Mooney M20J Glide Ratio and Distance

[PT20J]

On 4/26/2020 at 1:17 PM, jaylw314 said:

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.

The original design was for an Allison engine and 3 bladed prop. The switch to the Merlin and 4 bladed prop moved the CG forward and a weight was added to the rudder post to rebalance the airplane. When more range was needed to provide fighter escort for bombers deep into Germany, the 85 gal. fuselage tank was added behind the cockpit and the tail weight was removed. With the fuselage tank full, the CG was indeed aft of the neutral point and the airplane was longitudinally unstable. There was a prohibition against instrument flight or aerobatics until fuel was burned from the fuselage tank which was used first. However, if all the fuel were burned from the fuselage tank the airplane would be nose heavy and prone to noseover during landing since it no longer had the rudder post weight, so the admonition was leave 15 gal in the fuselage tank to only be used in an emergency.

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[PT20J]

If you like P-51s and are ever in Salinas, CA stop by Cal Pacific Airmotive (owners of the P-51 type and production certificates) and visit with Lori Atkinson. She loves to show pilots around. You'll probably see a few restorations in progress. The other cool thing is they have completely disassembled P-51s of each model with all the parts stored in racks and cataloged so that if they need to make a part and they can't read the blurry microfiche, they can take an original part and measure it.

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[PT20J]

On 4/26/2020 at 1:17 PM, jaylw314 said:

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

The glide ratio is numerically equal to L/D of the airplane (not just the wing). If one airplane has a better glide ratio than another, it simply means that it has a higher L/D ratio. The lift approximates the weight. As airplane size increases, weight increases more rapidly than drag since weight tends to increase with volume whereas skin friction drag increases more as a function of area. A paper airplane has low weight and a lot of area and doesn't glide very well.

Also keep in mind that best glide (max range) is different than minimum sink (max duration).

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[Ibra]

18 hours ago, PT20J said:

The glide ratio is numerically equal to L/D of the airplane (not just the wing). If one airplane has a better glide ratio than another, it simply means that it has a higher L/D ratio. The lift approximates the weight. As airplane size increases, weight increases more rapidly than drag since weight tends to increase with volume whereas skin friction drag increases more as a function of area. A paper airplane has low weight and a lot of area and doesn't glide very well.

Thanks for the explanation, yes the weight only go into max L/D picture via volume/surface proportions via some sort of "Square–Cube law"
For flying level cruise under power things get non)linear again: minimum speed to keep level varies as square root of length
https://en.wikipedia.org/wiki/Square–cube_law

In theory, max L/D relates to aspect ratio (wing span) divided by zero-lift drag coefficient (relate to parasite drag from skin friction)
P51 does look highly streamlined as much a modern B747, so the small difference in max L/D is probably the aspect ratio?

The zero-drag Coefficient for P51 is 0.0161 for B747 is 0.0184, for comaprasion the Fokker E-III fighter is 0.0771
https://www.sesarju.eu/sites/default/files/documents/sid/2018/papers/SIDs_2018_paper_75.pdf
https://history.nasa.gov/SP-468/ch1.htm
https://history.nasa.gov/SP-468/app-c.htm
https://history.nasa.gov/SP-468/app-a2.htm

Edited April 29, 2020 by Ibra

[PT20J]

A couple of fine points:

1. L/Dmax occurs at the angle of attack where induced drag and parasite drag are equal and each contribute 1/2 of the total drag

2. Strictly speaking, the induced drag is a function of wing span (b) rather than aspect ratio (AR). An infinite span wing has no vortices and no induced drag. As the span is progressively shortened, the vortices become stronger and the induced drag increases. For a given wing area (S), the aspect ratio will necessarily change as the span is changed since AR = b²/S.

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[jaylw314]

On 4/28/2020 at 8:48 AM, PT20J said:

The glide ratio is numerically equal to L/D of the airplane (not just the wing). If one airplane has a better glide ratio than another, it simply means that it has a higher L/D ratio. The lift approximates the weight. As airplane size increases, weight increases more rapidly than drag since weight tends to increase with volume whereas skin friction drag increases more as a function of area. A paper airplane has low weight and a lot of area and doesn't glide very well.

Also keep in mind that best glide (max range) is different than minimum sink (max duration).

Skip

Thanks for the clarification! 

It's still surprising that in terms of size and weight, the P-51 (max gross about 12,000 lbs) is much closer to an M20J (2800 lbs) than a B737 (160,000 lbs), yet the L/D ratio is much closer to a B737 than an M20J.  Surprising especially since there aren't really major airframe aerodynamic differences between the P-51 and M20J (non-feathering prop, low airframe drag, retractable gear, laminar flow and forward swept wings).  Are there any other factors you can think of?

[ArtVandelay]

9 hours ago, PT20J said:

 

2. Strictly speaking, the induced drag is a function of wing span (b) rather than aspect ratio (AR). An infinite span wing has no vortices and no induced drag. 

I thought induced drag was a result of providing lift, so even if the wingspan was infinite, there would still be drag?

[Ibra]

3 hours ago, jaylw314 said:

Surprising especially since there aren't really major airframe aerodynamic differences between the P-51 and M20J (non-feathering prop, low airframe drag, retractable gear, laminar flow and forward swept wings).  Are there any other factors you can think of?

I guess the main one is what @PT20J referred to L/D for "whole aircraft surface" not just wings L/D?
M20J wing has 7.4 aspect ratio vs 5.8 for the P51 wing still M20J under-perform slightly in L/D, my guess that has to do with wing/fuselage surface ratio and friction?
The zero-drag coefficient for P51 is 0.0161 while M20R 0.0298 (can't find for M20J) and that also helps for max L/D

 

Aspect ratio alone does not give max L/D, if you take B47 vs Vulcain, they both have same max L/D = 17, in one hand the B47 has a huge glider-like aspect ratio 10 but also a huge wetted surface fuselage/wing surface, in the other hand the Vulcan has 3 aspect ratio but the fuselage is basically "a whole wing"

http://www.meridian-int-res.com/Aeronautics/Burnelli.htm

 

[PT20J]

9 hours ago, ArtVandelay said:

I thought induced drag was a result of providing lift, so even if the wingspan was infinite, there would still be drag?

The induced drag is a product of the generation of wingtip vortices. So, an infinite span wing would not have tips (they would be infinitely far away) so it would not have vortices and thus would not have induced drag. Obviously, an infinite span wing is an abstraction. The oft-mentioned statement that induced drag is a result of producing lift is true for a wing of finite span where vortices are unavoidable.

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[PT20J]

11 hours ago, jaylw314 said:

Thanks for the clarification! 

It's still surprising that in terms of size and weight, the P-51 (max gross about 12,000 lbs) is much closer to an M20J (2800 lbs) than a B737 (160,000 lbs), yet the L/D ratio is much closer to a B737 than an M20J.  Surprising especially since there aren't really major airframe aerodynamic differences between the P-51 and M20J (non-feathering prop, low airframe drag, retractable gear, laminar flow and forward swept wings).  Are there any other factors you can think of?

A few possibilities come to mind. 

1. The P-51 has a wing loading of 43.4 lb/ft² whereas the M20J has a wing loading of 16.5 lb/ft². This means that the wing area for a Mustang is comparatively less than the wing area for a Mooney and wing area adds drag.

2. The general shape of the P-51 is relatively more aerodynamic because it is easier to form a smooth fuselage shape around a single occupant than around four. 

3.  Cooling drag is a significant drag contributor in piston powered airplanes. The Mustang uses a liquid cooled engine with a belly mounted radiator and inlet/outlet arrangement that takes advantage of the Meredith effect to reduce much of the cooling drag.

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[carusoam]

Any details regarding the Cross sectional area of the two subjects, Mooney vs. P51?

As the drag of a flat plate often gets used for estimating the drag of the fuselage....

The P51 has a very high glide speed compared to the Mooney... and drag is a squared function of speed...(?)

Best regards,

-a-

[bradp]

Skip interesting memory from 6th grade science class.  We were doing a unit  about lift.  Project was to create a flying paper airplane ie a glider.  The kids used paper, foam board etc. I remember my foam board airplane sucked, so I used a bit of thin balsa wood (3 ft span) that I warped a little to make it look like a wing on a real plane. Glued up a little t tail.  I didn’t think it would fly because it seemed too heavy.  I was wrong and that little plane made it across the gym.  Took it outside and it crashed promptly in the non-still air.  

[PT20J]

37 minutes ago, carusoam said:

Any details regarding the Cross sectional area of the two subjects, Mooney vs. P51?

As the drag of a flat plate often gets used for estimating the drag of the fuselage....

The P51 has a very high glide speed compared to the Mooney... and drag is a squared function of speed...(?)

Best regards,

-a-

From the link in @Ibra post, the equivalent flat plate area for the P-51 is 3.80 ft². From David Lednicer, for the Mooney it is 2.81 ft².

Remember that L/Dmax  occurs where induced drag = parasite drag for any airplane. Yes, the P-51 flies faster. But at best glide speed, the two drag components are still equal.

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[carusoam]

The single seat cockpit is pretty tiny... to get that small of a CSA.

Art and Al must have had an enjoyable time with the early Mooney Layouts...

They didn’t have YouTube to help...  

Best regards,

-a-