Laminar Flow Airfoil

[davebender]

I'm no expert in this area but found the web page interesting.
Laminar Flow Airfoil

The P-51 Mustang was the first aircraft intentionally designed to use laminar flow airfoils. However, wartime NACA research data shows that Mustangs were not manufactured with a sufficient degree of surface quality to maintain much laminar flow on the wing. The RAF found that the Bell P-63, despite being designed with laminar airfoils, also was not manufactured with sufficient surface quality to have much laminar flow.

The B-24 bomber's "Davis" airfoil was also a laminar flow airfoil, which predates the Mustang's. However, the designers of the B-24 only knew that their airfoil had very low drag in the wind tunnel. They did not know that it was a laminar flow airfoil.

The boundary layer concept is credited to the great German aerodynamicist, Ludwig Prandtl. Prandtl hypothesized and proved the existence of the boundary layer long before the Mustang was a gleam in anyone's eye.

Another interesting bit from the same web site.
Wing Dihedral

Highly maneuverable fighter planes have no dihedral and some fighter aircraft have the wing tips lower than the roots, giving the aircraft a high roll rate. A negative dihedral angle is called anhedral. The AV-8B Harrier II above has a negative dihedral or anhedral

 

[J.A.W.]

Big turbines providing excess thrust enabling VTOL/viffing/blown aero are in a different category, surely..although the Tempest was also WW2 era with a 'laminar-flow wing' claim.

 

[davebender]

Did any WWII era aircraft have this design feature?

 

[tyrodtom]

The He162 had anhedral on it's outermost wings.

 

[J.A.W.]

tail di-hedral.. One of the few 1st gen jet fighters that did not exhibit hi-speed porpoising dynamics to spoil gun-aim..

 

[GregP]

The He 162 outer wing panels are so small, one wonders why they are even there ... except to guide the tip vorticies. Really, there's almost no area there.

 

[tyrodtom]

I think the anhedral wingtips on the He162 was to compensate for too much dihedral in the main wing, it saved them from having to redesigning the whole wing.
They were in a rush.

 

[GregP]

They are so small I think it was to eliminate vibration in the tail more than anything else, by moving the tip vorticies.

And without VIFFing, the Harrier is not very maneuverable, though it does roll well.

 

[nincomp]

Davebender
OK, this thread finally got me to join the forum. I studied aeronautics in college and even had a part-time job at a windtunnel, so the continual references to the P-51's "laminar-flow airfoil" has been a sore point for me.

Yup, it is true that the P-51's wing did not have any significant laminar flow. Extensive laminar flow is extremely difficult to achieve and requires incredibly exact tolerances and smoothness to achieve. In fact, until recently, high-performance sailplanes were about the only "production" aircraft to actually have extensive laminar flow over their wings. Even on these high-performance sailplanes, laminar flow could be lost from something as minor as a few bug-hits on the leading edge. There are stories of sailplanes landing miles short of their landing strips because they encountered some mist or rain. The water droplets "tripped" the air boundary layer from laminar to turbulent - increasing the wing drag and drastically reducing the glide ratio. Similar stories exist about sailplanes that happened to fly into a small cloud (swarm?) of gnats or other insects. As a sidebar... the stall characteristics of some laminar-flow wings became very ugly (abrupt) when the laminar boundary was disturbed. This might explain some of the P-51's handling characteristics.

Boy, do I feel better now that I got that off my chest!

The first article that you mentioned also mentioned the "Davis wing" that was used on the B-24. It too showed extremely low drag in the wind tunnel, but not in the production aircraft. I actually feel very sorry for the design teams that used the wind-tunnel data in their performance calculations. I can't imagine what they (or their bosses) did when the actual performance was very much less than predicted.
You might recall that the original P-51's performance was good, but not spectacular. My guess is that the wing performance was well below predicted.
- Luckily for the P-51, the British had the better performing merlin engine to use in it. In my opinion, this made all the difference.

The main (unanticipated) advantage of the P-51's "laminar-flow" wing is now well known: it delayed the effects of compressibility, giving the plane a high critical mach-number. This allowed it to dive faster than most planes while staying in full control. - Remember that the P-51 was designed before much was known about compressibility (airflow at or near the speed-of-sound). The designers had no way to test for this, so these high-speed characteristics were a very fortunate accident.

There is a section (Chapter Five) in the NASA document sp-468 "QUEST FOR PERFORMANCE The Evolution of Modern Aircraft" that has a good discussion of the actual performance of early production laminar-flow wings vs. the perfectly shaped wind-tunnel models.
This is a wonderful .pdf book (500+ pages) that you can download for free. I have lost the link, but you can find it by searching for "NASA sp-468."

 

[J.A.W.]

Ah no, the P-51s V-max was not better than the thick-winged Typhoon [525 IAS] significantly less than the 'laminar flow' winged Tempest [545 IAS], in fact the P-51s developed a bit of a blue-note terminal-velocity dive wing-shedding rep..but the P-51 was fast for the power it had available, on the level...

 

[nincomp]

As for dihedral:
Your source is technically correct, but as is often the case, the full answer is more complicated.
The mention of inherent stability is important. It includes both the center of gravity and effective dihedral. Basically, if there is sufficient inherent stability the airplane will automatically return to flying upright. If, however, the wings have too much effective anhedral (negative dihedral), the airplane will continually try to roll itself inverted. The tendency of the plane to invert itself can give it an advantage in maneuvers that require rapid rolling. On the other hand, the pilot or other control system must always actively fight to keep the plane rightside-up.

Inherently unstable aircraft are a bad idea (especially near the ground) unless there is some automated control system that is very rapid and very reliable. In some cases, a human pilot would be incapable of reacting fast enough to safely fly the plane if the automated controls fail. During the WWII era, I do not believe that the required automated systems were advanced enough to deal with significantly unstable aircraft.

Now, however, on-board computers are used to control unstable aircraft. It is therefore much safer to go after that last little bit of maneuverablilty.

 

[nincomp]

Ah no, the P-51s V-max was not better than the thick-winged Typhoon [525 IAS] significantly less than the 'laminar flow' winged Tempest [545 IAS], in fact the P-51s developed a bit of a blue-note terminal-velocity dive wing-shedding rep..but the P-51 was fast for the power it had available, on the level...

No argument. Note that I said "most planes." The wing sections used in the P-51 were simply better than many others in use at the time, not optimal for (or even designed for) the highest critical mach. As I said, it was simply a fortunate accident.

It is important to remember that as the "sound barrier" was approached, many new and unexpected problems kept appearing. Wing sections were only part of the problem. There was a lot to learn about things like design and location of control surfaces, the wing-fuselage transition, effective wing sweep, etc.

The main point that I wanted to make in my first post was about the lack of actual laminar airflow on the so-called "laminar flow" wings of WWII fighters.

 

[tomo pauk]

Maybe the real-world 'laminar flow' wings were the best real world approximations of theoretical laminar flow wings? Some designers and producers doing their job better than another?

Ah no, the P-51s V-max was not better than the thick-winged Typhoon [525 IAS] significantly less than the 'laminar flow' winged Tempest [545 IAS], in fact the P-51s developed a bit of a blue-note terminal-velocity dive wing-shedding rep..but the P-51 was fast for the power it had available, on the level...

In the left corner we have a single engine fighter with 1150 HP, doing almost 400 mph. At the right corner we have a SE fighter with 2150 HP, doing 5 mp/h more. So let's trash the 1150 HP airplane???
The wing-shedding got nothing to do with wing being of this or that tape, it was connected with U/C doors opening in high G maneuvers, and was cured by installation of door locks.

Hi, nincomp,

You might recall that the original P-51's performance was good, but not spectacular. My guess is that the wing performance was well below predicted.
- Luckily for the P-51, the British had the better performing merlin engine to use in it. In my opinion, this made all the difference.

We can take a look at the available engine power the 1st Mustangs have at disposal. With engine same as at P-40D/E, the Mustang I was some 40 mph faster. The greater top speed was not something that could be attributed to the wing, but speed difference was simply to great.
The P-51B and later models, with Merlins comparable to the Spitfires VIII/XI, were some 30 mph faster than those Spits.

 

[J.A.W.]

True, the Mustang was fast, for the power it had, on the level..
The P-40 similarly powered was much slower, high speed in the thick sea-level air has to be a function of a balance between power/drag, if you`ve got a 2000+hp Napier Sabre to unleash you can go [ if you`ve got a 3000hp N.S. you can do 417mph at S-L - Tempest 6 @ 17lbs boost] .
Mind you, the much maligned P-39 went pretty fast on not much hp, so did the He 100..

 

[tomo pauk]

I'm not sure what do you mean by 'on the level'?
The high speed is always a function of a balance between power and drag (until the compressibility kicks in, that is; plus, we have the exhaust thrust, applicable for all altitudes. The P-39 was a small airplane (wing area akin to Fw-190, or considerably less than of Spitfire), the undercarriage was fully covered when retracted, the weight was not excessive. Once the wing guns were removed from the -Q, it was doing circa 400 mph. We can note that P-39 was never maligned by Soviets, they always clamored for more.

 

[J.A.W.]

On the level, as opposed to going up or downhill.. The Mustang was not the fastest zoom/dive fighter inspite of its clean airfame..

 

[nincomp]

Maybe the real-world 'laminar flow' wings were the best real world approximations of theoretical laminar flow wings? Some designers and producers doing their job better than another?

Well... not really. It was virtually impossible to get the required profile accuracy and surface finish with the aircraft building techniques of the time. The use of fiberglass and other composites has made production of these precision airfoils much easier. To give you an idea of how sensitive the NACA 6-series laminar profiles are to imperfection, sailplane owners would occasionally use 1000-grit sandpaper, attached to precision aluminum straight-edges to carefully smooth the leading edge and upper surface, even if they could not see any flaws. Some claimed significant performance gains.

We can take a look at the available engine power the 1st Mustangs have at disposal. With engine same as at P-40D/E, the Mustang I was some 40 mph faster. The greater top speed was not something that could be attributed to the wing, but speed difference was simply to great.
The P-51B and later models, with Merlins comparable to the Spitfires VIII/XI, were some 30 mph faster than those Spits.

Good points. The P-51 was a very clean aircraft. As a matter of fact, when I recently read a lament that there was not a greater attempt to improve the P-40, I realized that the North American engineers were familiar with the P-40 (having been asked to build it themselves) when they decided to design the P-51. In some respects, the P-51 was, indeed, an updated P-40.

The improvements would have been much greater, though, if the theoretical advantages of the laminar-flow wing were realized. On paper, there could have been a 25-30% reduction of drag from the wing. It should be mentioned, however, that even using today's construction techniques, such an improvement would be very, very unlikely. It turns out that the wind-tunnel data for those airfoils does not scale up accurately to the long cords and high speeds of the actual aircraft.
- Sailplanes (and military drones) have much narrower wings and fly at lower speeds.

 

[tomo pauk]

On the level, as opposed to going up or downhill.. The Mustang was not the fastest zoom/dive fighter inspite of its clean airfame..

Maybe some numbers, like diving speeds, would help us to qualify the Mustang?

Well... not really. It was virtually impossible to get the required profile accuracy and surface finish with the aircraft building techniques of the time.

I agree that it was impossible to achieve 100% laminar flow with wings produced back in ww2, that's why I've used the "best real world approximations of theoretical laminar flow wings" statement.

The use of fiberglass and other composites has made production of these precision airfoils much easier. To give you an idea of how sensitive the NACA 6-series laminar profiles are to imperfection, sailplane owners would occasionally use 1000-grit sandpaper, attached to precision aluminum straight-edges to carefully smooth the leading edge and upper surface, even if they could not see any flaws. Some claimed significant performance gains.

Thanks.

Good points. The P-51 was a very clean aircraft. As a matter of fact, when I recently read a lament that there was not a greater attempt to improve the P-40, I realized that the North American engineers were familiar with the P-40 (having been asked to build it themselves) when they decided to design the P-51. In some respects, the P-51 was, indeed, an updated P-40.

Mustang shared appearance with most of inline-engined fighter; apart the engine, it shared next to nothing with P-40.

The improvements would have been much greater, though, if the theoretical advantages of the laminar-flow wing were realized. On paper, there could have been a 25-30% reduction of drag from the wing. It should be mentioned, however, that even using today's construction techniques, such an improvement would be very, very unlikely. It turns out that the wind-tunnel data for those airfoils does not scale up accurately to the long cords and high speeds of the actual aircraft.
- Sailplanes (and military drones) have much narrower wings and fly at lower speeds.

We have "theoretical advantages" and "On paper" in one side, and the production and usage realities at another. Seems to me that NAA was able to get the grips of stuff and produce it as best as the technology of the day was allowing. The others following the suit, either as good as NAA, or worse.

 

[J.A.W.]

According to RAE evaluation the Tempest listed the following V-max limitations:
IAS Height
370 @ 30,000ft
410 @ 25,000ft
450 @ 20,000ft
490 @ 15,000ft
540 @ 10,000ft

Comments from Tempest evaluation, "The fact is that it has the best acceleration in the dive yet seen at this unit"[ ditto zoom].

 

[tomo pauk]

Indeed, the Tempest has better 'allowances' for dive, P-51 was redlined at 505 mph indicated at 9000 ft.