The AFB (Amazing FleaBike)

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rtfm

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While I ponder the various solutions you guys have suggested, here's a more aeronautical one...
  1. If the front and rear wing pivot equally, my suspicion is that the plane will rise vertically, like an elevator. Given that the rear wing may experience some downwash, the lift proportions may not be exactly 60/40, but I suspect pretty close to it. There may be some nose up, but not a lot.
  2. If this elevator rise happens, the plane will not be able to pull out of a dive, because there will be no nose-up attitude change with back stick
  3. Bugger.
  4. De La Farge experienced the same issue with his "Pulgas", so he added an all-flying control surface on top of the fin which he could operate independently from the cockpit. He used this successfully to alter the pitch of the airplane - specifically to pull out of a dive. That is an option of course, but a bit messy. Let's call this option "a"
  5. Another option is to split the control stick into two halves, (the left half controlling the front wing, and the right half controlling the rear wing) but join them with strong rare-earth magnets. To all intents and purposes, both sticks act as one under 99.9% of flying scenarios. However, in a dive, if pulling back on the control column does not raise the nose, all one has to do is to push the right stick forward, thus decreasing the incidence of the rear wing, and the nose will rise. An elegant solution, but it does require a bit of familiarisation on the part of the pilot. This is option "b"
  6. A third possibility is not to link the front and rear wing in a 1:1 ratio, but to (for example) link them in a 2:1 ratio instead. Thus pulling back on the controls would raise (for example) the front wing incidence all the way to 12 deg, but the rear wing only to 6 deg. This is option "c"
Which of these options - "a", "b", or "c" do you think is better? Or can you think of a better solution?
 

Vigilant1

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Which of these options - "a", "b", or "c" do you think is better? Or can you think of a better solution?
Since you have asked: I would strongly favor a fixed angle of incidence for the rear wing, the configuration that has proven to work well. No need for rare earth magnets, a changed control input scheme if the plane enters a dive, etc.
 
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TiPi

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Also remember the poor pilot! We are used to flying with reference to the horizon, “feeling” pitch changes before you have transited several vertical airways. If you remove that pitch change, you will fly like a lost puppy as the reference to the horizon is no longer valid. It won’t be like a helicopter.
 

rtfm

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Since you have asked: I would strongly favor a fixed angle of incidence for the rear wing, the configuration that has proven to work well. No need for rare earth magnets, a changed control input scheme if the plane enters a dive, etc.
Hi Vigilant1,
Why do you favour a fixed incidence rear wing? It has very little to recommend it other than the follow-my-leader design evolution of the Flea. A fixed rear wing means that the rear wing at quite a low airspeed will overpower the front wing. In fact, the fastest a Flea with a fixed rear wing set at 6 degrees can safely fly is the speed at which the front wing reaches 6 degrees to maintain level flight. Any faster, and the front wing incidence will need to be decreased, and the rear wing will start "overpowering" the front, so the stick will need to be pulled back to bring the nose up again.

Set the rear wing to a smaller incidence, and at takeoff its lift is decreased, so takeoff speed needs to be increased. The plane will be able to fly faster until the front wing and rear wing reach the same incidence. And that is the max speed of the plane. Any faster, and the rear wing will tend to lift the tail, and the pilot will need to pull back on the stick.

This fixed incidence rear wing is a built-in design flaw of all Fleas. It is a configuration which has "worked well" only within a very restricted flight envelope. Bigger engines and sleeker airframes mean faster planes. And as most Flea pilots will attest, the tail begins to rise as speed increases. In a dive, this configuration is still a danger.
 
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rtfm

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I have spent most of the day pondering the issue of flight attitude and the ability to pull out of a steep dive. I think the only solution is the one Jean de la Farge of Argentina settled on - a separate elevator.

A fixed rear wing incidence means that the flight envelope is limited to the speed at which the front wing is still able to produce more lift than the rear wing. Dual pivoting wings overcomes this, but because of the minimal nose-up attitude produced by this arrangement, pulling out of a dive is problematic.

So the simplest solution is to have a free floating elevator which can be manipulated by a separate control. I think the easiest would be a lever beside the pilot which he can reach down to. Need the nose to come up out of a dive? Pull on the lever, the elevator deflects downwards, and the nose rises. Let go the lever, and the elevator returns to free pivoting.

1613822290696.png
 

BJC

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This fixed incidence rear wing is a built-in design flaw of all Fleas.
the issue of flight attitude and the ability to pull out of a steep dive. I think the only solution is the one Jean de la Farge of Argentina settled on - a separate elevator.
That is why most airplanes fix the incidence of the wing that provides the majority of the lift, and vary the incidence of the other for pitch control.


BJC
 

rotax618

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The Cosandey Flap only went went up lessening the rear wing lift, it was to trim the nose down pitch that Fleas have as speed increases, as Duncan has described
 

Sockmonkey

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The Cosandey Flap only went went up lessening the rear wing lift, it was to trim the nose down pitch that Fleas have as speed increases, as Duncan has described
But if it's used as a takeoff flap, you can reduce the AOA of the rear wing. Then it's only something you have to use during takeoff and landings.
 

rotax618

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From my experience, it is safer and easier to leave the rear wing fixed and trim the aircraft with a Cosandey flap, a fin mounted third surface brings lots of structural, flutter and control issues.
 

Martin R.

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Cosandey put a flap on the rear wing to accomplish the speed/trim change, it is a much simpler addition.
Not only simpler but also easier and safer!
Many many years ago, Louis Cosandey had the possibility to examine "flea" models in the wind tunnel of the famous ETH Zurich - Swiss Federal Institute of Technology. Here you will find a conclusion from one of his tests:

Open this link: Pou-Guide - Contribution à l'étude et au réglage des aéronefs Mignet and search chapter "6° LE VOLET MOBILE AU BORD DE FUITE DE L'AILE ARRIÈRE".

There look for the part "AUTOSTABILITÉ" and then for "Fig.4" http://pouguide.org/uploads/images/impou/comprendre/ETUDES_Cosandey/Cosandey1Fig17.jpg

The text is written in French. But you certainly find a translator-programm that can help to understand Cosandeys explanations.

By the way: In the context of our thread the whole chapter 6 is very interesting.
 
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rtfm

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By the way: In the context of our thread the whole chapter 6 is very interesting.
Hi Martin,
I am familiar with Cosandy's work. In fact my browser translates the text very well. About on par with some English folks who post here... (Ha ha). In fact, I used some of Cosandy's work in my own paper.

My main concerns with his work is that he still clings to the notion that both wings act as one because of the "slot" effect, which has been shown not to exist (Barnaby Wainfan, I believe, quoting from wind tunnel tests).

My abiding regret, however, is that so little of de la Farge's work has survived. de la Farge built the "Pulga" in Argentina which had dual pivoting wings. About ten examples were built, and by all accounts flew very well. He fell out with Mignet over this innovation, even though Mignet had briefly experimented with this feature himself in an attempt to prevent the unrecoverable dives of his earlier designs.

I have a friend who is building an RC model of the FleaBike, and I have temporarily halted construction until the RC model takes to the air. I am waiting to see how the dual pivoting wings behave before proceeding.

Duncan
 

TFF

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No matter what configuration, one flying surface reacts to the other flying surface. If you move both, it only has a net effect. No different than if you move one double and the other is fixed. If you add a third, no matter if in front or rear, it’s job is to fight the natural configuration of both surfaces. Why not make them do their job in the first place.
 

rtfm

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<snip> If you move both, it only has a net effect.
Absolutely - no issues there.
No different than if you move one double and the other is fixed.
No. You are wrong there TFF. The basic reason for moving both wings is to maintain the 60/40 lift split, and to completely remove the tendency for the rear wing to overpower the movable front wing. Your contention is exactly what traditional Fleas do, and they all have a tendency to nose over at higher speeds.

If you add a third, no matter if in front or rear, it’s job is to fight the natural configuration of both surfaces.
Exactly. But because the third surface is a symmetrical airfoil which is free-floating, it has little or no overall effect on the main flying surfaces. The point is, we WANT this third surface to fight the natural tendency to nose over at high speeds. It starts fighting only this scenario.
Why not make them do their job in the first place.
The main wings ARE doing doing their job. But as I've outlined before, doing their job eventually raises the tail, drops the nose, requiring the pilot to pull back on the stick. Flight speed has been limited. Any significant increase in speed will see the nose dip significantly, and there is a race to get the nose up before the rear wing takes command completely. And this is where the third surface comes in. Pull up on the control lever, and the nose lifts. Once in safer territory, let go the tail control, and the third surface pivots freely again, minding its own business.

Regards,
Duncan
 

Protech Racing

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Show me an example of any nose over in the last 10 years , 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.
 
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