Wing Lift Coefficient (CL_Max) of P-51D

[Elmas]

P-51D ROOT (BL17.5) AIRFOIL (p51droot-il)

P-51D TIP (BL215) AIRFOIL (p51dtip-il)

 

[Laurelix]

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."

Does that mean I need to add 10mph stall on top of 102mph stall at 9700lb weight?

if I did that I would have 1.3 CL_Max which is what I would expect from a laminar flow wing.

 

[ThomasP]

I think that you may have to get a hold of a US P-51B or C manual and compare the listed stall speeds. Or maybe a RAF pilots notes, they usually have the airspeed correction numbers toward the rear of the manual.

 

[pbehn]

Does that mean I need to add 10mph stall on top of 102mph stall at 9700lb weight?

if I did that I would have 1.3 CL_Max which is what I would expect from a laminar flow wing.

Why do you expect what you expect from a wing, laminar or otherwise? Choosing values that give you the answer you want is anti scientific. The additional 10MPH is due to a different measuring system or air speed system to determine the Indicated Air Speed the true air speed is the same for the two set ups with these wings.

 

[drgondog]

I've read the post, read all the comments - only have a little to add.

First, here is 'stall' and there is Stall and 'it depends'. For the purpose of developing an operations handbook, flight tests will focus on three conditions - Gear Down/ Flaps down, Gear Down/Flaps up and level fight 'clean'. There were no flight tests for non-asymmetric flight conditions which is a whole 'nuther ballgame in theoretical study.

Hard to do analytically because 'real' analysis requires very sophisticated tools to model abrupt surface changes (like landing gear wells, gaps, protuberances, etc) to point to flow properties and discontinuities (like separation) - all required to model real aircraft instead of wind tunnel models.

So, for estimates of CL vs alpha, the wing section properties for 2-D Lift to drag shape (optimal) is the start. Next calculate 3-D effect of Aspect Ratio to degrade the CL vs alpha and L to D from Optimal - with stall characteristics and shape after CLmax. Next further degrade Lift to Drag as function of wing plan Geometry. The eliptical gives lowest Induced Drag, then look at Tip to root chord ratios for further calculated increases to Induced drag - with 1:1 being least effective in mitigating Induced Drag from optimal based on planform and MAC.

Either look up your airfoil for Section properties and wind tunnel 2-D (infinite wing span) data - take your wing to the WT and develop your own. If proprietary, like NAA/NACA 45-100 there are complex math solutions to 2-D section properties.

You are are getting closer to the expected clean wing aero properties - but nowhere close to Performance studies as the System Lift and Drag properties (wing/body) are not yet fully determined. You need both and recall that CL and CD in performance calcs are Normalized on Wing Area but, for example, are calculated for the entire airframe weight and in inclusive (but not stated) of all lift and drag contributions of the fuselage and empennage.

Not to mention Exhaust Thrust, Carb pressure drag, Cooling Drag and Vortex contributions to Profile drag to all the other Parasite/Profile Drag delta's to add to Induced Drag contribution (high in proportion at low speed). Prop and Prop Thrust is carefully scrutinized for the low speed conditions.

Then later, take to full scale wind tunnel and see how you performed your preliminary analysis.

The above is particularly important for a Mustang -

This preparation and assembly of data/estimates in the preliminary design stage shapes a lot of discussion regarding washout considerations (another increment to parasite drag), aileron area, flap type and size, landing gear location (in/out of vortex from prop), etc. Much of the same data is also required to estimate take off roll and landing roll runout.

All interesting, but - a pilot wants to know what to expect from his airplane at low speed, dirty and all the above discussion is mostly noise. Personally I was also interested in the characteristics of breaking to final approach in a banking turn.

 

[drgondog]

I think that you may have to get a hold of a US P-51B or C manual and compare the listed stall speeds. Or maybe a RAF pilots notes, they usually have the airspeed correction numbers toward the rear of the manual.

Interesting or not) the airspeed indicators in B/C were more accurate up to 300+ mph TAS and the D was more accurate above 300 mph TAS. The D/K had a different instrument. I would have to dig a little for the details.

Of course gross weight was the real determinant of landing speed recommendations. For given 'equal' load outs the fixed equipment for guns and ammo were the primary (as you know) deltas given both equipped with fuselage fuel tanks.

 

[pbehn]

I've read the post, read all the comments - only have a little to add.

First, here is 'stall' and there is Stall and 'it depends'. For the purpose of developing an operations handbook, flight tests will focus on three conditions - Gear Down/ Flaps down, Gear Down/Flaps up and level fight 'clean'. There were no flight tests for non-asymmetric flight conditions which is a whole 'nuther ballgame in theoretical study.

Hard to do analytically because 'real' analysis requires very sophisticated tools to model abrupt surface changes (like landing gear wells, gaps, protuberances, etc) to point to flow properties and discontinuities (like separation) - all required to model real aircraft instead of wind tunnel models.

So, for estimates of CL vs alpha, the wing section properties for 2-D Lift to drag shape (optimal) is the start. Next calculate 3-D effect of Aspect Ratio to degrade the CL vs alpha and L to D from Optimal - with stall characteristics and shape after CLmax. Next further degrade Lift to Drag as function of wing plan Geometry. The eliptical gives lowest Induced Drag, then look at Tip to root chord ratios for further calculated increases to Induced drag - with 1:1 being least effective in mitigating Induced Drag from optimal based on planform and MAC.

Either look up your airfoil for Section properties and wind tunnel 2-D (infinite wing span) data - take your wing to the WT and develop your own. If proprietary, like NAA/NACA 45-100 there are complex math solutions to 2-D section properties.

You are are getting closer to the expected clean wing aero properties - but nowhere close to Performance studies as the System Lift and Drag properties (wing/body) are not yet fully determined. You need both and recall that CL and CD in performance calcs are Normalized on Wing Area but, for example, are calculated for the entire airframe weight and in inclusive (but not stated) of all lift and drag contributions of the fuselage and empennage.

Not to mention Exhaust Thrust, Carb pressure drag, Cooling Drag and Vortex contributions to Profile drag to all the other Parasite/Profile Drag delta's to add to Induced Drag contribution (high in proportion at low speed). Prop and Prop Thrust is carefully scrutinized for the low speed conditions.

Then later, take to full scale wind tunnel and see how you performed your preliminary analysis.

The above is particularly important for a Mustang -

This preparation and assembly of data/estimates in the preliminary design stage shapes a lot of discussion regarding washout considerations (another increment to parasite drag), aileron area, flap type and size, landing gear location (in/out of vortex from prop), etc. Much of the same data is also required to estimate take off roll and landing roll runout.

All interesting, but - a pilot wants to know what to expect from his airplane at low speed, dirty and all the above discussion is mostly noise. Personally I was also interested in the characteristics of breaking to final approach in a banking turn.

Before the first flight was any advice, guidance or estimates of performance given to the pilot or is it safer not to?

 

[drgondog]

Not quite clear what you mean. Predicted performance is about estimating boundary conditions for stall, top speed vs altitude, ROC, take off roll, etc. but not handling characteristics possibly predictable from Stability and Control analysis, etc.

So, advice might be "use a lot of runway and get your airspeed up, rotate and depart - to acquire a feel for rudder and elevator authority." or "keep gear down for first (several) test flights", or "perform some power off stalls at 5K to determine stall speeds while clean to compare against estimates.".

 

[Aeroweanie]

The answer to your question is contained in NACA TN-1302 "The Effect of Modifications to the Horizontal-Tail Profile on the High-Speed Longitudinal Control of a Pursuit Airplane" and NA-5548 "Wind Tunnel Data for XP-51B Airplane". I credit the aircraft with a CLmax of 1.49 at M=0.3 and this declines with increasing Mach number:

 

[GregP]

Did we ever come up with a lift coefficient for the entire wing?