Why is the top wing swept on some biplanes?

[RobZ]

Why is the top wing swept on some biplanes? What purpose does this sweep serve?
Rob.

[Tom Harper]

Probably depends on the spedific design.

However, it can provide a dihedral effect without the structural problems of raising the wing tips.

Whatever isuues are associated with positive and negative stagger are distributed over the span and may cancel out.

[vintage1]

Apart from the dihedral issue, there may have been structural or visbility issues - maybe the best place to strap the cabane struts was well forward, but the CG was too far aft for a straight wing there...maybe the pilot wanted more upward visibility in a fighter..

[hul]

easier access to the front cockpit

Hans

[RobZ]

I guess what I mean is aerobatic biplanes like the Pitts and models like Aeromaster. There must be some aerodynamic advantage to the sweep or they wouldn’t do it.
Rob

[vintage1]

I guess what I mean is aerobatic biplanes like the Pitts and models like Aeromaster. There must be some aerodynamic advantage to the sweep or they wouldn’t do it.
Rob

Not necessarily.

It takes more than just aerodynamics to make a plane fly

Having the pilot able to get in is also a bit useful...:D

[towgadget]

I guess what I mean is aerobatic biplanes like the Pitts and models like Aeromaster. There must be some aerodynamic advantage to the sweep or they wouldn’t do it.
Rob

You're correct. Sweeping the wings enhances the aerobatic capability of the airplane. Snap rolls are faster and easier to start and stop in an airplane with sweep. The Pitts is a good snap roller and the Jungmeister with swept top and bottom wings is even better.

FS Pitts driver.

[RobZ]

You're correct. Sweeping the wings enhances the aerobatic capability of the airplane. Snap rolls are faster and easier to start and stop in an airplane with sweep. The Pitts is a good snap roller and the Jungmeister with swept top and bottom wings is even better.

FS Pitts driver.
Ahh. So that's it. Thanks towgadget.
It made no sense in terms of pilot access because the root does not change position when the wing is swept, only the tips move rearward.
Rob.

[coosbaylumber]

I have been digging into the original Vought Corsair. Their reasoning for bending the wing was pilot visability and cockpit accessability.

They originally wanted to construct the upper and lower wing from one jig. To do this, meant the upper wing would have to be shifted quite forward. This then upset the C.G. for the A/C. The secondary solution was to bend the wing and thus bring the C.G. within reason.

Oddly, this increased the total A/C speed, and made it substantially more flexible. The whole two seat airplane then became faster and more manueverable than several front line fighters of the biplane era. It also meant the 1928 design lived much longer than it was intended to have been. Into the early 1940's.


Wm.

[Jinker]

It made no sense in terms of pilot access because the root does not change position when the wing is swept, only the tips move rearward.
Rob.

Bzzzt.

The DeHavilland Tiger Moth is another example of a biplane with a swept wings. If you sweep the wing, to keep the center of lift in the same location with respect to the center of gravity, you have to move the root of the wings fowards.

In the Tiger's case, this allowed them to place the front cockpit right on the center of gravity (so it doesn't change with only one pilot in the rear of the airplane) without having the wing right above the front cockpit, which would make ingress and egress very difficult.

-Greg

[RobZ]

You're correct. Sweeping the wings enhances the aerobatic capability of the airplane. Snap rolls are faster and easier to start and stop in an airplane with sweep. The Pitts is a good snap roller and the Jungmeister with swept top and bottom wings is even better.

FS Pitts driver.
Look. I'm not trolling for a discussion. I am sure there are lots of examples of planes sweeping wings for pilot access but, I wanted to know why do some aerobatic biplanes have a swept upper wing, the Pitts, Christian Eagle etc. and I had a feeling that it was more than making access easier. I thought it must have something to do with how it performs. Now I know.
Rob.

[Work in Progress]

RobZ

I'm sure this isn't your intention but it does sound a little as if you are only interested in hearing things which confirm your pre-conceived ideas. Curtis Pitts was a highly practical man and real-world engineer, and had no interest in building an aeroplane that introduced unnecessary risks into aerobatics.

Your statement "It made no sense in terms of pilot access because the root does not change position when the wing is swept, only the tips move rearward" is simply wrong. If you keep the MAC in the same place, the root MUST go forward.
Try moving the upper wing root back to where it would have to be to keep the MAC in the same place on a Pitts S2 or Eagle, and see if you can get into the front cockpit.
Even if you extended the centre-section struts to make some sort of horrible entrance possible, you would not be able to bale out, nor would you be able to see very much looking upwards and ahead. Both of these are prime requirements for aerobatics aeroplanes and Curtis would very likely have done it whether it helped the flick roll or not.
There are some minor aerodynamic advantages associated with leading-edge sweep concerned with flick rolling. There are some disadvantages in other pasts of the flight envelope, and in maintaining strength. Certainly when I flick-roll my Yak-52 I don't feel disadvantaged not having a swept wing, nor do the competitors in today's Unlimited class monoplanes which wipe the floor with the Pitts in all aspects of their handling and performance. No disrespect to the Pitts, I also fly a Pitts S2 as well as the Yak, and like it very much, but it must be seen in context as a vintage design, albeit an excellent one. The science of aerodynamics has moved on a great deal in the 60 years since Curtis laid down the basic outline of the Pitts S1.
Incidentally, the straight-wing Dh60 Moth family has significantly nicer aerobatic handling than the Tiger Moth with its swept upper wing.

[Bill Mixon]

Yep,
The sweep and stagger have a lot to do with placing the pilot. The pilot needs to be on or near the CG, yet he also has to be able to get in and out of the plane. ;)

[yakman]

:cool: looks good

[dgfisher2004]

RobZ

I'm sure this isn't your intention but it does sound a little as if you are only interested in hearing things which confirm your pre-conceived ideas. Curtis Pitts was a highly practical man and real-world engineer, and had no interest in building an aeroplane that introduced unnecessary risks into aerobatics.

Your statement "It made no sense in terms of pilot access because the root does not change position when the wing is swept, only the tips move rearward" is simply wrong. If you keep the MAC in the same place, the root MUST go forward.
Try moving the upper wing root back to where it would have to be to keep the MAC in the same place on a Pitts S2 or Eagle, and see if you can get into the front cockpit.
Even if you extended the centre-section struts to make some sort of horrible entrance possible, you would not be able to bale out, nor would you be able to see very much looking upwards and ahead. Both of these are prime requirements for aerobatics aeroplanes and Curtis would very likely have done it whether it helped the flick roll or not.
There are some minor aerodynamic advantages associated with leading-edge sweep concerned with flick rolling. There are some disadvantages in other pasts of the flight envelope, and in maintaining strength. Certainly when I flick-roll my Yak-52 I don't feel disadvantaged not having a swept wing, nor do the competitors in today's Unlimited class monoplanes which wipe the floor with the Pitts in all aspects of their handling and performance. No disrespect to the Pitts, I also fly a Pitts S2 as well as the Yak, and like it very much, but it must be seen in context as a vintage design, albeit an excellent one. The science of aerodynamics has moved on a great deal in the 60 years since Curtis laid down the basic outline of the Pitts S1.
Incidentally, the straight-wing Dh60 Moth family has significantly nicer aerobatic handling than the Tiger Moth with its swept upper wing.

Right on the money. Couldn't have said it better.

[tommy321]

Speaking of wingsweep and CG location… there are several full size two-seat glider designs (Schleicher K-13, Puchacz) that have forward sweep on the wings. These are mid-wing gliders with tandem seating directly in front of the wing. The main reason they do this is to move the wing aerodynamic center forward closer to where the rear pilot sits. If they didn’t sweep forward, the main spar would have to pass through the rear pilots chest (ouch!). With the forward sweep, the spar passes immediately behind the rear occupant and the rear cockpit can hold anywhere from 0lbs to 250lbs without busting the CG range. (Solo flight is done from the front seat)

You certainly wouldn’t design forward sweep into a wing unless you absolutely needed to for some reason. You need a much stiffer wing to avoid flutter if there is forward sweep… (but that’s another thread :P)

Tom