barit1

Interesting point. The P-51 was initially supplied to the British as the APACHE, with an Allison engine. Carbureted and non turbo, it was a failure. The a/c was re-engined with the Spit's Merlin, (eventually made under license by Packard) then fitted with its Belly tank, and there you have it.

Sharing the specs and patents was less than important when the B-17s couldn't reach Berlin w/o losing escort (fighter range issues). The Mustang arguably ended the European War. It certainly gave Hitler's laundress some repellent work, eh??

Yeah, but the Brits called their first batch the Mustang Mk I not Apache. The Apache was the US dive-bomber A-36 version (which was a political animal, apparently, funded from a different wallet...).

The Mustang name stuck for all other versions and customers. :ok:

P-51D root

http://www.ae.illinois.edu/m-selig/a...s/p51droot.gif

Tip

http://www.ae.illinois.edu/m-selig/a...ts/p51dtip.gif

The cusp on an aerofoil (not all have them) refers to an aerofoil where the trailing edge is reflexed down forming a concave profile on the lower surface at the trailing edge. Typical of super critical aerofoils as here.

http://www.aerospaceweb.org/question...l/whitcomb.gif

barit and Jane,

Of course my suggestions are also ambiguous, because of where the "point" might be located - on the surface, or on the chord (which is normally taken to be the axis of coordinates).

I asked by aerodynamics guru, who said he understood it as meaning the highest point on the upper surface of the wing, and pointed me to Shevell, (his edition p193; my, first, edition, p188, first page of Chapter 12), where it is defined as the "point on the airfoil upper surface to which the freestream is tangent".

So, yes, it seems to be a common term, but it means the second of my proposed definitions, not the first!

PBL

barit1,

Yeah, it is unusual. Regardless, the P-51 ended up one as the USAAF's best fighters. Fast, nimble, and possessed sufficient range to escort strategic bombers to and from their targets.

They did if I recall, buy the plans for the Curtiss P-40 though. I don't know if any of the P-51 was derived from the P-40, though I suppose there are some superficial resemblances.


bearfoil,

Yeah, the V-1710

Why was the USAAC/USAAF so averse at first to using turbochargers? They work better than superchargers, and the technology existed since at least 1918.


Brian Abraham,

Yeah, the instant I saw the cross section I noticed the parallels. A supercritical foil is flatter on the top, a little blunter in the front, with the trailing edge drooped, and the cusp on the bottom side.

I assume with the shape of the foil being what it was, they weren't just trying to produce laminar flow over the wing, but to also weaken shockwave formation at high speeds (That or they just got lucky with the latter...)


PBL,

I actually thought that was a common aerospace engineering term.

I suppose when I said it I was thinking of the point of maximum upwards deviation from chord as well as the thickest point on the wing. I didn't actually give it as much thought (the fact that the two are not necessarily the same) as I probably should have.

Jane-DoH

I think at first, North American wasn't into the "Spirit" of the fight.

Superchargers were of course in service. The Lockheed "Lightning" or P-38, had turbocharging, each unit was aft of the Engine mounted on the Boom. The engines were contra rotating (There was a Left, and a Right, problematic in the field). This a/c was developed pre War, by Kelly Johnson, one of his first masterpieces. So turbo was not on the Apache, but one will have to admit the P-51 iterated into one beauty.

The wing is essentially symmetrical. The Angels are in the details.

cheers

bear

barit and Jane,

It seems I just munged my original note, but it doesn't matter.

Of course these suggestions are also ambiguous, because of where the "point" might be located - on the surface, or on the chord (which is normally taken to be the axis of coordinates).

I asked my aerodynamics guru, who said he understood "crest" as meaning the highest point on the upper surface of the wing, and pointed me to Shevell, (his edition p193; my, first, edition, p188, first page of Chapter 12), where it is defined as the "point on the airfoil upper surface to which the freestream is tangent".

So, yes, it seems to be a common term, but it means the second of my proposed definitions, not the first!

PBL

Not exactly. Shevell's depends on the angle of attack. Thinking of the airfoil as such, I think your (and your aerodynamics guru's) definition is better than Shevell's.

regards,
HN39

Good point! I'd missed that.

The aerodynamic significance of the crest, according to Shevell, is debated: "largely an empirical matter". One idea is that, in the transonic regime, supersonic flow forward of the crest does not contribute much to drag, whereas supersonic flow behind it would contribute a lot to drag, because of the "resulting low pressures (suction) on the aft-facing surface". Another possible explanation is that the crest represents the minimum distance between airfoil upper surface and (undisturbed) free stream, and if Mach > 1 at this point the "flow will accelerate in the diverging channel behind the crest, [leading] to a high supersonic velocity, a strong suction and a strong shock".

This suggests that the AoA-dependent definition of "crest" is aerodynamically significant in the transonic regime. Whereas I don't see at present that my second definition has much aerodynamic meaning at all - but I am willing to be educated!

PBL

Does this count? Circa 1923..

Autogyro-C4 | Aircraft |

"The rotor blades were of Eiffel 101 symmetrical section 28 in wide"

1929..

Albatros L 79 - Wikipedia, the free encyclopedia

1926..
Google Books also finds an edition of Flight from 1926 that appears to refer to a monoplane with a symetrical section and full span flaps tested at Farnborough...but only snippet vew prevents me being sure and identifying which aircraft.. Page 613..

Flight International - Google Books

Jane-DoH:
The Army Air Corps was NOT averse to turbos. Republic, Lockheed, and Curtiss all built prototypes using Army-specified turbos, and the P-47 and P-38 (both with innovative layouts) won production contracts. Likewise the B-17 and B-24.

Had NAA proposed the Mustang first to the US Army, it likely would have had a turbo just to win the contract; but the British were first to the table, and (at that time) they had little use for the turbo. The British even bought "P-38s" (Lockheed 322) hobbled by lack of turbosupercharging.

In fact, the Army flight-tested a LaPere bipe with a GE-turbosupercharged Liberty engine in 1919, setting a new altitude record in the process. In 1936 they had Lockheed build the XC-35 research aircraft with turbos and a pressurized fuselage - the first aircraft to used the monocoque skin as a pressure vessel.

New question: What was the first aircraft to use supercritical airfoil AND turbosupercharged engine(s)?

My apologies for introducing cusps into the conversation when I should have been researching crest.

You sure you don't mean laminar airfoil and turbocharging barit? I know the answer to that one.

You are absolutely right barit & don't forget the Bell XP-39 that had excellent performance with a turbo, but so equipped had no room left for guns & ammo. The AAC were so enamored of turbos that they refused to let Allison spend any money to develop two-speed or two-stage superchargers for their V-1710. All high altitude applications would be turbo-charged. When no one except Republic, with their huge P-47, was able to develop a successful single-engine turbo fighter, the US was left with P-39, P-40, P-51A that were dogs at altitude.

Of course, the pre-war AAC bomber boys thought their high-flying formations of B-17's & B-24's didn't need fighter escort. They paid the price in Europe with greater losses than the Marines in the S. Pacific.

As a non-engine freak, can I ask a rapid question?
"What's the difference between a supercharger and a turbocharger?"
As I understand it, a "supercharger" is a compressor driven from an auxiliary gearbox on the engine shaft, while a "turbocharger" or "turbosupercharger" is a compressor driven from a turbine in the engine exhaust, both feeding the compresed air back into the engine inlet manifold.
Right or wrong?

Comet
 

Slightly tangentially, I've read that the Comet 1 had a symmetrical aerofoil. Why would a jet transport require that? It seems to have been the cause of its ground stall.

ChristiaanJ

Hi. To me, a supercharger is a device used to increase gas pressure in the combustion chamber, any device. Turbocharger, or Turbosupercharger are self explanatory. A purely mechanical device, driven by sapping some of the available energy from the crankshaft, is a Blower.

A nifty combination is the Turbo compound system, used in something of reverse fashion. The Exhaust turns a turbine that drives an intake turbine, and excess energy is replaced via a geared shaft at the crankshaft, making theoretical use of "all" the tapped energy, and increasing dramatically the net Power available to drive a Propellor. It was used on the Corn Cob engine, the four row seven cylinder per bank on the Connie. The exhaust from the exit to the airstream of the system would turn the tip of the exhaust White. It was quite dramatic and prompted more than one passenger to inquire, "Is that normal?".

Iron Duck I want to say that the Bell X-1 wing was symmetrical.? The body had a purpose built shape of a .45 caliber bullet, the designers agreed that that projectile went super, and I think that is a true story.

You are correct ChristiaanJ. Supercharger is mechanically driven, turbocharger is exaust driven. Turbocharger is more efficient since it does not take power from the crankshaft.

BA asks me -Well, OK, I misspoke.

And offhand, I DON'T know the answer.

bearfoil:

You have a couple different engines/systems confused.

A "baseline" supercharger is a centrifugal compressor, gear-driven from the crankshaft, at perhaps 10:1 drive speed.

A turbosupercharger uses exhaust gases to turn a turbine and thus a centrifugal compressor. It's mechanically separate from the engine, and has been used on many installations including the P&W Wasp Major R-4360 "corncob". But that was NOT the Connie engine.

The Super Connie (and DC-7) used the unique Wright Turbo Compound engine, with exhaust-driven power recovery turbines (3 of them, each fed from six cylinders) geared into the crankshaft to send more "free" horsepower to the engine, gear-driven supercharger, and prop.

The advantage of the turbocharger is that its speed is determined by the density of the air it is compressing, to some extent independently of the engine rpm. This means as you fly higher, and the incoming air is less dense, it will rotate faster until the air exiting the compressor is at approximately the pressure it would be at a lower altitude.

The crankshaft driven supercharger on the other hand will run at the same speed regardless of altitude (for a given engine rpm) so its effectiveness reduces with altitude. The most efficient way round this would be a variable speed drive to the supercharger, so it speeds up with altitude. Some Merlins had a two speed supercharger as a way of partially overcoming this. An less efficient method is to dump some of the output of the compressor at low altitude, and close the dump valve as altitude increases.