Bi-Fli (oz823)


Bi-Fli (oz823) by Phil Kraft 1959 - plan thumbnail

About this Plan

Bi-Fli. Early RC stunt model. 47in RC Stunt .15+, 3-chan escapement bipe, Phil Kraft, Oct 1959 MAN.

Update 24/11/2019: Added article, thanks to MikeFoster.

Quote: "Full-scale biplanes are no longer produced except for special purpose types such as Grumman's new crop duster. Their relatively low efficiency clue to the higher induced drag of two wings has relegated this oldest aircraft type to rapid extinction. However, for our purposes, the lower lift-drag ratio is not a disadvantage and, from the theoretical standpoint, the biplane seems to be the most logical approach to RC stunt. There is little question that the biplane is the most maneuverable type of aircraft. Virtually all of the full scale airplanes used in stunt competition were biplanes.

The reason for the biplane's inherently superior maneuverability lies primarily in its shorter moments and concentration of mass and area which reduce the effects of inertia. Thus, the biplane can respond more quickly and smoothly to control movements. Besides maneuverability, the biplane offers greatly increased stall resistance. This is because the rear wing is generally set at a slightly lower angle of attack than the front. In a stall, the rear wing retains its lift longer than the front and since it lies aft of the center of gravity the nose of the airpla'ne will drop to regain flying speed without the abrupt loss of lift and sharp dive associated with monoplanes. Ground handling, take-off, and landing characteristics are also excellent. From the model standpoint, the reduced overall dimensions of the biplane provide a stronger, lighter aircraft which is easier to transport.

Theory and practice do not always coincide, but the apparent advantages of the biplane layout prompted the design of the Bi-Fli. A great deal of thought went into the design in an attempt to produce a maneuverable yet stable, smooth-flying model suitable for any type of control from rudder-only to multi-channel equipment. For flight smoothness, a moderate wing aspect ratio (6-1) and fairly long tail moment with plenty of stabilizer area has been used. Logically, the two wings should be the same size, but for appearance sake the lower wing was designed 10% smaller. While free-flight experience has taught that an unduly scientific approach to airfoil selection is rather senseless, the NACA 23015 section was selected because its characteristics seemed ideal as to center of pressure travel, etc. More important, the section looked 'right.' Negative stagger was seriously considered, but the more conventional positive stagger was decided upon to simplify landing gear installation.

The first Bi-Fli weighed only 37 ounces and was powered with the K&B .09. The ship was a beautiful flier, living up to all expectations, but it was a bit slow due to the very light wing loading..."

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Bi-Fli (oz823) by Phil Kraft 1959 - model pic


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Bi-Fli (oz823) by Phil Kraft 1959 - pic 009.jpg
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User comments

AFAIK it was single-channel with 3 *cascaded* (Bonner Varicomp) escapements, as mentioned in the plan.
UStik - 08/01/2017
Please find attached photos of my recently completed & flown Phil Kraft Bi Fli built from your OZ plan 823 [more pics 003-005]. When I first saw the Bi Fli in an old magazine found at our local hobby shop I was hooked on the design. Must have read over the article a dozen times. Quite some time later I found the plan on your site, downloaded, printed it off & started construction right away. I made a few changes to the construction of the nose area to suit modern motor mounts & fuel tanks. I also intended to use servos instead of three escapements! The engine used is an OS 15FP with 8-5 prop - which worked out well - not over powered but enough to allow takeoff's from our grass field. This model has a terrific flat stable glide. The fuselage is covered with silk span & dope, wings & tail with Solartex. After the colored dope was applied the entire model was then sprayed with automotive two part clear in order to fuel proof. This summer I will fly it with my 2.4Ghz converted Kraft Reeds transmitter for nostalgia sake (and a step up in technology from rubber driven escapements).
MikeFoster - 07/01/2019
Added another nice photo of the completed model, thanks to MikeFoster [more pics 006].
Mary - 21/01/2019
Please find attached build article to go along with your Bi Fli plan oz823 [see suppl. file]. After several flights with my Bi Fli off our grass runway I made new landing gear with the axle 1-1/2 in farther forward placing it in line with the #4 bulkhead. This made it much more difficult to nose over on take off and landing on the grass runway.
I found a video on you tube showing Phil Kraft (and others) flying off their club field which was all hard surfaced, see - the original landing gear configuration would have worked well on that!"
MikeFoster - 25/11/2019
Hi Steve, Hi Mary, I bought this Bi-Fli at the 2023 Old Warden Mayfly event. Unusual for me, as building model aircraft is such a joy that I normally do not buy models that have already been built by someone else. The first thing I did when I got back home was to download the Outerzone plan (oz823). A quick check showed that the model was an 80% Bi-Fli with a span of 38ins instead of 47ins. I had to do a rebuild of the interior, which made me happy, as I had then actually put some creative work into the model. It appears to be a scratch build as there are no part numbers to be seen. The Outerzone plan was a big help as I was able to use the centre of gravity location shown on the plan. The flying weight of 3-1/2 lbs mentioned in the article was also a big help as this scaled down to 1016g for the 80% model. The finished model came out at 884g, so I was well pleased with that [pics 008-010]. I usually stick a couple of smileys on American models. I couldn't do it on the 1959 Bi-Fli as smileys weren't invented until 1963! Just waiting for the weather to clear up for the maiden flight. Best regards as ever,
Chris Pinn - 24/08/2023
I'm just now getting started on Phil Kraft's classic Bi-fli, from 1959. Yes I even remember reading the Model Airplane News issue that it appeared in. No possible way I could afford the magazine, read it at the library. Even though I admired the airplane, most of the text could just as well have been written in Klingon, couldn't understand a word. But now it's time. I plan to add ailerons, cut out of the original wing as barn door versions near the tips, should look more vintage that way. Servos are so cheap now that four more won't matter. I'll use four 9g servos. Last night, I pasted up a pile of "tiles" to assemble the whole sheet, not all that easy. I got it done but a few problems cropped up. No matter what brand of printer you have, a few little errors are bound to creep in, mostly toward the bottom of the plan sheet where they all add up. The 16" ruler at the bottom gained about 1/16", not a big deal but cause for worry. The original file was not that great, appeared to be an enlargement from the magazine sheet. All the lines look fat and unclear, usable but not ideal. I didn't want to cut up my recently taped together sheet to build parts of the model, and so I went back to the original file and cropped out the pieces I needed, saving them as individual PDF files [see zipfile supplement: 'print_doug']. Also, I made mirror images of the wing panels, so whole wings can be built at once, assuring matching halves. When you print the various files, there will be only a few sheets to tape together, minimizing errors. Saves the effort and expense of finding a print shop who will print you a good plan. The fin and rudder looked a little small for the Bi-fli, built back when few models had ailerons, so I enlarged the fin twice at 10 and 20 percent. I'll probably use the 20 percent one, looks about right. When printing, be sure to print the "cut marks". All flies attached, hope this will help somebody to build this part of our history.
Doug Smith - 06/02/2024
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