Wing Lift Coefficient (CL_Max) of P-51D

[Laurelix]

You see what I see across many planes is...

Laminar Wings offers less lift but produce less drag and have lower CL_Max 1.28-1.32

Elliptical Wings provide lift but are efficient in producing low drag for lift ratio. CL-Max is roughly 1.36

Normal Wings usually have about 1.4-1.5 CL_Max.

——————

Now... P-51D has laminar wings, This is why it's so aerodynamic and achieving 604km/h at Sea level with only 1630 horsepower whilst having 21.65m2 wing area.

However I was caught off guard with this...

A guy showed me this claiming P-51D's laminar wings had 1.49 CL_Max which I instantly was skeptical about. If they produce that much lift they would also give a lot of drag and 21.65m2 isn't example small wing area. Then achieving 604km/h at SL with only 1630 horse power is quiet the achievement as it is.

Then he posted from P-51D manual the stall speed which is 102mph (164km/h) at 9700lb weight (4400kg)

Lift Force (Newton's) = 4400kg x 9.81 (Gravity)
-
Air Density at Sea Level = 1.225kg/m3
-
Velocity = 164km/h (stall)
-
Wing Area = 21.65m2

This however gives me CL_Max of 1.57 which makes no sense bevause it's far too high and yet I feel it's not high enough to be with flaps deployed.

 

[Laurelix]

A friend of mine says its because the stall speed has no PEC

I was told as well that the 1.49 is the maximum Lift in a section and not the overall wing

 

[pbehn]

You see what I see across many planes is...

Laminar Wings offers less lift but produce less drag and have lower CL_Max 1.28-1.32

Elliptical Wings provide lift but are efficient in producing low drag for lift ratio. CL-Max is roughly 1.36

Normal Wings usually have about 1.4-1.5 CL_Max.

.

Laminar type profiles refers to the aerofoil shape, preserving laminar type flow over more of the wing chord.

"Elliptical wing" refers to the planform, the Spitfire was "elliptical" but in fact a series of ellipses. The P-51D was trapezoidal. FWIK the issues are with span wise lift distribution, a trapezoidal wing being almost as good as an elliptical wing but a whole lot easier to make and also easier to make without as many joints and rivets.

I don't know what you call a normal wing but a Hawker Tempest had a laminar flow elliptical wing. The comparative speed advantage of a P-51D against a Spitfire with the same engine was only partly due to the wing, it also had a better cooling set up with Meredith effect and was cleaner overall with smoother joints etc.

 

[mad_max]

The biggest attribute to the speed vs HP in the Mustang is it was a very clean airframe with much less drag than most.

A very good article on Aerodynamics; well worth reading:



Here's a link to my google drive that has the NACA test on the prop-less P-51.



I have more articles like this on other airframes if anyone is interested I'll post links.

 

[fubar57]

drgondog

 

[pbehn]

dr

Fubar I have a strange feeling that it isn't quite as simple as in the OP, prepare for some incoming angles of attack, parasitic drag, and equations.

 

[fubar57]

 

[swampyankee]

The P-51D did not use a 6-series airfoil; it used a NAA/NACA 45-100 (see https://m-selig.ae.illinois.edu/ads/aircraft.html); the F, G, H, and J models used the NACA 66-(1.8)15.5 (root) and NACA 66-(1.8)12(tip) (https://history.nasa.gov/sp4409-vol2-3.pdf). I've not been able to find polars for the NAA/NACA 45-100; I suspect that they're deeply buried in a Boeing archive, digitized, and on an 8 inch, hard-format floppy disc no longer readable by current technology (this has already happened to my MS capstone project , only with 3.5" floppies).

In any case, the NAA/NACA 45-100 airfoils are probably fairly close in performance to the 66-(1.8)12 and 66-(1.8)15.5 airfoils.

 

[Algernon]

The P-51D did not use a 6-series airfoil; it used a NAA/NACA 45-100 (see https://m-selig.ae.illinois.edu/ads/aircraft.html); the F, G, H, and J models used the NACA 66-(1.8)15.5 (root) and NACA 66-(1.8)12(tip) (https://history.nasa.gov/sp4409-vol2-3.pdf). I've not been able to find polars for the NAA/NACA 45-100

P-51 airfoil 45-100
P-51 NAA/NACA 45-100 airfoil graph

 

[Skyediamonds]

Laurelix mentioned "..... because it doesn't have PEC." What is that?

 

[mad_max]

Laurelix mentioned "..... because it doesn't have PEC." What is that?

If it's what I think: Position Error Correction

 

[Laurelix]

If it's what I think: Position Error Correction

it is yes

 

[pbehn]

Below are some drawings of a P-51, with undercarriage extended for landing, how much of the wing conforms to the NACA profile? It has a slot for the gear, the leg sticking out, guns sticking out of the leading edge, holes for shells/links to drop out, ailerons with trim tabs and all sorts of rivets and panel lines. The sections taken at W1, W2 and W3 show the difference in size between root and tip, but they are all to the NACA profile. Not clear on the drawings is the washout on the wings, the whole wing doesnt have the same angle of attack, when the aircraft stalls that is the last phase, some of the wing already had.

 

[thunderbird]

Stall speed is flaps down. P-51 had partial span plain flap which doesn't increase the lift all that much. The B-52, designed a few years later with an odd flap, (moves down first like a split flap before travelling aft to the trailing edge, similar to the final configuration of a fowler flap, the difference being that a fowler flap moves aft first), goes from Clmax clean of 1.4 to Clmax flaps down of 1.7, reference my professional work on B-52 flight manuals a few years ago.

Clmax as I recall, is a complex subject that is only slightly related to 2-D sections. Most wings have a relatively thick root chord of 15%-18% on subsonic planes, tapering down to 8%-10% at the wingtips. Wings are twisted towards the tip to move the onset of stall inboard, because who wants one's ailerons to be stalled out first. Then the flow itself is complex, flowing towards the wing tip rather than flowing straight back on the lower surface, for straight and conventionally aft swept wings. Then throw in a little aeroelasticity, especially on swept wings, even predicting Clmax is a tricky problem, and is (was? in my day), only finalized by flight test.

 

[Skyediamonds]

Mad and Laurel, thank you guys for your response. Just one more question only because the above detailed drawings called to my attention something I've long forgotten. What exactly are the recognition lights under the starboard wing for? Are they all "on" at once, or to flash a code or what?

Gary

 

[mad_max]

Good question and after doing a search found this posted in another thread on this board.

(As memo serves these three lights were for group flights.)
(Also used to flash the 'colours of the day' to ground forces, for recognition purposes. The switch unit was on the starboard wall, with a switch for each colour, with positions for On/Off, Steady and Flashing, and I believe their was also an interval timer incorporated, to enable a timed combinations of light sequences.)

RAAF P-51D Mustang underwing recognition lights?

 

[Laurelix]

Well I can easily get CL_Max value if we found a source stating P-51D stall with PEC and a certain weight.

 

[mad_max]

Well I can easily get CL_Max value if we found a source stating P-51D stall with PEC and a certain weight.

Don't know if this will help you as I'm no Aero Engineer, but here is something I found:

Mustang IV

M.S. gear, 3,000 R.P.M. Radiator flap adjusted to give 8 1/2" gap Corrected to 9,000 lb. High Level speed.

From see level to high alt. the average is +5 mph ASI. From +5.25 to +4.75 mph. Who knows about if the low speed PEC is different or the same.

Mustang IV Flight Trials

 

[pbehn]

Well I can easily get CL_Max value if we found a source stating P-51D stall with PEC and a certain weight.

You can make a calculation, but what is that calculation of? If you take the wings off a P-51 and put the fuselage in a wind tunnel at a high angle of attack it will generate some lift, even a javelin does in the Olympic javelin event, and that is just a pole with pointed ends, to reduce the distance travelled they moved the CoG of javelin so it stalled earlier and kept the competition in a stadium. With the engine running the prop also generates some lift as well as thrust at a high angle of attack.

 

[ThomasP]

Hey Laurelix,

The manual the stall speed charts are from can be found on the wwIIaircraftperformance website, located here:

(http://www.wwiiaircraftperformance.org/mustang/P-51D-manual-5april44.pdf)

The page is 29 of 78, and off to the right of the chart (not visible in your attachment) it says:

"Warning: Airplane stalls at approximately 10 mph higher IAS than previous P-51 airplanes, due to a change in the airspeed system."