Hello:

About fifteen years ago I built a Precedent BiFly (48" span), it was fun but heavy. So I built a light weight version with ailerons on both wings, symmetrical wing section and a bigger fin - and it was wonderful! Now back in the present day I am thinking about building another aerobatic bipe. Unfortunately I have lost my bi-fly plan and am therefore not sure what would be the best wing / stabilizer incidence combination to go for.

Firstly does anyone know what the By-Fly stabilizer and wing incidences were set for?

Secondly what are the recommendations out there? I see that 0-0-0 is not uncommon nowadays, but I’m pretty sure the Bi-Fly was not.

also I think I'm a little confused on how to properly state incidences. A sport scale Hawker fury (30's biplane) plan I am current studying states incidences are stabilizer negative 2 1/2 degrees, upper wing positive 3 degrees and lower wing positive 2 degrees. When I tried to measure them for myself using the centre datum line as 0 degrees, incidences are stabilizer negative 6 degrees, upper wing positive 5 1/2 degrees and lower wing positive 4 degrees!

Anyway I'm pretty sure I want the top wing to have slightly lower incidence than the bottom. Whatever I build should be very aerobatic and neutrally stable.

Any advice would be appreciated!

Hello
afraid I know nothing about the Bi-Fly (and some will argue I know nothing about aerodynamics when they read this!) but I have always understood that the idea of a greater incidence on the top wing was that, due to the upper wing being forward of the lower one (stagger) the upper wing would stall 1st and bring the nose down. The other way round, as you mention, would by the same theory put the plane into a deeper stall.
Just my thoughts - anyone else?

cheers
Rick

I have built and flown 7 Aeromasters (60-90 size), two Phaeton90's, two Phaeton40's, three Tiger Moths, several others and all flew best with the upper wing at about -1.5 to -3 degrees incidence relative to the lower wing. I have made several experiments where I was able to vary the incidence on the Aieromasters and Lazy Aces and found that this was always the best setup for any airobatics. If you can, build the model in such a way that you can reliably vary the upper wing incidences and do your own experimenting. You will find a point where the plane suddenly grooves just the way you like it.

I like to use zero lower wing incidence and zero horizontal stab incidence on my own design aerobatic bipes, and no zero thrust offset. A degree, or at most, two degrees negative upper wing incidence seems to slightly help aerobatic line holding. I try to get as much of the vertical tail area under the thrust line by use of large subfins and subrudders, removing the need for right thrust.

Wind tunnel tests that were conducted back in the 1920s revealed that giving the upper wing slightly more incidence reduced the induced drag of the combination a bit, whether stagger was used or not. Very little effect was noted on stall characteristics as a result of positive stagger. This may be because when a wing stalls, its lift decreases slightly, but its drag goes through the roof, which would tend to produce pitch-up, opposing the pitch-down caused by the loss of lift.

The same tests also revealed that downwash from the upper wing evidently kept the lower wing in business long after the upper wing had stalled, even with zero stagger, although positive stagger slightly increased the effect. Negative stagger, where the upper wing is slightly rearward of the lower wing produced only slightly more abrupt stall and slightly increased induced drag. The lower wing was found to remain largely unstalled up to about 30 degrees angle of attack.

A bipe with positive stagger has, in effect, negative stagger when flying inverted, and I have noted only slightly more abrupt stall when inverted, and a faster rotation rate in inverted spins.

dunno about tchaikowsky but that has certainly helped me, lads. I have a Wots Wot on the books to build next and this will help me from making a complete horses backside of myself!

Thanks for that

cheers
Rick

Thank you all for your advice. I think I'm going to go with the following: Stabilizer 0.0, Lower wing 0.0, upper wing 1.0 negative. Engine 0.0 down thrust, 1.0 or 2.0 degrees right thrust.

Happy flying!

Cheers.....

I recently completed a Phaeton 90 with wings and stab at 0. Engine was given 2 deg down and 2 deg right thrust.

It required 8 clicks of down trim that was nuetralized by shimming 3/16 at the aft upper wing struts to yield abt -2 deg upper wing incidence. Rolls are better, sink rate for landings is better and very little down is now needed for inverted.

However, Balsa USA on their web site cites the top wing should have 1 - 1.5 deg positive relative to the bottom wing. To accomplish that would require releiving the dowel hole and shimming the front of saddle and forward N struts at the lower wing, harder than shimming the upper wing but doable. I don't however want to do it unless someone makes a very good arguement for doing so as currently I deem flight trim to be good and very well in line with what Rodney says has worked for him.

Hi all !

Does anyone have any idea where to buy a BiFly 25 kit ?

thanks

Thank you AA5BY for your comments: I think I better incorporate a bit of down thrust! I'm probably going to stick with 1 degree negative because the top wing on the bipe I'm building has about 30% larger area than the bottom.

Strykaas I'm afraid that what I have found is that the bi-fly has been out of production for some time - I've seen a few on Ebay but that's about it. I must say my original bi-fly (built from a kit) was VERY heavy and consequently did not go all that well. It was only once I had built another one from the plan with some major modifications (including losing 1 3/4 lbs) that it performed well.

Right now I'm building my own design but I must say of all the kits I've been looking at the Phaeton sounds like an easily obtainabe and excellent option.

Cheers!

AA5BY, your experience with setting the upper wing negative jibes with all of mine. I also found that this greatly improves the inverted flight characteristics, in fact two of my bipes will fly hands off inverted as well as right side up with no trim changes. This was accomplished by judicious choice of upper wing incidence.

Regarding the down thrust, I agree with the expressed thought that biplanes suffer a positive pitch moment caused by the drag of the upper wing. Some of that has to be countered by negative wing incidence but it is reasonable to conclude that drag increases with speed so that increase should be dealt with by down thrust, which increases negative pitch with speed.

The decision to add the down thrust is a good one.

However, I should clarify that after more flights, the two degrees recommended down thrust that I gave to my recent Phaeton 90 build is possibly slightly too much. When power is fully reduced for landing (necessary for this floater), it pitches up a very slight amount, which seems to me to be saying, I need a slight bit more negative incidence and slightly less down thrust.

When getting around to the changes, I'll offer comment but wanted to report that I think two degrees down thrust was slightly too much.

Hello all,

I have bought a 100% (poorly) built BiFly25 and I find it very heavy indeed. What could I do to make it lighter, especially the wings ? I already plan to replace the stock heavy balsa ailerons by built ones. I could gain a lot by replacing the wire strut holding the upper wing and replacing it with some ply...
I do not have the plans.
What do you think ?

Hello
I have bought a BiFly .25 2nd hand.Could you please tell me where I should set the center of gravity ? I have no set of plans for this bipe. Thanks a lot !

I can tell you If you suspend the plane from the wingtips at the high point of the curve on the top off the wing you'll be close.If the wings are not directly on top of each other meaning one is forward from the other you may heve to do it from both points and it should be between the two

OK thanks !! CG is by definition located around 25% of MAC. I'll go that route first, and average it for both wings which are not strictly laid one above the other.

Actually nothing could be further from the truth. CG is set by a few things and seldom does the ideal CG position come out at 25% MAC. Look at one of the online CG calculators and all the various numbers that are not related to the MAC of the wing to see what goes into finding the CG location.