swept-back top wing

[east6008]

I've always been curious about the reason for sweeping back
the top wing on the Pitts Special biplane.
It spoils the unity of shape, and looks lousy.
Another example is the Great Lakes Trainer.
Maybe it was to achieve balance, but I think it was
for better snap rolls?

[biber]

It makes it more easy to access the cockpit.

biber

[JetPlaneFlyer]

It makes it more easy to access the cockpit.

biber

Yes, and improved upward visibility for the pilot.

[MCarlton]

Could also have a function for bringing the CG back so less trim changes with 1 or 2 pilots, or pilots of different weights?

[coosbaylumber]

I have been doing some digging on the original Vought Corsair, the biplane. It was stated that the reason the two different upper and lower wings was for sight and C.G. as mentioned here.

Wm.

[macboffin]

Could also have a function for bringing the CG back so less trim changes with 1 or 2 pilots, or pilots of different weights? No way no day Hosea! Design doesn't work like that! With most fairly light aircraft, the second pilot or the passenger in a two place fore and aft seater sits on the CG, or very close to it. The aircraft design allows for the single pilot weight to be part of the total weight and balance equation.
Ever noticed that in the Tiger Moth, for example, if only one is flying, it's from the rear seat.

[MCarlton]

Well thats pretty much what I was getting at. Which comes first? The fuselage or the wing?

So is the fuselage designed so that the front pit is at the CG?

Or is the wing designed to the CG is at the front pit?

Look at full size sailplanes with forward swept wings. To my mind, the FSW is there so that occupying the rear cockpit makes no odds to the CG because its occupant sits ON the CG.

Are not 2 seat sailplanes usually flown solo from the front seat?

Therefore, the wing is designed to allow the CG to be sufficiently far forward that it can be as near to the LE of the wing root as possible. Surely that could work with occupancy on or behind the CG?

[macboffin]

Refer to Tiger Moth pic.Note fuel tank in center of top wing, like passengers/second pilot's seat, on CG. No trim change as fuel used.No trim change if flown solo or dual. Most biplanes, except fast/heavy ones, usually have bottom wing behind top wing to enable easier access to front cockpit,(Beech "Staggerwing" has "side" entry, and retractable undercarriage which is housed in bottom wing, so for it to be in correct position bottom wing in front.) Airplanes are designed as a whole, not piece by piece.Interesting to note that the "Tiger" is highly aerobatic, but only has ailerons on bottom wing.
(I learnt to fly on Tiger Moths many years ago at Croydon Airport, when it was London Airport!).

[MCarlton]

I think we're saying the same thing in different ways here. An old aircraft designer I once knew told me "the best designer is often the one who makes the best compromises" and I think this is the flip side of that coin, one design feature, for example, a swept wing, can yield and permit other design features and advantages.

What I was getting at was that say when the TM was designed, was the wing designed specifically to put the CG at the location where the front seat was to be, or was the front seat put where it is so as to be at the CG?

From what you say, it is both at the same time, which makes sense, I suppose in the early design visualisation stages, the whole thing is "imagined" in 3D and worked out from there.

Interesting regarding the TM though, did it not evolve from the original Gipsy Moth? Which had straight wings, with a fairly identical fuselage?

Leading me to think that when the TM evolved, some bright spark thought "hey, if we sweep the wing, then we'll avoid all those trim problems the last one had with different payloads and fuel loads"

[biber]

There are different approaches to the design task.
Either you try to improve a design when you encounter its weak points, but that often is just a kind of a repair.
Quite common, once a design gets actually built that the CG does not locate as planned, the most typical bugfix is to change the sweep by some degrees.

But the general goal is to hit the sweet spot more or less precise just by a sensible design.
And that involves a very foresighted approach to it.
You might decide for a high mounted wing or for a low mounted one just to better get the CG sorted without compromising the pilot's comfort.
Since nearly any design parameter in a plane affects all other ones, there is not the one and only way to do it right.

biber

[BMatthews]

If you look at any small plane that seats the pilot and passenger in tandem you'll see that the pilot is always in the seat furthest away from the CG. Otherwise there would be too large a CG shift with no passenger.

But yeah, when you see wings of this sort with moderate sweep it's most typically to allow easier access for the PASSENGER in the case of the Pitts and in the case of the old transition of the Gypsy Moth to Tiger Moth when it went from the straight to swept wings.

Back when I got a ride in a Murphy Renegade biplane if it had not been for the sweep in the upper wing putting the center section ahead a little I would not have been able to fit into the cockpit.

In other cases with monoplanes it may be due to moving the wing spar location so the pilot doesn't have it passing through his kidneys or it could even be a pure styling factor on the part of the designer or it could be a "quick fix" to make up for an unexpected shift in the CG location for some reason that prevents other solutions.

[Zor]

Along with all the mentioned reasons in this thread no one talked about lateral stability.

Sweeping the wing(s) back provides for lateral stabiliy along with dihedral.

[BMatthews]

Along with all the mentioned reasons in this thread no one talked about lateral stability.

Sweeping the wing(s) back provides for lateral stabiliy along with dihedral.

But typically with prop biplanes the sweep is less than 5 degrees.

It depends on where you find it but sources I've seen equate a sweep angle of 20 degrees with something like 3 or 4 degrees of dihedral. Basically the typical swept wing biplane isn't swept enough to make any noticable difference.

[Brandano]

Looking at pictures of the Gipsy Moth and the Tiger Moth I have the impression that not only the top wing is swept, but it's also more staggered. I suspect in this case the main reason for both the sweep and stagger increase was caused by the need to fit a heavier engine, and therefore shift the wing surface forward. The Fairey Swordfish seems another plane that has undergone the same "cure" http://richard.ferriere.free.fr/3vues/swordfish1_3v.jpg

[Bax]

Well thats pretty much what I was getting at. Which comes first? The fuselage or the wing?

So is the fuselage designed so that the front pit is at the CG?

Or is the wing designed to the CG is at the front pit?

Look at full size sailplanes with forward swept wings. To my mind, the FSW is there so that occupying the rear cockpit makes no odds to the CG because its occupant sits ON the CG.

Are not 2 seat sailplanes usually flown solo from the front seat?

Therefore, the wing is designed to allow the CG to be sufficiently far forward that it can be as near to the LE of the wing root as possible. Surely that could work with occupancy on or behind the CG?

Modern two-seat sailplanes have a compartment in the vertical fin that allows you to add weight when you have a second person aboard...or a rather heavy solo pilot. That's how they get the airplane to balance...and most modern two-seat sailplanes don't have forward-swept wings.

[biber]

Yea, but that makes the weight and balance thing a bit more complicated, which is always a thing that can hurt safety.
However, the Duo Discus and DG 1000 are two of the most modern two seaters in production and both have at least medium forward sweep in the inboard wing sections.
Still both need sensible ballasting, but less of that, than what would be necessary if the wings were perfectly straight.
For open class ships that's another story, but you'll see rarely any of them flown solo, anyway.

biber