The P-51 and its laminar-flow wing

[Stoney]

Which aircraft in WWII had laminar flow wings?  P-51 I know and I seem to recall reading that the Tempest and Ki-84 did as well.

Just the Pony...  According to Dave Lednicer, the Ki-84 shared the 23000 series airfoil with most of the other WWII fighters.  The Tempest used a proprietary airfoil developed by Hawker, but if the plots I've seen of it are correct, it isn't going to be a laminar airfoil.  The P-63 used one, but I don't really count it in the mix.

[Stoney]

As to the cl/cd graph... this one compares the P-51 versus the Spitfire (I believe 14 model). You can clearly see that the lift created by the Spitfire wing exceeds the lift of the P-51 but only after a given AOA is exceeded does the drag begin to effect the Mustang.

Do you have the data points for those drag polars?  That chart looks like an illustrative comparison between a polar that demonstrates the "laminar bucket" and a turbulent airfoil.  Its good for showing the concept, but can't be used for quantitative comparison.  If you look at my graph, you can see the laminar bucket on the P-51 plot.  However, every airfoil I've ever seen, except for those designed with very high design lift coefficients will have more profile drag at 8 degrees of alpha versus 0 degrees of alpha.

However, remember that effective AoA is computed from the relative wind.  In a near 1G climb at high speed, it is possible to have a situation where there is 8 degrees of nose-up pitch on the aircraft, but the effective AoA is close to cruise AoA.

[Brooke]

Your going to lose that pile of money... but your large sum probably equates to about $15.

So the bet is whether this statement is true or not:  "The P-51 at 8 degrees angle of attack has more drag than the P-51 at zero degrees angle of attack," where we are talking about real-world P-51's as in service in WWII.  We'll give our wagered money to a mutually trusted third party and equally chip in to hire three aerodynamics experts to opine on it.  Winner is paid by the third party based on decision of the panel.

How much do you want to bet?

[Stoney]

So the bet is whether this statement is true or not:  "The P-51 at 8 degrees angle of attack has more drag than the P-51 at zero degrees angle of attack," where we are talking about real-world P-51's as in service in WWII.  We'll give our wagered money to a mutually trusted third party and equally chip in to hire three aerodynamics experts to opine on it.  Winner is paid by the third party based on decision of the panel.

How much do you want to bet?

I thought I had already nipped that one in the bud...   

[Brooke]

I thought I had already nipped that one in the bud...   

Only if Chalenge believes your modelling or opinion on it.  I'll take bets on that one, too.

[Brooke]

Which aircraft in WWII had laminar flow wings?

The B-24 had the Davis Wing, which provided some amount of laminar flow.

[Stoney]

The B-24 had the Davis Wing, which provided some amount of laminar flow.

Its imprecise to characterize "wings" as laminar flow.  Laminar flow is not a characteristic of wing design, it is a characteristic of airfoil design.  The Davis Wing was unique because it emphasized aspect ratio, chord thickness, and reduced wing area, not because it generated laminar flow.  The airfoil used on the Davis Wing created the laminar flow associated with the wing.  In most circumstances though, the amount of laminar flow created by the airfoil would still put it in the "turbulent" category.

[Stoney]

Only if Chalenge believes your modelling or opinion on it.  I'll take bets on that one, too.

Well, an actually, you don't even need the airfoil analysis I did for that, as you can derive the induced drag coefficient of the P-51 at 8 degrees AoA without it.  If the Cdi is greater at 8 AoA, then it must create more drag at 8 degrees AoA than at zero.  Of course, you know this Brooke.  (Actually, the design lift coefficient of the P-51 gives it zero lift at ~2 degrees AoA)

 

[Chalenge]

Stoney I believe the problem you are having is that you are unable (due to limitations of XFoil) to get away from the region of flight in which vortex drag (the poorly named "induced drag") is the overwhelming drag on aircraft. What you cant see in the comparison chart I submitted is the AOA points and airspeeds (constant CV) which were in the range where profile drag and parasite drag overwhelm vortex drag significantly.

Mapping a laminar profile at such a low speed (outside of radio control airfoils) is a little unfair and not at all practical considering the full range of these aircraft.

[Brooke]

[edited to get rid of debating terminology]

The Davis Wing was unique because it emphasized aspect ratio, chord thickness, and reduced wing area, not because it generated laminar flow.

The Davis Wing emphasized less drag for a given amount of lift.  To do that, it had those characteristics including a laminar-flow design -- intentional or not, I have no idea, but without the laminar-flow characteristic, according to the following, the Davis Wing probably would not have been adopted, so that part was perhaps most important.

http://en.wikipedia.org/wiki/Davis_wing

In most circumstances though, the amount of laminar flow created by the airfoil would still put it in the "turbulent" category.

Yes -- that was my point above.  Dirt, paint, bugs, footprints, design variation, skin wrinkles, scratches, airframe vibration, turbulence from the propeller, etc. all mean that in practice (not theoretically, or with a perfectly smooth version in a perfectly clean lab) those laminar-flow wings (or "wings based on laminar-flow airfoils" or whatever folks want to call them) did not fully produce laminar flow.  However, they did seem to have significantly lower drag than other non-laminar-flow designs and so were good wings in that regard.  (That might be because they still had laminar flow for at least a portion of their chord, as Chalenge says -- that is not the point I was debating with him.)

[Charge]

Pretty nice definition of laminar flow wing here too: http://www.aviation-history.com/theory/lam-flow.htm

-C+

[Chalenge]

Brooke I will agree with you that the majority of wings instill additional profile drag due to flow seperation and increased turbulence as the AOA increases. This is also true of the P-51s laminar wing but only after a point. I believe I recall that point being 8.8 degrees with the coefficient of lift dropping below a Spitfire at 10.2 degrees and drag increasing dramatically thereafter. The reason most people cannot picture this as possible is due to the image of general flow coming from directly ahead and level with a zero AOA line which is not the case. In order to picture the effects of a wing upon open air flow (non-wind tunnel) you can think of a wing as being a rotating vortex like a tornado as seen from above. As air flows into the vortex it is curved by the approach of the varying pressure zones. This is not what actually happens with a wing (a rotating vortex I mean) but the effect of oncoming air flow is the same. For the majority of wings the stagnation point where approaching flow meets wing leading edge will drop below the leading edges center point significantly further than it does with a laminar design. I think if you see that much clearly you can see the reason that profile drag does not increase for a laminar airfoil until the low drag limitation is exceeded.

I also do not believe the B-24 had any laminar flow (beyond 2-3mm). Please site anything other than wikipedia if you have it. From what I recall of airfoil theory the Davis wing was designed with a smoothly increasing profile in such a way that even though turbulence occured it did not cause rapid separation and the onset of the separation bubble occurs well aft of the minimum pressure point. The problem with this design is the profile is so large it causes issues at high altitudes and probably the only fix would have been more horsepower. I belive this fact is born out in historical accounts as well as the performance in AH.

[Stoney]

[edited to get rid of debating terminology]

The Davis Wing emphasized less drag for a given amount of lift.  To do that, it had those characteristics including a laminar-flow design -- intentional or not, I have no idea, but without the laminar-flow characteristic, according to the following, the Davis Wing probably would not have been adopted, so that part was perhaps most important.

http://en.wikipedia.org/wiki/Davis_wing

Yes -- that was my point above.  Dirt, paint, bugs, footprints, design variation, skin wrinkles, scratches, airframe vibration, turbulence from the propeller, etc. all mean that in practice (not theoretically, or with a perfectly smooth version in a perfectly clean lab) those laminar-flow wings (or "wings based on laminar-flow airfoils" or whatever folks want to call them) did not fully produce laminar flow.  However, they did seem to have significantly lower drag than other non-laminar-flow designs and so were good wings in that regard.  (That might be because they still had laminar flow for at least a portion of their chord, as Chalenge says -- that is not the point I was debating with him.)

I'm not trying to be obstinate Brooke--its important to understand the difference between the characteristics of wing design and the airfoils that are used on them.  Wings by themselves shouldn't be considered as "laminar" as wing design does not generally contribute towards laminar or turbulent flow.

Wing design considers planform shape, aspect and taper ratios, wing tip design, wing to fuselage placement, anhedral or dihedral, sweep, etc.  The P-51 wing design was, like the Davis wing, a remarkable step forward in aerodynamic development.  They further increased the efficiency of both wings by using airfoils with low profile drag.

Airfoil use introduces the concepts of "laminar" versus "turbulent" airfoils.  All airfoils experience laminar flow.  Generally speaking, any airfoil that can maintain theoretical laminar flow beyond the 30% chord region is considered a "laminar" airfoil.  Any airfoil who's boundary layer is interrupted short of that 30% region is considered a "turbulent" airfoil. 

The P-51 has a laminar airfoil, regardless of whether or not it achieves it in the service condition, because it fits the definition.  That same airfoil on a wing made of composites would definitely generate laminar flow, even in the service condition.  You are correct that service P-51s suffered from dings and dents that would have interrupted laminar flow in flight, but it still is considered, from a design perspective, to have a laminar airfoil.

The B-24, on the other hand, achieved most of its low drag characteristics as a result of wing design.  I haven't analyzed the airfoil in XFOIL, but from the Wiki description you linked, it would appear that the airfoil itself was a very efficient, turbulent (by definition), airfoil.

This may sound like semantics, but its not.

@Chalenge, I'll respond to your post later today...

(good discussion regardless...)

[Have]

I came by a MIT lecture video where Dale D. Myers, who worked on the P-51 design, mentions that scoop solution before starting his actual lecture about space shuttle systems engineering.

Here's the link if you're interested: http://www.youtube.com/watch?v=iiYhQtGpRhc

He talks about the Mustang briefly at about 16:00 mins into the video.

So if it wasn't the laminar flow wing that gave it it's high speed and
extensive range, what was it?

The most prominent speed secret was the dramatic reduction of cooling
drag.  Placing the airscoop on the belly just in front of the rear edge
of the wing removed it as far as was practicable from the turbulence of
the prop and placed it in a high pressure zone which augmented air
inflow.  Tests in the wind tunnel with the initial flush mounted scoop
were disappointing.  There was so much turbulence that cooling was
inadequate and some doubted that the belly scoop would work.  The
breakthrough was to space the scoop away from the surface of the belly
out of the turbulent boundary layer of the fuselage.  Further testing
showed that spacing it further out would increase cooling but at a cost
to overall drag.  Various wind tunnel tests established the spacing at
the current distance which represents the best compromise between
spacing out from the turbulent flow of the fuselage, drag and airflow.

With the flow into the scoop now smooth and relatively nonturbulent,
the duct leading to the radiator/oil cooler/intercooler was carefully
shaped to slow the air down (the duct shape moves from narrow to wide,
in other words a plenum chamber) enough from the high external speeds
to speeds through the heat exchangers that allowed the flow to extract
maximum heat from the coolant.  As the air passed through the radiators
and became heated, it expanded.  The duct shape aft of the radiator
forced this heated and expanded air into a narrow passage which gave it
considerable thrust as it exited the exhaust port.  The exhaust port
incorporated a movable hinged door that opened automatically depending
on engine temperature to augment the airflow.  The thrust realised from
this "jet" of heated air was first postulated by a British
aerodynamicist in 1935.   The realization of thrust from suitably
shaped air coolant passages is named after him and called the "Meredith
Effect".   Some have said that at certain altitudes and at a particular
power setting the Meredith effect was strong enough to actually
overcome all cooling drag; this is not regarded as being accurate by
most aerodynamicists.  It greatly contributed to overall efficiency of
the cooling system but never equaled or overcame cooling drag.

[Captain Virgil Hilts]

The radiator scoop on the P-51 is what is commonly called a boundary layer scoop, the same design was used years later as a hood scoop on race cars. It creates less drag than many scoops for the same amount of frontal area and air intake, creating something of a ram effect. Ramming air into the radiator with an efficient low drag scoop, then using the heated air to create thrust is almost like free HP. The gain is from taking a necessary function that is usually pure drag, and lowering the drag, while increasing the thrust, so instead of being almost pure drag, it actually contributes thrust, with lower drag as well.