Author Topic: Biplane Lift Characteristics? (Read 432 times)

Warspawn · « **on:** September 21, 2007, 07:48:18 PM »

Well, our Aerodynamics professor tossed one at us today to think about. I'd like to know if anyone has thoughts or experience about it. We're supposed to describe Biplane flight characteristics and lift; he kept asking questions about it, hinting that the lift isn't the sum of both wings.

The only thing I can figure out is that the struts on early biplanes spoiled the lift of the bottom wing, and that if the wings were too close together the low-pressure flow of the bottom airfoil would interfere with the higher-pressure air on the underside of the upper wing.

Also, would the vortices from the lower wings cycling up off the tip to the lower airpressure cause any kind of extra lift for the upper set?

Is the interference from the airflow of the wings the reason for some of the 'staggered' biplanes we see at airshows?

Thanks! I really appreciate any input here!

eagl · « **Reply #1 on:** September 21, 2007, 07:59:37 PM »

The top wing is often at a higher angle of incidence than the bottom one. That is one thing to investigate when trying to figure out what he's talking about. It means the top/front wing usually stalls first, and the lift remaining from the lower/aft wing will help lower the nose to aid stall recovery.

There are a ton of other issues as you say... Each wing has a different moment on the center of lift so each wing contributes differently to induced drag and drag from tail surfaces, even if the wings had the same angle of incidence.

Wing-fuselage interference drag and as you said, pressures between the wings and wingtip vortice effects, will change things too.

If you have time, see if you can look up why the beech staggerwing had the wings staggered backwards from most other biplanes.

eskimo2 · « **Reply #2 on:** September 21, 2007, 08:15:37 PM »

Would there be any sort of “ground effect” from the bottom wing acting like the ground to the top wing?

Warspawn · « **Reply #3 on:** September 21, 2007, 08:47:58 PM »

Quote

Originally posted by eskimo2
Would there be any sort of “ground effect” from the bottom wing acting like the ground to the top wing?

I hadn't thought about that. Perhaps it reduces drag for the top wing like being near the ground surface does? Would the trailing vortices from the top wing be eliminated by the bottom airfoil?

AquaShrimp · « **Reply #4 on:** September 21, 2007, 08:51:32 PM »

In the aircraft and vehicles section, I ran across a link that described the aerodynamic forces that caused biplanes to breakup in mid-air. It was always the top wing that broke. The forces of lift were much greater on the top wing, and on certain sections. Let me see if I can find it for you.

Dago · « **Reply #5 on:** September 21, 2007, 08:52:15 PM »

Don't forget to take into account the fuselage effect on the airflow on the low wing, and the reduced wing surface area.

AquaShrimp · « **Reply #6 on:** September 21, 2007, 09:02:48 PM »

Check out this link, it explains why the top wing gets so much more lift:

http://www.aviation-history.com/theory/dr1wing.htm

Warspawn · « **Reply #7 on:** September 21, 2007, 11:03:41 PM »

Found something interesting:

Interplane Interference: The interference between the airflows round the planes of a biplane. This is one of the serious drawbacks to the biplane arrangement. Interplane interference (sometimes termed "biplane effect") may be reduced by having the gap between the planes at least equal to the maximum chord and by the use of positive stagger, that is to say, by having the top main plane slightly in advance of the lower.

From: http://www.aeroplanemonthly.com/glossary/IJ_news_70041.html

So positive stagger and gap distance seem to help quite a bit here...

Halo · « **Reply #8 on:** September 22, 2007, 12:25:29 AM »

The propeller or propellers forces or force air between the two roughly parallel wings where it is compressed and then flows in a required higher temperature over the horizontal stabilizer, elevating the nose of the biplane until it ascends to its design altitude limit, at which point cooler air prevails, the nose falters, and the biplane descends straight into the ground. Which is why you never see jet biplanes and rarely even prop biplanes anymore.

WilldCrd · « **Reply #9 on:** September 22, 2007, 08:40:31 AM »

an interesting side note, The german tri-plane design was actually a 4 wing design. if you look at the landing gear there is a small wing between them which actually helped the plane in low speed maneuvering

Halo · « **Reply #10 on:** September 22, 2007, 12:47:47 PM »

Good nugget, WilldCrd.

Tac · « **Reply #11 on:** September 22, 2007, 01:58:07 PM »

since we are in topic.. is there any software out there that simulates a wind tunnel?

I remember reading a LOOOONG time ago there was a program that let you design a plane or whatever and put it through the virtual wind tunnel to test your idea out.