straight wing aerodynamics

[downwind]

hello all,

Quick question up to what speed or mach number is a straight able to operate efficiently. (ie wings on a dash 8, saab 340, king air 200 etc...).

Thank you.

[Intruder]

Depends on the wing. The F-104 had straight wings and was supersonic...

[airfoilmod]

Bell X-1 1.21 F-104 2.2 Delta F102 SUBsonic. If Power is not an issue, straight works just fine. When the wings sweep, they present less frontal to the airstream, less parasitic drag for equal induced and lift. Some form of hybrid airfoil is preferred as in F-15 or F-22 ("double Delta"). The wing is a very large Library of text. What's the Mission? Low Speed Handling? Constant chord, straight. Except for Intruders F-104.Wings milled from solid Billets of Aluminum and bolted on in Anhedral, the Starfighter could kill you quickly on short final below 200 knots. Engineered Instability? B-2. Efficient hauling? 74, 75, AB, etc. This could get to be a long discussion.

Airfoil

[jb2_86_uk]

I read once about spitfire pilots breaking the sound-barrier during dives in dog-fights - I bet that was a brown-trouser moment

JB

[ChristiaanJ]

Quick question up to what speed or mach number is a straight able to operate efficiently. (ie wings on a dash 8, saab 340, king air 200 etc...).
Your question is too vague, really.
Define "efficient" ....

The aircraft category you mention runs out of "puff" way before you run into Mach effects.
In that case a straight wing is the simplest and most efficient solution.

The only exception is when for some reason you want to get rid of the horizontal tail, with such extreme examples as delta-plane hang-gliders and microlights, the B2, the Me163, the Vulcan or the Rutan Vari-Eze. Swept-back flying wings, and tailless deltas can be made stable in pitch, unlike a straight wing.

At what point you run into "efficiency" problems with a straight wing depends mostly on the wing profile and the wing thickness.

Strictly off the top of my head, without looking up references, I'd say with a straight wing, a modern wing profile and about 10 to 12% thickness (your examples), you will find the trouble starts at about M0.65, where the airflow over the wing will go locally supersonic (Mcrit) causing increased drag.
Depends on the wing and the aircraft.... it would appear a Spitfire has been dived to Mach 0.92, although its critical Mach number was certainly less.

So when you run into "Mach" problems, you have two solutions.

Make your straight wing thinner. Oops. Less space for fuel, structurally heavier and more difficult to manufacture.

Or use a swept-back wing, which "looks like a thinner wing to the airflow", but isn't in reality. So you can still house the fuel.
Swept-back wing have less ideal flying qualities than straight wings, but we've learned to deal with that for a very long time.

So.... a Fokker F-27 has a straight wing. Anything else would have been a joke.
A Boeing 737 has a swept wing. Again, today, anything else would have been a joke.

Beyond Mach 1?

The F-104 and Concorde were two extremes. Yet the currently proposed SSBJs (supersonic business jets) are still looking at those extremes.
So the jury is still out.

CJ

[henry crun]

jb2_86_uk: What is the title of the novel you read that in ?

[flynerd]

It has always made me wonder why there are not more forward swept wings in aircraft design. This would (presumably) increase the inward flow of air under the part of the aircraft rear of the COG and thus provide some "free" lift. Might also provide some pre-compressed for engine intakes.

With wings swept back, pressure eddies would be swept away from the rear of the aircraft and not provide any of the "free" lift.

Any informed thoughts?

flynerd

[airfoilmod]

I think google X-29? It debuted at Edwards when Fly By Wire was new.The rumor was that the initial TP was full of himself, strutting around claiming to be "quarterback" (Leader) of the test programme. On one flight,he was describing his role as "quarterback" one too many times, an engineer on frequency was heard to mutter "Quarterback? more like the Football." Tips forward means FBW since flex precedes load rather than following it, and Load can snap the wings clean in a millisecond. Not disqualifying of an advanced design, but Aft Load is preferable in manual to preserve high speed stability.

Airfoil

Also, adding airflow under the Fuselage/Tail is counterintuitive, more Drag than Lift results, and Lift is never Free. Trick Question: You have a Hose to blow high speed air at an A/C in Flight. Would you direct it under or on top of the wing? (To increase Lift)

[jb2_86_uk]

OK, I cant for the life of me remember where I read/heard about spitfires going supersonic during dog fight.

I wonder if I was "got" by some sensationalised documentary perhaps?

Aaanyway, done a quick google search and the closest I can find to anything like that is a post from someone on another forum (on the same subject) explaining that although the aircraft would not actually be exceeding mach 1, his instruments may say that he is, inaccuracies and what-not. This would lead to pilots genuinly believing they had broken the sound barrier.

Sorry for being naive, I shall be sure to check for substance behind these random "facts" that I seem to memorise, before posting next time :ok:

JB

[airfoilmod]

You're not alone. Those accounts are plentiful over here as well; on this side the speedy one is generally the P-47. Can one imagine that truck 1+?

On another tack, the account of Yeager's feat is challenged by USAF tales of Flight Test involving the new F-86 in 1947. The day before Major Yeager's Mach odyssey, a two ship of the brand new swept wing Saber (Yeager's X-1 was definitely straight wing) were dive tested and produced at least three very loud bangs in the afternoon at ~14,000 feet. Yeager still bristles at discussion of this "challenge" to history. General Yeager still flies, but as of today, only his friend Bart's Aviat Husky. He lives closeby in a suburb of Sacramento, 0-17, Grass Valley is our airport. He still commands awe in a room, and doesn't hesitate to assume command there-in.

Airfoil

[FE Hoppy]

One thing missing from this thread so far is the airfoil profile. The speed at which Mcrit occurs can be increased or at least the severity of the shock wave reduced by choosing a supercritical profile. Developed by Whitcomb at Naca along with area rule. If you compare the average sweep angle of new airliners to those designed in the 50's and 60's you can see the tendency towards less sweep. This is due to the use and development of supercritical Airfoils which also have a large internal volume.

So if you wanted a straight wing to fly at high subsonic speeds it would need to be as thin as your internal volume requirements would allow and use a supercritical airfoil.

The Saab 2000 shoots along and in the real world I would say thats about as fast as you will get without some new technology being introduced.

[henry crun]

jb2_86_uk: It is easy to be misled by what appear to be facts in print.

The question of instrument inaccuracies might have applied later but at the time of Sptifire dog fights in WW2 there were no mach indicators, the ASI's were calibrated in MPH and, IIRC, only went to about 550.

I also think one would have been hard pressed to find an RAF pilot at that time who even knew what the speed of sound is, let alone the effect of compressibility.

[ChristiaanJ]

OK, I cant for the life of me remember where I read/heard about spitfires going supersonic during dog fight.
I would say the confusion stems from the fact that towards the end of the war several fighters (not just the Spitfire) had enough power to run into Mach effects in dogfights, such as trim changes, buffeting, or even loss of control, caused by the airflow going supersonic locally, without the aircraft itself going supersonic.

I'll have to find a source for the max. Mach reached by a Spit in a dive. This was after the war, as part of high speed research.

[chksix]

I've read about pilots experiencing Mach tuck and being unable to pull out of the dive during WW2 so I don't think it's a myth.

(doesn't mean the plane or wing is supersonic though, just entering the transonic region)

[ChristiaanJ]

chksix,
I can assure you it's no myth!

The P-38 "Lightning" was known for its low critical Mach number. I've seen figures as low as M 0.65 quoted. This was due to its thick wing.

Figures for the Spit can be found here:
http://en.wikipedia.org/wiki/Supermarine_Spitfire#Speed_and_altitude_records

That lists M 0.89 as the maximum in a controlled dive, and M 0.94 in a somewhat less-than-controlled dive, which tallies with the figures I remembered from other sources.

[keith smith]

jb2 86uk
I know that there were trials at Farnborough towards the end of WWII involving Spitfire in vertical dives. The indicated speeds were close to M=1. However, at that time there were lengthy pipes between the static pressure head and the recording instruments, and at that time our knowledge of the resulting pressure lags was limited, and in steep dives could be appreciable. The Spitfire wing had a higher critical mach number than the Meteor, because of a thinner wing section