# The AFB (Amazing FleaBike)

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#### rtfm

##### Well-Known Member
Hi TiPi
Why would a stable 60/40 lift distribution lead to PIO? I'm not saying it wouldn't, but I don't understand why it might occur.

Duncan

#### TiPi

##### Well-Known Member
HBA Supporter
Log Member
If you have 2 lift-producing surfaces that are both moveable (= changing lift), how are you going to control your lift distribution between the 2? A bit of slop in the control mechanism, a gust upset and all bets are off.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
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?
Take a step back and draw out a basic statics diagram. Lift is not the only force to be considered. To be statically stable there also needs to be some force that increases or decreases to return the craft to neutral. This is generally a change in overall pitching moment.
To climb the lift force needs to be increased. That requires either more speed at the original lift coefficient or more lift coefficient at the original speed, either from more AOA or flaps.
Adding flaps, and keeping the same balanced 60/40 split, will result in an 'elevator like' climb at the original airspeed. This presumes the there is still some stabilization force in play.
Changing the AOA of the wings also means changing the pitching moment. IF - the sum of all the forces retains the 60/40 split then the craft could climb in 'elevator mode' relative to the fuselage. Desinging such a system is more work than I wuold be willing to do.

With simple mechanical pitch control you will probably see some change in fuselage AOA with any change is speed or climb rate.

Now that I've completely confused you I'll leave it to others that are much better teachers than I am to do damage control.

#### Protech Racing

##### Well-Known Member
The wing loading is determined by the CG, not AOA. AS I have said in the past , the Fleas are best at around 65 -69%front . Pitching the rear wing more will not change the load,just the flying angle of the fuse .
AOA is made by a relative change between the front wing and the rear wing. Pitch the front wing up RE to the rear and the plane points up. Pitch the rear wing down RE to the front wing, the nose goes up.
The craft will fly with the rear wing near 2-4Degrees AOA at all times,unless you stall the rear wing . So , if you pitch the rear wing up 4, the nose goes down, pitch the front wing up 4, for level, and you will get the level flight with the fuse pointed down along with the power vector .
If the rear wing moves at 10% of the front wing in the same direction, you may not die.. But I dont see anything good from pitching the rear wing at all.
1/4 scale model it . If it flies as a model it will fly larger.99% of the time . I have 100\$ that it wont fly well enough to scale up. until you reduce the rear wing travel to nil.

#### Victor Bravo

##### Well-Known Member
The experienced engineers all unanimously say to only change one thing at a time. So IMHO leave the aerodynamics of the Flea alone, it is known to work safely, and you are already designing a new fuselage and construction method.

#### Protech Racing

##### Well-Known Member
Right , If in doubt, Copy known good value . Save yurass.

#### rotax618

##### Well-Known Member
Duncan it seems you are trying to re-invent the wheel.
When I built my Flea I started out with the HM293 plans and the Sport d l’Air book, I wanted the simplicity of the HM14 and the safety of the HM293. I superimposed the HM14 on the wing placement and thrust line of the HM293.
My Flea was perfectly stable in pitch, it was the delayed pitch/roll couple that made it uncomfortable to fly.
You will find that you don’t need a very high angle of attack on the ground, the variable incidence of the front wing gives you that.
As for your ply, I couldn’t get the ply to bend around the LE and used a vey light timber for the LE radius and D box sheet top and bottom.
To get an Idea of just how long ago it was, my daughter is almost 50 years old now.

#### rtfm

##### Well-Known Member
Thanks for the generous input guys.
I'm not going to build an RC model, because I can't fly them. But what I might do is to decouple the rear wing, and fix it at somewhere around 2 to 4 degrees. Then once the initial flights have been made and I'm (hopefully) happy with the results, I'll start experimenting with the rear wing pivot. I don't share your pessimism about pivoting the rear wing, however. De La Farge used this to great effect on his "Pulgas", and the numbers certainly bear out the aerodynamics of this approach. But I'm not here to discard your collective caution (and wisdom). So I'm going to meet you half way with the approach above.

Regards,
Duncan

#### Protech Racing

##### Well-Known Member
Sounds reasonable.

#### Victor Bravo

##### Well-Known Member
I'm sorry to push so hard on this, but you don't need to be able to fly the R/C models. There are plenty of people who can fly them in Australia and every other corner of the civilized world.

You can start with small free flight models like Mr. Whittaker's photo shows. I guarantee that he learned significantly from this simple model and the lump of clay stuck on the nose.

All you have to do is build the free flight and R/C models, and be the researcher that is inquisitive enough to collect data and learn from the model testing.

#### nickec

##### Well-Known Member
Burt Rutan made hand launch gliders to investigate planforms. He threw them from a roof.

Remember: his company was named Scaled Composites. A reference to scale models.

Toss some simple gliders. It is something aeronautical engineers do.

I sometimes take 3x5 notecards and make micro-gliders. You quickly learn what works and what fails.

Such tiny aircraft have single-surface wings. I harbor no illusion that they predict everything. Yet they can quickly provide guidance.

#### Sockmonkey

##### Well-Known Member
Yeah, that pivoting rear wing just seems like complication you don't need. I'd really move that fuel tank back a bit due to it's variable weight.

#### Speedboat100

##### Banned
I have built 4 models of my design...and being able to enhance the design considerably with these free flying models.

#### rtfm

##### Well-Known Member
Yeah, that pivoting rear wing just seems like complication you don't need. I'd really move that fuel tank back a bit due to it's variable weight.
Two things:
Pivoting rear wing - have you read my design paper? rtfm-aero.com/pou/technical-resources/ It analyses why a pivoting rear wing is a huge improvement for the Flea.

Moving the tank back a bit.
Is this an instance of eyeball engineering, or is it based on something more substantial? The CG is exactly where it's supposed to be. If you want to check the calculations showing this, the link to the calculations spreadsheet is on the same page as the above link.

Regards,
Duncan

#### rtfm

##### Well-Known Member
I have built 4 models of my design...and being able to enhance the design considerably with these free flying models.
I have a friend who flies RC. I may make a model and get him to fly it for me.

#### Tiger Tim

##### Well-Known Member
Pivoting rear wing - have you read my design paper? rtfm-aero.com/pou/technical-resources/ It analyses why a pivoting rear wing is a huge improvement for the Flea.

#### Protech Racing

##### Well-Known Member
Duncan, I have a few disc or tapes / videos({from a Cub,) of Jack's 293, mixed in with some other Flea vids. Send me your address and I'll send them to you. I'm not sure what all is on the tapes or disc.

#### Sockmonkey

##### Well-Known Member
Two things:
Pivoting rear wing - have you read my design paper? rtfm-aero.com/pou/technical-resources/ It analyses why a pivoting rear wing is a huge improvement for the Flea.

Moving the tank back a bit.
Is this an instance of eyeball engineering, or is it based on something more substantial? The CG is exactly where it's supposed to be. If you want to check the calculations showing this, the link to the calculations spreadsheet is on the same page as the above link.

Regards,
Duncan
I can't access the paper.
You want the tank at the combined COL SO the CG doesn't change as you use fuel.

Hi,
Duncan

#### rtfm

##### Well-Known Member
Duncan, I have a few disc or tapes / videos({from a Cub,) of Jack's 293, mixed in with some other Flea vids. Send me your address and I'll send them to you. I'm not sure what all is on the tapes or disc.
Hi,
That's very kind of you. I've PM'd you my address.

Cheers