# The AFB (Amazing FleaBike)

### Help Support HomeBuiltAirplanes.com:

#### rtfm

##### Well-Known Member
Repositioned rear wing, verified front wing position (subject to actual weighing).

#### Protech Racing

##### Well-Known Member
Center of the front wing lift, pitch input, must be in front of the CG. It is a couple percent behind the high point of the airfoil . , for a reflexed airfoil.
Your new pic may work if you put the fuel behind the seat.
Engine = 90# FWF is the rear wing and fuse another 80#?

#### rtfm

##### Well-Known Member
Hi,
I'm sorry, but I just don't understand what you're saying.
"Centre of the front wing lift" - by this do you mean the centre of lift of the front wing?
"Pitch input" - I can only guess this means where the control rods attach to the front wing so that it can be pivoted? If so, this has nothing to do with the CG. I'm confused.

In the sketch above, the small red arrow depicts the centre of lift of the front wing (and of the rear wing).
The large red arrow shows the combined centre of lift of both wings.
The grey circle is the calculated CG
As you can see, the combined centre of lift is BEHIND the CG - as it must be.

Putting the gas behind the seat (besides interfering with the control linkages) moves the CG rearward - why do you want to do that?

Regards,
Duncan
PS FWF = 53kg Don't know what the rear wing/fuse will weigh

#### Protech Racing

##### Well-Known Member
The front wing center of lift is your pitch input point. It has to be well in front of your CG

#### Protech Racing

##### Well-Known Member
That wing spread may work. The rudder gets less effective as it gets near Cg and aerobcenter of side area
I can’t see how you estimated CG without weights for the tail vs engine.
I just don’t see how this will fly

#### Protech Racing

##### Well-Known Member
That wing spread may work. The rudder gets less effective as it gets near Cg and aerobcenter of side area
I can’t see how you estimated CG without weights for the tail vs engine.
I just don’t see how this will fly
If you move the front wing foreword about 1/4 chord, it may work. Imho
My scale test.

#### Attachments

• 61.1 KB Views: 53

#### Vigilant1

##### Well-Known Member

Probably beside the point, but that's an interesting bit right there. The AoA of the wing is in no way limited to the pivot angle, unless that 12 degree pivot angle is somehow measured from the relative wind (and not, say, the fuselage, the rear wing, etc which can be very misaligned with the relative wind).

Last edited:
BJC

#### Protech Racing

##### Well-Known Member
The 12 * is RE to the fuse.

Also. Unless your rear wing is 110% of your front wing; The 25% of total chord is faulty. The wing described is smaller and shorter than the fore wing, thus the CG needs to go foreward also . Not even considering the efficiency of the rear wing being diminished by many factors. Bent air, slowed air, turbulent air for the fuse, pilot.

The stuff that wont scale perfect; rudder
The light loading of the model wont give the full scale turbulent flow from the wings. The CG willbe very close, only a little to the rear due to lack of turbulence.

FWIW ;My test model shows the CG at the TE of the front wing, little tiny blue dot. 37% of total chord maybe . The full scale liked around 34%. When I went back any more the rudder was too small /close to CG, pressure center etc.

Last edited:

#### rtfm

##### Well-Known Member
... Both have the same 43015 foil.
Hi,
I've gone looking for the 43015 airfoil, and so far have drawn a blank. You wouldn't perhaps have the DAT file would you?

Regards,
Duncan

Thanks BJC

HBA Supporter

#### Protech Racing

##### Well-Known Member
The Fraser and the 43015 are very similar. Both should have CP around the 27- 29 % point . The Fraser is fine . Using a smaller rear wing makes the calculations difficult . for me anyway.

#### rtfm

##### Well-Known Member
Thanks.
Here are my stability calcs:

Terms:
Wf/Wr = Wing front/Wing rear
c/4 = quarter chord
C = combined chord
CoL = Centre of lift of both wings combined
MoS = Margin of Stability (difference between CoL and CG)

As you can see, by ensuring the CoL is behind the CG by a margin of stability between 5% and 10%, I have arrived at a similar CG position to the "old" 25% of combined chord method. Bottom line, as long as the CoL is behind the CG we're good to go.

As the build progresses I'm able to more accurately weigh stuff, and more closely estimate the CG. The front wing is then able to move to accommodate this. This is where I am at the moment. The fuselage is largely built.

#### rtfm

##### Well-Known Member
Hi guys,
A question for you aero-engineering types: With both front and rear wings pivoting to maintain a 60%/40% lift split, will the plane rise like an elevator? By this I mean, will there be no nose-up attitude in the climb? And if so, do you consider this a good/bad thing?

I suspect the elevator ascent scenario. If this is an undesirable thing, and easy fix would be to fit a small "trim/attitude" surface to the tail, linked to the rear wing - like this: Max deflection about 2.5 deg. Minimal complexity, and an easy "aftermarket" fit should it be required.

#### Victor Bravo

##### Well-Known Member
HBA Supporter
Duncan, welcome to the Mission Creep Death Spiral... pay no attention to those cries of despair you hear in the distance... here we go, right this way

#### rtfm

##### Well-Known Member
Ha ha. I know it all too well. What I've done is to pre-drill the holes for the fitments, but they are not going to be part of the initial build. They're there for insurance purposes.

But what do you think of the elevator vs nose-up climb question?

Duncan

#### rtfm

##### Well-Known Member
All planes are flying collections of compromises...
When rubber hits the road (or in this case, when wood hits the cut-table) some compromises have to be made. No way round it.

For example, 1.5mm plywood comes in 2500 x 1220 sheets. The front wing has a 1200mm chord. That places (ideally) the front spar at 25% chord (or 300mm from the leading edge. So far, so good. But that makes the leading edge piece more than 600mm, leaving too little remaining plywood to cover the top or bottom without significant wastage - and at \$120 per sheet, wastage is a sin.

So I have had to reposition the main spar in a sub-optimal position just to stay solvent. Here's what I've ended up with:

As you can see, each section is under 600mm, meaning I get to waste very little plywood, but the main spar is slightly forward of where I'd have liked it to be. Bugger.

Such is life...

Duncan

#### TiPi

##### Well-Known Member
Log Member
Hi Duncan, I'm no aerodynamicist but I think having 2 variable lifting surfaces will lead to unstable situations eg the lift distribution between front & rear starting to oscillate (PIO). Look up some of the old stories from the early gyros. Some modern aircraft might use that, but with fly-by-wire technology to eliminate the PIO.