The AFB (Amazing FleaBike)

[rtfm]

Hi Martin,
Excellent photos. So I'm not that crazy after all. (Which is comforting news). Well found!

Yes, I have two prospective customers so far, even (as you say) I have little more than a fuselage and dreams. I think what they find attractive is the fact that the entire fuselage arrives in six cnc-cut panels, which they simply have to join together. Similarly, the wings arrive in eight completed sections, which again have to be joined together with hinges (mostly). Also the fact that it is now a completely standard Flea so they know what they're dealing with. Quick, simple and very inexpensive. The first five sales will be simply the cost of materials and $1000 for my labour.

Duncan

 

[Tiger Tim]

I have a question about tipping both wings on a flea: does incidence with the fuselage matter at all or is decalage between the surfaces all that’s necessary?

 

[rtfm]

Hi Tiger Tim,
The way I see it, having two lifting surfaces means we have to think of them (first) as a single entity so that we can find their combined centre of lift. This will apply to a fixed or a movable rear wing. Once we have that, we need to make sure the CG is ahead of this by some percentage.

Then we need to think about their relative lifting power. In a fixed rear wing, the decalage is changing all the time as the front wing pivots. But we've been through this before. In a pivoting rear wing, the decalage remains the same, but the lifting ability of the rear wing increases/decreases as it pivots.

So changing decalage = the traditional Flea configuration. Constant decalage = dual pivoting wings.

Duncan

 

[ragflyer]

Then we need to think about their relative lifting power. In a fixed rear wing, the decalage is changing all the time as the front wing pivots. But we've been through this before. In a pivoting rear wing, the decalage remains the same, but the lifting ability of the rear wing increases/decreases as it pivots.

So changing decalage = the traditional Flea configuration. Constant decalage = dual pivoting wings.

Duncan

RTFM, what happens in the case of conventional FW tail back configuration when you have an all moving tail? Is that a case of changing decalage as well? Does that make all moving tail less stable? As I said you are confusing AOA with incidence wrt fuselage. Here is an example that hopefully helps.

Assume straight and level flight. A flying flea forewing typically carries between 2/3 and 3/4 of the total weight. Let us go with 2/3rd being carried by the fore wing and 1/3 by the aft wing for this example. This means at close to stall the fore wing is at AOA 16deg and aft wing would have to be at half that i.e. 8deg for the aircraft to be balanced about the CG. Now if we set up the rear wing at a 4 deg nose up incidence with fuselage then the fuselage will be 4 deg nose up so the aft wings sees 8deg AOA and the fore wing will be at 12 deg (16 - 4) nose up wrt to fuselage so it sees 16deg AOA. Every thing is nicely balanced.

Now if we double the speed. The AOA would have to drop for both wings by a factor of 4. For everything to be balanced as before the fore wing will now have to be at 4deg AOA and the aft wing as before will have to half that at 2 deg. Now we know the aft wing is fixed at 4deg nose up wrt to fuselage which means the fuselage will have to fly 2 degrees nose down so that the aft wing experiences a 2 deg AOA. The fore wing in turn will be at 6 deg nose up with respect to the fuselage so it has a 4 deg AOA (as fuselage is 2 deg nose down AOA). You can do this for all speeds until you run out of control range of the fore wing. I have ignored downwash on the aft wing for simplicity but the principle is the same, the only difference is the fuselage angle will change a little.

As you can see the main effect of the changing the rear wing incidence is to change the AOA of the fuselage at any given speed (not stability). This is important in general at two speeds - landing so that the landing gear is aligned correctly with the ground at touchdown and at cruise so that the fuselage is aligned at near zero AOA to minimize drag. The only reason you will have to have a pivoting rear wing in a stable flying flee is if:

1. you are unable to reconcile the landing (or parachute decent) attitude with cruise attitude due to landing gear geometry
2. have multiple cruise speeds you want the fuselage to be at near zero AOA
3. need more control power if the fore wing range is limited for some reason

Hope this helps.

 

[rtfm]

Let us go with 2/3rd being carried by the fore wing and 1/3 by the aft wing for this example. This means at close to stall the fore wing is at AOA 16deg and aft wing would have to be at half that i.e. 8 deg for the aircraft to be balanced about the CG.

This initial statement is where I disagree with you. The relative lifting capacity of the fore/aft wings is a function of their size (assuming both have the same airfoil, and the rear wing is relatively free from downwash). Their load, on the other hand is a function of the placement of the CG. You are now confusing lift with load.

Duncan

 

[Martin R.]

The first five sales will be simply the cost of materials and $1000 for my labour.

Australian $ or US$? ;-)

….. Assume straight and level flight. A flying flea forewing typically carries You can do this for all speeds until you run out of control range of the fore wing. …..

To be honest: As a non-aerodynamicist I've never really understood why the fore wing of a Flew should run out of control. Because the pilot can at any time increase the incience of the fore wing in relation to the rear wing (in our example e.g. +6 degrees) . This means in every case: higher lift for the front wing. In my "aerodynamic world" just three exceptions are imaginable:

a) a false CG or
b) a much greater surface of the rear wing (but that's no longer a Flea and you should apply canard-rules) or
c) a high-speed stall of the front wing (what I never heard)

….. The only reason you will have to have a pivoting rear wing in a stable flying flee is if:

1. you are unable to reconcile the landing (or parachute decent) attitude with cruise attitude due to landing gear geometry

2. have multiple cruise speeds you want the fuselage to be at near zero AOA

3. need more control power if the fore wing range is limited for some reason

I fully agree with you. That’s why I showed the photos of the Milvius
The AFB (Amazing FleaBike)
I think he will just use 2 configurations: (slow and cruise). But is this additional design effort worth it?

I have very very little flying experience with Fleas (I just flew a HM-1000 BALERIT). But if effort is to be made, then in my opinion the first for rotation around the longitudinal axis (e.g. ailerons or aileron-effect).

 

[Martin R.]

... Their load, on the other hand is a function of the placement of the CG. You are now confusing lift with load.

Hi Duncan,

Perhaps you should explain once why, in which direction and how far the CG moves in your Flea when you push or pull your stick. But with real values (kg/m^2 and cm etc.) please. Otherwise you'll be arguing in circles

But I stand by my statement in my posting 286 ".....if effort is to be made, then in my opinion the first for rotation around the longitudinal axis (e.g. ailerons or aileron-effect)."

 

[Hot Wings]

You are now confusing lift with load.

In un-accelerated flight, about the pitch axis in this case, they are both identical.

 

[rtfm]

Hi,
OK - I think this has gone on long enough (partly my fault). We're going in circles, confusing terminology and in danger of getting pissed off with each other. Why don't we leave it here, and pick it up again once the FleaBike is airborne?

Regards,
Duncan

 

[rtfm]

I've updated my build log: Bonding the tDeck

 

[rtfm]

Build log updated - bonding the fin.

Check "Build Updates" for details.

Duncan

 

[Protech Racing]

The Farge Flea stands so high .. to accommodate the single blade prop. I think that it was used only a little due to vibration. No way to keep the thrust line from shaking the plane .
Speed control line models also used a single blade for a short while to increase disc size and efficiency. I dont think they do any more for the same reason.

 

[rtfm]

Single blade props? I've never understood this...

 

[Tiger Tim]

Single blade props? I've never understood this...

In short, I think we can all agree that whether it’s wings or propeller blades, tips generate losses. Fewer tips mean fewer losses so in theory a single blade propeller is the most efficient use of blade (disk?) area. So far, single blade props have brought with them enough baggage and compromise to not be worth it.

 

[Protech Racing]

The thrust vector travels with the rotation, thus shaking the air frame.

 

[rtfm]

Just looks weird...

 

[Sockmonkey]

I've been told that at the speeds prop tips move at, the air doesn't have much of a chance to flow spanwise before the blade is done with it anyhow. Plus prop blades tend to have a fairly high AR in the first place.

 

[TFF]

Properly one blade are counterweighted and they need to teeter like a helicopter blade. If the following blade is not in the wake of the first, it’s a little overboard. Control line speed with an engine turning 45,000 rpm with a 7” diameter prop or smaller have a cavitation problem. Those are small enough not to worry about the teetering.

 

[Martin R.]

Single blade props? I've never understood this...

Just read de La Fargues original (French) explanations: Pou-Guide - Les pou(x) de Jean de la Farge (Suite)

He describes the efficiency of a single-blade prop in comparison with a multi-blade prop and concludes: "..... Cela explique pourquoi de la bipale nous sommes passés à la monopale qui pour absorber la même puissance devra avoir un rayon de 25 % supérieur à la bipale. La monopale permet de balayer pratiquement un 50 % de plus de surface active, donc d'expulser un 50 % en plus de masse d'air. ..... "

= "....... This explains why we have switched from two-blade to single-blade which must have a 25% greater radius than the two-blade propeller to absorb the same power The single-blade sweeps almost 50% more active area, thus expelling 50% more air mass. ......"

PS Please excuse my "strange" English. It's not my mother tongue :-(

 

[Protech Racing]

The real world issue is again, the center of thrust with 2 blades is the hub. The center of thrust with one blade is around 45-55- percent up the blade . This thrust vector moves with the Blade , offset on the airframe and shakes the craft.
I believe that the model racers decided that shaking the airframe disturbed it enough to slow it down more than the higher thrust sped it up.
I may be wrong by stating they are not using a single blade today ,as I have not flown UC in over 10 years .