The AFB (Amazing FleaBike)

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rotax618

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Doesn’t the application of a Cosandey flap have the same effect as lowering the incidence of the rear wing, it certainly is far simpler to implement.
 

rtfm

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Doesn’t the application of a Cosandey flap have the same effect as lowering the incidence of the rear wing, it certainly is far simpler to implement.
Yes it does, but it is subtractive, and does nothing to address the loss of lift at takeoff and landing.

Both wings pivot to 12 degrees:
1613959635421.png

Rear wing set to 2 degrees (as seems the fashion nowdays) You have lost 96kg of lift, and takeoff/landing is now 45kts

1613959755276.png

My dual pivoting wings provide almost a 100kg of additional lift at takeoff/landings, and removes any tendency for the plane to nose over - at ANY speed.

Difficult to implement? I don't agree. The Cosandy flap had to use a lever, control rods, a spring, and a control horn. Mine requires an additional control rod attached to the control stick, and a bellcrank. Arguably simpler in fact.

Duncan
 

rtfm

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Show me an example of any nose over in the last 10 years
As long as a Flea flies within its self-imposed speed limits, there is no danger of a nose over. Exceed this speed limitation and all Fleas experience a dropping of the nose. Get into a strong dive (in excess of 15 degrees) and the results will be hairy, because full back on the front wing is only the differential between 12 degrees and the set angle of the rear wing.

, and with the wing gap more than 10 in. It just wont happen if you have sufficiant pitch authority, as I have said for about a year to you. Spread the wings a little and fly.
Overlapped wings may nose over as the lift center moves past the CG.
Your stipulation of a "wing gap more than 10 inches" is meaningless (please excuse my bluntness, I'm not trying to be provocative - just trying to be clear). The placement of the rear wing has nothing to do with an arbitrary wing gap, and everything to do with the airflow it encounters. In the screenshot below, overlapping wings would make no difference at all. If this is your major objection, then (with all due respect) you've been wrong for over a year on this matter.

Check out the following screenshot. It is footage from the NACA Langley Wind Tunnel, superimposed on the wing layout of the FleaBike. Please notice my horisontal wing gap. Also the vertical gap. But most of all, notice the quality and direction of the rear wing air at the front wing max pivot of 12 degrees. It is every bit as clean as that arriving at the front wing.

1613961648567.png
How much further do you want me to spread the wings?
 

rtfm

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Hey Protech Racing:
I've just re-read my post to you, and I must apologise if it sounds harsh. I didn't mean it to be. I don't want to initiate a flame war with you. We're all mates here. But my views arise from a protracted period of study and reflection. I need to stand up and be counted, as it were. Please PM me if you feel offended, and we can sort this out over a virtual beer.

Regards,
Duncan
 

rtfm

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Hi. Yes, the model will be most illuminating. Sorry, I don't know the dimensions of the RC model, but David tends to build them on the big side.

As far as the debate is concerned, I have had a change of heart. Not a change of mind, because I firmly believe I'm right. But a change of perspective. I have decided to build the FleaBike completely (and I mean completely) standard. No dual pivoting wings, and therefore no third attitude correction flying surface required. No foot activated rudder. No aileron (or aileron-like activity via differential deflection of the rear wing). A true 2-axis Flea.

Why? Because two very keen customers were getting cold feet about the number of innovations I was proposing.

So Serial #001 will be a stock-standard Pou du Ciel, with the only "innovations" being the "sit-on" fuselage, the novel way of cutting the airframe on a CNC router and (possibly) a ground-adjustable rear wing incidence mechanism.

I think once I have Serial #001 up and flying, and have some serious hours under my belt on it, I'll be in a position to start introducing some of my innovations. Until then, it'll be "FleaBike AX2" and later, the "FleaBike AX3"

Regards,
Duncan
 

Tiger Tim

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I fully support your decision, Duncan. You have an awesome innovative method of construction and too many innovative things in the same place usually leads to an unfinished project (in your case I assume the finishing line is the design reaching actual production).

Get some development time on an airworthy FleaBike, then get a bunch of customer ones launched, then start developing your own example into the Super FleaBike 2.0 if that’s what you really want.
 

Protech Racing

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Your rear wing will fly at about 3-4 AOA. Less if you spread the wings more. Mine was about 2 AOA as the wing spread was further.
DO you believe that adding rear wing AOA will increase the rear wing loading? The only way to change the loading is to move the CG.
 

Martin R.

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Your rear wing will fly at about 3-4 AOA. Less if you spread the wings more. Mine was about 2 AOA as the wing spread was further.
DO you believe that adding rear wing AOA will increase the rear wing loading? The only way to change the loading is to move the CG.
A propos wing loading: When I open the file "Design Paper spreadsheet calculations" via Technical resources - The Backyard Builder's Forum
I find e.g. 16.8 kg/m^2 wing loading for the front-wing.

wing_loading_2.jpg

In posting #107 (Nov 2020) The AFB (Amazing FleaBike) I learn, that the values in this cell are "lbs/ft^2".

And when I look at the EXCEL formula for this cell (D20), I don’t understand anything anymore. However: wing loadings are quite important values in terms of what is being discussed here!

What do I misunderstand?
 
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rtfm

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Hi Martin,
Good catch. I went over the formula and I have no idea how I arrived at it. Doesn't make sense to me at all. So I redid the formula for wing loading, and added a little diagram for future reference. It now looks like this. I must try to find some time tomorrow to update the spreadsheet on the web site. I'm a one-man show here - It's nearly 11pm here, and I have to get to bed before the day ends...

Regards,
Duncan

1614084609512.png
 

Sockmonkey

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Question: Would using a symmetrical or mostly symmetrical airfoil on the rear wing set at a positive AOA be a viable fix for the rear wing making too much lift at high speeds?
Then if the tail lifts more than it should, the airfoil starts producing less lift as it becomes parallel to the airflow.
 

rtfm

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Question: Would using a symmetrical or mostly symmetrical airfoil on the rear wing set at a positive AOA be a viable fix for the rear wing making too much lift at high speeds?
Then if the tail lifts more than it should, the airfoil starts producing less lift as it becomes parallel to the airflow.
Hi,
I don't follow. If the rear wing (whatever the airfoil) is set to a fixed incidence, by the time the rear wing is parallel to the airflow, the plane is already nose-down by the rear wing incidence set.

Duncan
 

rtfm

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So the issue with the pivoting wings on the fleabike was because of the spacing?
Hi,
No, it was because if both front and rear wings are at the same AoA, the nose of the plane can't rise. In takeoff, this would be quite nice (but different), with an elevator-type ascent. But if the plane gets into a dive, what then? No nose-up attitude means you can't pull out of the dive.

A better (but less sexy) option would be to implement a manual rear wing incidence adjustment from the cockpit. Sort of like flaps on a non-Flea plane. Three notches (max incidence) for takeoff/landing, zero notch for cruise.

Duncan
 
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Sockmonkey

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Hi,
I don't follow. If the rear wing (whatever the airfoil) is set to a fixed incidence, by the time the rear wing is parallel to the airflow, the plane is already nose-down by the rear wing incidence set.

Duncan
Since a symmetrical airfoil needs positive AOA to create lift, the amount of lift it creates decreases as the plane starts to nose down. It won't ever raise the tail enough to be completely parallel to the airflow because the tail still has to support weight. There won't be more than a degree or two of pitch change for the fuselage.
 

rtfm

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Since a symmetrical airfoil needs positive AOA to create lift, the amount of lift it creates decreases as the plane starts to nose down. It won't ever raise the tail enough to be completely parallel to the airflow because the tail still has to support weight. There won't be more than a degree or two of pitch change for the fuselage.
It would be interesting to plot the lift against airspeed and AoA.
 

poormansairforce

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It won't ever raise the tail enough to be completely parallel to the airflow because the tail still has to support weight.
If the plane is nosing over where do you think think the airflow is coming from? And as the wing now has to support less weight it gets even better at doing its job. 😳
 
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