The AFB (Amazing FleaBike)

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rtfm

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Jan 3, 2008
Messages
3,409
Location
Brisbane, Australia
Hi Gentlemen,
After a rather long silence while I beavered away in the RTFM shed, I have some news to pass on regarding the FleaBike.

1610174640219.png

You will notice:
(1) Significantly beefed-up under-the-seat section
(2) Adjusted geometry to improve rigidity

Progress to date:
  • Both sides panels completed
  • All 6061-T6 hard points cut and attached
  • Main bulkheads cut
  • Engine mount cut and attached
Imminent:
  • Bond firewall, cowl floor, tailstock, turtledeck floor, turtledeck bulkheads
  • Bolt on control panel, seat, seat, seat-back, turtledeck floor
The brakes (MX) have arrived, as have the tyres and the three bearings carrying the load of the aileron torque tube (in red)

On the way: the pulleys, cables, turnbuckles for the rudder

In case anyone is wondering... This is a three-axis Flea.
The FleaBike is designed for today's pilots, ALL of whom have trained and qualified on 3-axis airplanes. To my mind, building a 2-axis plane removes almost all current pilots from the potential buyer pool. So the rudder is foot-controlled, and the aileron function is provided by the rear wings being able to swivel an extra 1 deg to provide a total of 72kg of aileron force.

Many will say that this is not a true Flea - but that's OK - I don't mind. The Flea community is dying off anyway (by the passing of the old pilots, and by lack of interest among the rest of the flying community). If this new design is to make any impression at all on the general flying community, it HAS to be 3-axis. When Mignet designed his original HM14, it was with a view to making flying accessible to as many as possible. In today's world, every single pilot who takes to the air has (1) been trained and (2) can only fly 3-axis planes. Those who choose to fly weight-shift, or Part 103 are the minutest minority. Maybe this new design will get "ordinary" pilots in the air in a Flea.

Regards,
Duncan
 

pwood66889

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Feb 10, 2007
Messages
1,831
Location
Sopchoppy, Florida, USA
G'day, Duncan. Do take exception to:
"In today's world, every single pilot who takes to the air has (1) been trained and (2) can only fly 3-axis planes. "
Though I have (1), I fly what is in my avatar, and am working with friends to get it back into the air.
Regards, Percy in NW FL, USA
 

Protech Racing

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Jul 10, 2020
Messages
406
AS do I
None of mine will have pedals for the rudder. Nose wheel if it has one .( my feet work poorly)
FWIW the strongest virtue of the Flea is that it is crash/stall resistant as long as the front wing stalls first. Using roll control is fine and has been done. Acouple of those that tested it found it unnecessary and removed it's function. I set my Tandem up for roll control with split front wings and locked it out. 2 degrees is plenty tho if you chose to use it .
Roll control on the rear wing may increase the AOA to the stall value before the front wing.
 
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rotax618

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Joined
Oct 31, 2005
Messages
1,071
Location
Evans Head Australia
I have built an flown a Flea, you should attempt to fly one before you spend a lot of your time and cash to build one.
Fleas fly but the are inferior to 3 axis types, If you have flown a rudder only RC model compared with conventional 3 axis model you will quickly see the limitations.
The Wright brothers were aware that you need 3 axis control to manoeuvre in 3D space, although yaw and roll have a secondary effect on the other the delay or lag renders that effect less than useful in many flight regimes.
It is possible to couple the rudder and ailerons together in a proportion that provides a simpler though less effective way as with the Ercoupe.
The Fleas are all very short coupled in yaw and could be described as a super staggered tailess biplane, the rudder has a very powerful effect but there is quite a lead and lag on the secondary effect, this is very unnerving when you need to pick up a dropped wing.
 
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rtfm

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Jan 3, 2008
Messages
3,409
Location
Brisbane, Australia
Progress this week:
  • Recut the engine mounts (third time)
  • Recut the under-seat doublers (also third time)
  • Bonded the doublers in place
  • Added a strangely overlooked structural airframe member to the top of the engine mount
  • Attached the metal hard points (trial fit only). Discovered that inserting the captured nuts before sealing the inside was a terrible idea. Enough epoxy seeped into the threads that I had to remove and replace a significant number of them.
  • Cut the firewall, tailstock, cowl floor, seat and seat-back
One of the issues I am facing is that I'm cutting parts while building. Partly because I'm an idiot, partly because I'm impatient and partly because I'm discovering that there is a world of difference between CAD drawings and real pieces of wood. Many adjustments have had to be made along the way to ensure the physical plane reflects the CAD. But at least it's now repeatable.

Regards
Duncan
 

rtfm

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Jan 3, 2008
Messages
3,409
Location
Brisbane, Australia
Update:
I can't find my original calculations for the additional "aileron" force which will be applied by deflecting the rear wing halves, so I can't verify the figure in the drawing above. So I re-did the calculations, and (of course) the differential aileron effect of pivoting the rear wing differentially is dependent on airspeed. It turns out that at cruising speed (78kts - which is wildly optimistic, I know), 0.5 deg of "aileron" movement amounts to 21kg per side. The trick will be managing such small incremental pivots. Linear actuators? Servos? A piggy-back lever on the rear wing bellcrank?)
 

Beragoobruce

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Joined
Oct 16, 2014
Messages
69
Location
Blue Mountains, AUSTRALIA
I'm using West System epoxy. I thought of using T88, but the stuff is so bloody thick, it makes it difficult to work with. The West system is very runny, and goes on very easily with a squared-off tongue depressor.
I'm enjoying your postings on the Flea - love your pioneering construction philosophy.

I've used WEST for many years in both boat building & in construction of my MiniMax. It is excellent stuff, plus you can get slow hardeners, useful in Oz temps. I much prefer it to T88.

But as regards your comment above on its viscosity, you will get better results if you thicken it a little. The Gougeon brothers do (did?) an excellent book on using WEST, & they recommend using a thickener to improve gap filling. Ideally, you'd coat each surface with unthickened epoxy, then apply an extra coat of epoxy thickened with microfibres. This not only fills voids, but the microfibres wick resin-rich areas into resin lean areas. Prolly too much hassle to use both modified & unmodified resin, but in general the addition of microfibres will usually improve your bonds.

Apologies if I am preaching to the choir. . .

Good luck with your build, & keep us in touch with your progress posting.

Bruce
 

Protech Racing

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Joined
Jul 10, 2020
Messages
406
Update:
I can't find my original calculations for the additional "aileron" force which will be applied by deflecting the rear wing halves, so I can't verify the figure in the drawing above. So I re-did the calculations, and (of course) the differential aileron effect of pivoting the rear wing differentially is dependent on airspeed. It turns out that at cruising speed (78kts - which is wildly optimistic, I know), 0.5 deg of "aileron" movement amounts to 21kg per side. The trick will be managing such small incremental pivots. Linear actuators? Servos? A piggy-back lever on the rear wing bellcrank?)
It will work better /simpler, splitting the front wing .
 

Protech Racing

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Joined
Jul 10, 2020
Messages
406
A picture is worth etc etc.
View attachment 106243
The CG is about at your head . The front wing center of lift is about at your head. Bad. Have you 1/4 scaled this ? Has anyone run this on any sim? This is why I spread my wings apart, to move th4e pitch authority further from the CG.
Please model it or sim it before you leave Earth.
The CG placement models/scales well, BTW.

1610590175716.png
 

rtfm

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Joined
Jan 3, 2008
Messages
3,409
Location
Brisbane, Australia
The CG is about at your head . The front wing center of lift is about at your head. Bad. Have you 1/4 scaled this ? Has anyone run this on any sim? This is why I spread my wings apart, to move th4e pitch authority further from the CG.
The CG has been very carefully worked out. The traditional method for calculating the proper CG was to at least start with it at 25% of the combined chord. I have written extensively on a more scientific way to establish the proper CG in my paper: Basically, the CG needs to be ahead of the combined centre of lift of both wings by some factor between 5% and 10% of the combined chord. As it turns out, the 25% rule isn't all that bad as a starting point.

(From the paper:)

1610590816074.png
In addition, I am building in 150mm fore/aft adjustment of the front pivot point.

I think I've got my bases covered.
 

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