What's the purpose of Biplane "STAGGER"

[JonB]

Hi Guys:
Just looking for a CLEAR understanding of Bi-Plane stagger. I have seen bipes that have NO stagger and ones that have REVERSE stagger. Also, I understand that the upper and lower wings should/must? be set to different incedence angles? I presume this is to SOFTEN the stall point of the aircraft.....But I might be wrong here...
Sincerely,
Jon B.:confused:

[Ollie]

When a wing is producing lift, there is an upwash in the flow ahead of the leading edge and a down wash in the flow behind the trailing edge. The wings' influence on the flow around it extends several chord lengths above and below the wing (theoretically, all the way to infinity). The influence weakens with distance.

A biplane's wings influence each other.The leading wing is flying in the upwash of the lagging wing and the lagging wing is flying in the downwash of the leading wing. The up wash and downwash angles increase as the square of the angle of attack so that the inteference is strongest at high angles of attack just before a stall. These influences affect the angle of attack of both wings because the wings' angles of attack are not aligned directly to the line of flight. This has to be taken into account when setting up the incidence angles of the wings so that one stalls a little before the other.

Some airfoils have very soft stalls and their use can obviate the need to use the wings' incidence relationships to soften the stall. For example, the S4233 airfoil can increase its angle of attack by about 6 degrees after the onset of stall before much lift is lost! By steepening the glide angle or increasing thrust, the extra drag in this "mushy" region of partial stall can be overcome and the conditioned maintained for long periods of time.

[Captain Ed]

That was a great explanation, Ollie. I'm building a biplane from a Mountain Models Dandy Sport. I have an extra wing. I'd like your suggestion on the incidence of the upper vs lower wing. I have the upper wing about 1 1/4" forward of the lower one. Ed

[Ollie]

Ed,

It depends on the chord of each wing, the span of each wing, the airfoils of each wing, the sweepback of each wing, the taper of each wing and the vertical spacing of the wings in addition to the stagger you mention.

The desirability of a soft stall also depends on the priorities of the handling characteristics desired. If snap rolls are desired over a forgiving stall then you get one answer. If the otherway around you get a different answer.

If you want to use the stagger to set pitch trim you get a third answer which also depends on the horizontal tail area and moment arm.

I don't know all the specific answers or even how to get them without a little research on the web.

An airplane is a set of compromises flying in formation. How one strikes a balance between conflicting objectives depends on the purpose of the aircraft and the priorities and intentions of the designer. So the first burden on you is to give all the dimensions and set all the priorities. If you do that I will try to come up with the mathematical model and make some recommendations.

[winmodels]

Since the beginning of non- and motorised flight people have tried to find ways to keep wing aerea loading in a confined space (span) low.
Lilienthal (1895) came to the conclusion that he could not copy the stork's performance, so he doubled the wing aera (for the same span) by inventing the biplane!
Well, he got killed in the process - so you may call it a success story or not.
Ever since then the damn' bloody "double-decker" has been with us!
Or have you ever seen an animal with this strange arrangement?
Now, since it was there, the poor scientists had to deal with it.
And so they did!
Prof. Prandtl at the University of Goettingen developed the mathematically applicable "design codes" in ca' 1914-1917..........
and nothing has changed or added (of any importance) ever since.
"Stagger" is the horizontal displacement of the two wings of a biplane.
Prandtl developed codes to design the minimum resistance of an arrangement of two or more wings for an aeroplane. He proved, that a multiple wing arrangement allways has a higher resistance than a single surface wing of the same load carrying capacity. More importantly he developed calculus to determine the influence of stagger, vertical distance of the wings, the influence of dihidral and sweep.
The exploits of my HERO are now nearly forgotten but if you search the web for"Ludwig Prandtl" you will not only find the extaordinary history of that man, but also the mathematical solutions to your querries.
The reason for the physical arrangement of the wings on a biplane is always "physical" or may be we should call it "geometrical": low wing loading, position and range of the CG in respect of load, visibility for the pilot, ooops!! and that's it. Not much ....... but they look so nice!!!!!!!!!!!!!!!

Regards

Klaus

[frankenfoamy]

Stagger can also provide better visability (for the full scale pilot) and appearance in some planes.

http://rcguy.tripod.com/

[winmodels]

so we see that there are several reasons for stagger
and here you find information on how to deal with it:

http://naca.larc.nasa.gov/reports/1924/naca-tn-182/

Have a go

Klaus