Designing a Biplane

I think a lot of it depends on the size of the plane. You can scale a plane but unfortunately air is not scaleable. If the original plan was built and flown successfully then you can probably stick with it even with changed airfoil. Tips shouldn't make that much difference unless you are doing something really radical.

My wildfire 36" span, 7" top wing cord, 6" bottom wing has a separation of

5" between the wings while my 45" Tiger Moth has 100% spacing.

Red S.

I have some hazy recollections, take them for what they're worth:

Air is happy being compressed, but when you ask it to diverge it tends to get turbulent. In a biplane, particularly one with thick wings, this translates into more turbulence on the upper surface of the lower wing (whichever one happens to be 'low' at any given moment), and consequently higher drag. Many designers compensate for this effect by rigging the wings such that the upper wing has a bit higher angle of attack than the bottom -- this ends up 'asking' the air to diverge less, resulting in reduced drag.

I honestly don't know how much of this is true, and how much is only good for fertilizing a dairy field. It would certainly be interesting to rig a plane so that you could easily change incidence and see the results.

On Mon, 02 Oct 2006 09:02:20 -0700, Tim Wescott wrote in :

That doesn't sound totally false.

There is definitely some compression coming off wings up to one chord below them. That, I think, is the cause of "ground effect," where the wing provides more lift close to the ground.

Having one wing too close to the other, while improving looks, may cause both wings to be less efficient.

Of course, the difference may not be that noticeable in an RC aircraft.

Marty

"Martin X. Moleski, SJ" wrote in message news:4521479f$0$24245$ snipped-for-privacy@news.astraweb.com... | On Mon, 02 Oct 2006 09:02:20 -0700, Tim Wescott wrote in : | | >Charlie wrote: | >

| >> I am building a biplane from a plan and have reached the point of | >> attaching the wings. This is a heavily "kit bashed" airplane. The | >> original plan had the wings set at about 70% chord apart. My changes to | >> the wing were to use a different, slightly thicker airfoil and to change | >> the wing tip shape which increased the wingspan a little. Most of the | >> design data or commentary I have found says the wings should be

100% to | >> 150% of upper wing chord apart, no explanations or discussion. That | >> much separation looks terrible. Maybe 80% would do but keeping near the | >> original 70% would be best for appearance. What effects are likely in | >> flying this plane with the wings this close together? | | >I have some hazy recollections, take them for what they're worth: | | >Air is happy being compressed, but when you ask it to diverge it tends | >to get turbulent. In a biplane, particularly one with thick wings, this | >translates into more turbulence on the upper surface of the lower wing | >(whichever one happens to be 'low' at any given moment), and | >consequently higher drag. Many designers compensate for this effect by | >rigging the wings such that the upper wing has a bit higher angle of | >attack than the bottom -- this ends up 'asking' the air to diverge less, | >resulting in reduced drag. | | >I honestly don't know how much of this is true, and how much is only | >good for fertilizing a dairy field. It would certainly be interesting | >to rig a plane so that you could easily change incidence and see the | >results. | | That doesn't sound totally false. | | There is definitely some compression coming off wings up to | one chord below them. That, I think, is the cause of | "ground effect," where the wing provides more lift close | to the ground. | | Having one wing too close to the other, while improving looks, | may cause both wings to be less efficient. | | Of course, the difference may not be that noticeable in | an RC aircraft. | | Marty

In theory, when the wings are not separated enough they act as one wing. Many of the older biplanes and one crop duster airplane, that I know of, has a more positive angle of incidence (angle of attack) on the top wing to aid in stall recovery. The top wing stalls first and since it is ahead of the lower wing the airplane pitches nose down on its own, aiding stall recovery.

Although it is a reverse stagger wing here is an example for the "Hot Canary" R/C model by Top Flite around 20 years ago:

Here is the full scale pylon racer that preceeded the model:

Charlie --

Remember this first: You will probably get all kinds of technical theory, but it's still best to use the LAR principal (Looks About Right). Good grief, man it's a model! Models are fun! So experiment and don't get caught up in dribble :-) If you lose some efficiency, so what? Your time is better spent building it straight and true and light.

As a sidenote, I just checked the only bipe I have flying now, a vintage Custom Live Wire as designed by the late Hal "Pappy" deBolt. He was a famed model designer. It flies great and the upper chord is 13" with the gap measuring only 7".

Cheers -- \_________Lyman Slack________/ \_______Flying Gators R/C___/ \_____AMA 6430 LM____ / \___Gainesville FL_____/ Visit my Web Site at

Thank you for all the responses to everyone who offered information and commentary. The information and commentary you provided have given me confidence in continuing along the lines of the original design.