ft on Altimeter at various speeds (vario is probably less reliable on low rate of descent)?

On height loss during a 360 turn at 90ias in M20J, I recall getting 1000ft on 45deg and 1600ft on 20deg, hard to see how to make runway back with that just after takeoff unless I am at 2000ft agl, that height will not get lot of clicks on YouTube 

Edited April 24, 2020 by Ibra

[Ibra]

On 4/15/2020 at 11:03 PM, EricJ said:

This discussion reminded me to dig this up:

 

When did they had that cover printer 1961 or 1986? I am sure they want to convey "efficient & higher"...

http://www.khmetals.com/robert_harris.htm

[jaylw314]

I've been playing around with the P-51D in DCS, and I found this from the P-51 manual:

Glides
The P-51 can be glided safely at speeds down to 25% above stalling speed. With average loads, this will be around 125 mph IAS at any level – this speed increasing with the weight of the aircraft. The optimum power-off gliding speed is 175 mph.
With the landing gear and flap retracted, the glide flight path is fairly flat. In this condition, however, the nose is held high and forward visibility is poor. Lowering either the flaps or the landing gear reduces the safe gliding speed and greatly steepens the gliding angle and increases the rate of descent.
The table below demonstrates the horizontal glide distances obtained with a power-off glide at 175 mph IAS from various altitudes:

Holy smokes, that's almost 50% better distance from a glide...  How is it that much better than a Mooney?

[1980Mooney]

24 minutes ago, jaylw314 said:

I've been playing around with the P-51D in DCS, and I found this from the P-51 manual:

Glides
The P-51 can be glided safely at speeds down to 25% above stalling speed. With average loads, this will be around 125 mph IAS at any level – this speed increasing with the weight of the aircraft. The optimum power-off gliding speed is 175 mph.
With the landing gear and flap retracted, the glide flight path is fairly flat. In this condition, however, the nose is held high and forward visibility is poor. Lowering either the flaps or the landing gear reduces the safe gliding speed and greatly steepens the gliding angle and increases the rate of descent.
The table below demonstrates the horizontal glide distances obtained with a power-off glide at 175 mph IAS from various altitudes:

Holy smokes, that's almost 50% better distance from a glide...  How is it that much better than a Mooney?

Full feathering prop.  The distances are the same as a Mooney with a Rocket or Missile conversion which includes a full feathering prop.

[jaylw314]

12 hours ago, 1980Mooney said:

Full feathering prop.  The distances are the same as a Mooney with a Rocket or Missile conversion which includes a full feathering prop.

From the manual (well, the DCS one, FWIW):

The P-51D propeller is a Hamilton Standard four-blade, hydraulic, constant-speed propeller with a diameter of 11 ft, 2 in and a blade pitch range of 42°, set to 23° at low pitch and 65° at high pitch. The propeller RPM is controlled by the Propeller Control lever on the throttle quadrant in the cockpit. The propeller governor automatically controls propeller pitch to maintain a constant speed between 1800 and 3000 RPM, depending on the Propeller Control setting. The propeller cannot be feathered.

[carusoam]

don’t expect the P51 data to be too accurate...

The Mooney is designed to be real efficient at light weights... compared to how stout the P51 is...

The P51 when completely empty is still very heavy, combined with short wings... it can’t be that good of a glider...

It does have great wing shapes for gliding... Just Completely over built to be an efficient glider...

Even when empty, the giant weights of construction and huge engine.... can’t possibly make a good glider...

It would be amazing to know I’m wrong on this... 

Best regards,

-a-

[jaylw314]

13 minutes ago, carusoam said:

don’t expect the P51 data to be too accurate...

The Mooney is designed to be real efficient at light weights... compared to how stout the P51 is...

The P51 when completely empty is still very heavy, combined with short wings... it can’t be that good of a glider...

It does have great wing shapes for gliding... Just Completely over built to be an efficient glider...

Even when empty, the giant weights of construction and huge engine.... can’t possibly make a good glider...

It would be amazing to know I’m wrong on this... 

Best regards,

-a-

I found an AOPA article on the P-51D

https://www.aopa.org/news-and-media/all-news/2007/august/01/north-american-aviation-p-51d-mustang

They suggested a glide ratio of 15.3:1, which is just about 3 statute miles per 1000 ft of altitude with a non-feathering prop.  That still beats my M20J glide ratio of about 11:1 by a pretty hefty margin! 

[carusoam]

19 minutes ago, jaylw314 said:

I found an AOPA article on the P-51D

https://www.aopa.org/news-and-media/all-news/2007/august/01/north-american-aviation-p-51d-mustang

They suggested a glide ratio of 15.3:1, which is just about 3 statute miles per 1000 ft of altitude with a non-feathering prop.  That still beats my M20J glide ratio of about 11:1 by a pretty hefty margin! 

Still something not right...

really heavy even when empty...

wingspan not much different than a Mooney...

Same airfoil design basically...

Best glide speed is ridiculously high... with the drag associated with Speed...  175mph!

Glide fast, before you run out of altitude.... 

If I were a glider pilot... I would be placing a call with the author of that story to find out where I Got lost...

Could be something in the wing loading...? I didn’t compare ours to the P51...  but expect the longer wings to be better for glide...

We have a few glider pilots resident on MS...

Best regards,

-a-

[Ibra]

8 minutes ago, carusoam said:

Still something not right...

The B747 does 1:17 at 200kts, go figure  I guess mainly Lift-to-Drag decide on the glide ratio while weight decide on best glide speed 

I think P51 has silver aluminium colour that helps a bit 

Edited April 25, 2020 by Ibra

[Fry]

5 hours ago, Ibra said:

The B747 does 1:17 at 200kts, go figure  I guess mainly Lift-to-Drag decide on the glide ratio while weight decide on best glide speed 

I think P51 has silver aluminium colour that helps a bit 

My understanding is:

The glide ratio is indeed independent from weight. Lift equals weight in unaccelerated flight anyway, and both drag and lift scale with the square of (air)speed, so the ratio remains unchanged. That is at a given angle of attack (AoA), which in this case is the "best glide", or maximum L/D, AoA. And far enough below the speed of sound.

Put simply, at a given configuration and AoA, all aerodynamic forces - including the one driving your airspeed indicator needle - scale with the same underlying physical quantity (ram air pressure, rho v^2). That is the reason why at MTOW, you always stall at the same airspeed, regardless of density altitude, and this is the property that makes the airspeed indicator probably the greatest invention in aviation.

(The "air speed indicator" is actually an indicator of ram air pressure, just the scale is drawn in units of "equivalent speed at 0 MSL in standard atmospheric conditions").

Just for completeness, the above breaks down when approaching Mach=1 because then, air compressibility enters the stage (the nature of sound), and with it a fixed speed scale (the speed of sound), destroying the nice scale law. Then, the Mach number becomes equally or more important than ram air pressure (=airspeed).

But most here probably knew all that already....

Edited April 25, 2020 by Fry

[Ibra]

Yes, at low Mach best L/D is independent of weight, speed, density it simply relate to wing aerodynamics geometry coefficients, hence a specific AoA for best L/D glide, everything else will just cancel out from Lift/Drag ratio including weight, speed, density, same as the fact that one stalls at max AoA and you fly best L/D at some best glide AoA (so wing shape mainly including flaps), the ASI speed at which one stalls depends on his weight (or G*weight for completeness), so I guess the same applies to speed at which you have best glide ratio, it will increase with weight (Lift = C(AoA)*ASI*ASI = G*Weight, should give ASI speeds for stall at max AoA and best glide AoA)

Being heavy should increase indicated ASI speed for best glide but yes it will not change the wing best glide L/D ratio, this is why competition gliders take extra water to go faster on cross-currency, to change L/D ratio high performance gliders have "Speed or Negative Flaps" to be used to increase L/D at high speeds, 

Just guessing, B747 has higher L/D ratio even at high speeds probably because of its extreme swept wings (airflow over wing never get supersonic to create any shockwaves/drag as air can't travel faster), but then it may need load of "lift flaps" for landing at slow speeds (Krueger flap in front reduce sweep factor and triple slot flaps in the back to stop the beast)

Or maybe those big swept wings just look cool and simply fly higher L/D? 

Edited April 25, 2020 by Ibra

[jaylw314]

I've found a number of different sources on the internet that suggest the P-51D's glide ratio is around 15:1.  It sort of begs the question what makes single-engine prop aircraft like ours so poor gliders.  I always thought it was the stuff related to having a propeller, but that doesn't seem to be the case if the P-51 is in the same league as the B747.

Incidentally, the P-51D also regularly flew with fuel in the internal fuselage fuel tank which sounds like it actually put CG dangerously rearwards.  The manual even describes some amount of negative stability with a full fuselage tank, although they call it "reversibility" of stick forces if you pull the stick back far enough.  Yikes.  Presumably, this also decreases drag from the elevators, but dang.

[carusoam]

Speaking of DCS...

Being hunkered down...

Lots of free trial offers for a month...

https://www.digitalcombatsimulator.com/en/index.php

Best regards,

-a-