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Propeller Dynamics

Essential reading for model aircraft contest fliers. This is the only book on the market explaining propeller theory in non-mathematical terms. A rattling good read, I know, I wrote it.


F2B: Proof-of-concept radical design .. “Heart of Gold”

After 5 years frustration trying to get one of my old 1970’s era F2B motors running properly, success was finally achieved by doing my own piston/cylinder/ring fits. My Eather-built Firecracker test ship is now a mere pale shadow of its former glory, the result of many engine/tank changes, plus hitting a tree branch. After repairs, it weighed 2 kg, so a new model was long overdue. My Enya 45 6001 with Supercool 11X7 has about 1.2 kg static thrust, so its expecting a bit much for the old Enya to drag the 2 kg model through safe reverse wing-overs in the wind.

My thoughts returned to the old 1973 Anna Domini design, long a cherished favourite, and well remembered by my associates of that era. While it is a truly magic design (plan still available from Supercool!), the building time, like most F2B models, is at least 2 months. Also, wood selection to keep the weight down is very time and cash consuming. Both remaining life and cash are quantities of which I am running low. So how to build a light, competitive model in a short time became my principal concern.

The Dominator/G15 combat model of 1965 has always been a favourite of mine. Easy to build straight, cheap, quick to build and rugged, just a bit too small for F2B and no wheels or muffler. But if you are quick on the handle, it will do nice squares, in fact, the whole pattern with no problem at all: so how about a big version, with wheels, to take a 45?

I am further influenced by Bob Fry’s Open Combat model “Laminator”, also a flying wing. With an OS40 FSR it flies in excess of 100 MPH, turning so tightly on such a small amount of elevator movement that I found it hard to believe. If by marrying Bob’s ideas on aerodynamics with the ease of construction of a Dominator, then I was going to be at least half way there.

I didn’t want Bob to get all the credit, so I made some superficial planform changes to end up with the beast in the photo. The undercarriage was mounted on 2.5mm ply plates, which were glued straight onto the massive Dominator style leading edge, solving that problem quite elegantly (says I). With no fuselage, flaps or tailplane to build, the weight, covered in very heavy Vilene, came in at the desired 1.23 Kg, without effort made in saving weight anywhere! This was promising!

Having no real idea where the C/G should be, I drilled extra engine mounting-boltholes, to give the option of placing the C/G well forward. With real terror at the prospect of an unstable rearward C/G for the first flight, the engine was put fully forward, plus a big bar of brass was mounted behind the engine on the spare rearward boltholes. Well, that sure worked out OK, the model could barely loop! In the event, I was able to remove the brass bar, the place the engine fully back: this got the C/G close (no ballast, how is that for blind luck?), but wasn’t the end of the problems.

With full down elevator, I managed to do a 46 degree bunt from 45 degrees: the grazing descent into terrain at full speed ripped a wheel leg out by sheer inertia, breaking off the ST needle at the spray-bar. More business for Bill Swan! After the minor repairs, a rethink was needed. This was embarrassing.

Even raw beginners were coming up to me, saying “hey, old guy, your elevator is too small”. When my beard was long, a 4 year-old rode up to me on his trike and asked me if I was God, which seemed to me a reasonable thing to ask. But 16 year olds telling me that the elevator was wrong is a bit depressing. So I slunk off, having admitted nothing, then put on a much bigger elevator. As much as I hate to admit it, they were right: the ship started to do nice square manoeuvres, with prospect of F2B performance within reach.

At this point, it was time for a public appearance of the model, so it was off to the 30 year celebration of the formation of TARMAC club by Jim Stivey, Fred Tower and the boss, Lorraine, held at Lumen Christi College in Gosnells. No sooner had I plonked it on the ground, than up walked Norm Kirton, who announced to all and sundry, “you gotta call that the BAT”. Gee Norm, I thought “Heart of Gold” or “Light of the World” or even “The Infidel”, but surely not “The Bat”!

My mind flashed back to the 50’s. My school chum and good mate Harry Cooper, now TV-star vet, had built a flying wing model from Model Airplane News called the “Bomb Bat”. This flew quite well, but was not really a stunter. The only other flying wing stunters I could think of were Charles Mackey’s “Red Wing” and Wild Bill’s “Fierce Arrow”. Harry’s dad had brought back from Japan an Enya 60, one of the first, with the 5 (or was it 6?) bolt crankshaft retention. Wonder if he has still got it?

I remember flying the Bomb Bat quite well, as Harry had a rowing boat and we needed to row across the Lane Cove river to get to the flying field. Being a weak swimmer, this worried me a bit, but it was fun dragging the wingtip of my scaled up OS29 Max II powered Aeromodeller design “Pedro” in the water, while watching the wake.

At this point it was time to do some serious trimming. First off, the model was rolling too much in squares. As the tip weight was only held on with sticky tape, the one ounce of lead was easily removed and moved inboard. Well, when I say inboard, it really is; I now have _ ounce lead halfway along the INBOARD wing!

Things were looking pretty good, the one problem remaining being that the engine was going rich on outsides: risky. So I changed the motor to my Enya 45 6002. This is a new policy of mine. Anything goes wrong, then change the motor.

Well it didn’t work this time, so I cut out the tank and remounted it in a box, held down with rubber bands. Tank level could now be adjusted with shims under the tank. The tank also was an experiment. By keeping the uniflow vent to the front, it was hoped to emulate Peter Smith’s beautiful 4-2 break, but that didn’t work either. Having previously had problems at the end of the run, the tank was angled out at the rear, while the uniflow vent went into a bubble on the side of the tank. There is nothing worse than aborting the clover due to engine cutting in the first loop, then having the engine re-catch and run for another 10 laps. Been there, done that.

In the event, it was necessary to raise the tank 6mm, which was a bit weird. Still, it worked, so if you go this route it might be best to make your tank movable as well.



To conclude, here are the vital specs:

Span: 47”

Wing area: 650 sq. in

Weight: 1.23 kg (43 ozs)

Root chord: 22”

Tip chord: 14”

Ribs: 4mm balsa, carbon capped

Spar: none

C/G: 20% of root chord

Line rake: 1” behind C/G

Elevator span” 13”

Elevator chord 2.5”

Section: Dominator

Covering: Vilene (.0055”, heavyweight)

Engines: Enya 45 6001/6002 (ring motors, crossflow, mid 70’s)

Muffler: Supercool F2B, side mount, 2 chamber

Prop: Supercool petal-type 11 X 8.5

Line length: 65’

Lap time: 4.8 - 5.2 seconds

Tank: 100cc, Palmer uniflow type: uniflow vent in bubble


By the way, if you have any of Enya 45 6001, 6002 or Enya 35 II in surplus, then I would like to make you an offer for them. Till next time, stay Supercool.

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