# The AFB (Amazing FleaBike)

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#### Sockmonkey

##### Well-Known Member
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.
I'm talking about when the tail lifts due to an increase in speed. Even in a dive, there's a limit to how fast the tail would lift because nosing down fast would put the rear wing at neutral or negative AOA. You still need to pull back the stick to pull out, but it's not going to flip too fast for you to correct.

#### Martin R.

##### Active Member
Hi Duncan,

Iâ€™m sorry to bother you again. But Iâ€™m trying desperately to follow your Flying Flea considerations. (At least some of them ;-)

If I compare the MAUW in your todayâ€™s "Flea Design Paper spreadsheet calculations" (Technical resources - The Backyard Builder's Forum) with the MAUW in your posting #243 The AFB (Amazing FleaBike) I take note of another serious difference:

Is this the consequence of no longer following the ideas of Jean de la Farge (Pulga) or are there other reasons?

#### Speedboat100

##### Banned
Hi. The fin is 36mm (two 16mm layers of ply, covered with 1.5mm sheet + epoxy = 36mm) and the rudder is one layer of 16mm + 1.5mm skins. About 19.5mm

Duncan

Sonja Englert seems to prefer 12% thick...I am for the 10% thick as it could be supersonic.

I can get pretty sturdy yet lite spar inside there.

#### ragflyer

##### Well-Known Member
RTFM, I am sorry but there are lots of misconceptions in this thread.

In the first order, the stability of any two surface airplane (whatever the relative dimensions of the fore and aft wings are) is a function of:
1. CG position,
2. size and relative position of the two wings wrt CG
3. the ratio of the lift slope of the two surfaces(which in turn is a function of AR)
However It is not a function of the incidence of the surfaces wrt to the fuselage. IOW changing the relative incidences, in the first order, will not make an otherwise stable airplane unstable or visa versa. This is an important concept to fully understand. Otherwise you can talk yourself round in circles and try to fix phantom problems.

In the first order, the control range (speed range) is a function of the relative incidence range of the surfaces. You can change the fore wing incidence or the rear wing incidence or both. You can decrease the speed by either increasing the incidence of the fore wing or decreasing the incidence of the aft wing or some combination of the two.

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.
It should be clear now this cannot be true. There is nothing special about how any stable flea derivative flys. When you reach the end of the fore stick control range you cannot go any faster. That is the fastest speed you can fly. It will not suddenly dive because of some underlying peculiarity of the flea configuration relating to rear wing overpowering the fore wing and relative incidence. Of course this assumes the flea is stable to begin with (some of the early fleas where not). Also there are some second order factors that are more relevant in flea type designs but this does nor preclude from the above principle.

If you are not satisfied with this description, I would recommend building a spreadsheet to calculate the pitch equilibrium of a flea design. You will find it very illuminating and will uncover many of the misconceptions in this thread. If you have already done this and found it to support your theory then please share your spreadsheet and I will be happy to review and correct it for you. The basics of FW stability have been very well understood for about a century.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
I would recommend building a spreadsheet to calculate the pitch equilibrium of a flea design
This^
Start with chapter 5 of Perkins an Hage. Just the first 4 pages up to Figure 5.3 should be enough study to start. The derivatives involved are pretty basic. If you have no calculus background the resulting stability derivative formula(s) can be plugged into an online solver to get formula(s) for a spreadsheet.

Other than the interaction of the 2 wings, and this phenomena also applies to biplanes, there is nothing magical* about the Flea concept.

* excluding the charm, uniqueness and practicality.

#### Protech Racing

##### Well-Known Member
The Flea is perfect for leaving the water . Pitching the front wing lifts the body without depressing the tail .

#### Sockmonkey

##### Well-Known Member
The Flea is perfect for leaving the water . Pitching the front wing lifts the body without depressing the tail .
Yep. Posted a few "sea flea" concepts on here.

#### rtfm

##### Well-Known Member
Hi Duncan,

Iâ€™m sorry to bother you again. But Iâ€™m trying desperately to follow your Flying Flea considerations. (At least some of them ;-)

If I compare the MAUW in your todayâ€™s "Flea Design Paper spreadsheet calculations" (Technical resources - The Backyard Builder's Forum) with the MAUW in your posting #243 The AFB (Amazing FleaBike) I take note of another serious difference:

View attachment 107940

Is this the consequence of no longer following the ideas of Jean de la Farge (Pulga) or are there other reasons?
Hi Martin,
The initial figure of 320kg was an estimate of what the MAUW might be taking the worst case, and simply hard-coding it in that cell. Since then, I've managed to update the weight estimates, (mainly the weight of the wings and the airframe) and now the 243kg reflects the empty weight plus the expected load (pilot, baggage etc) - a calculated weight. This is probably the most optimistic estimate. Both are estimates, and I expect this number to finally end up somewhere between them.

Interestingly, I had a visit from a very keen prospective buyer of Serial # 002, who was asking about using a much lighter engine outputting about 35hp (the Polini Thor). He's a big lad (110kg) so we played around on the spreadsheet to see what the nett result would be. Based on the latest weight estimates for the airframe and the wings, MAUW = 260kg, but the CG moves over 150mm rearwards, seriously close to the combined centre of lift. But fortunately, being a Flea, one can move the front wing forward/rearward by adjusting the cabanes, so it's doable.

#### rtfm

##### Well-Known Member
Sonja Englert seems to prefer 12% thick...I am for the 10% thick as it could be supersonic.

View attachment 107947

I can get pretty sturdy yet lite spar inside there.
I thought long and hard about the fin and rudder. I was going to use a 12% symmetrical airfoil. But my stumbling block was working out the compound curve of where the fin meets the turtledeck. I just couldn't work it out. Having built my fin now, I see that there was no need for all those hours in front of my CAD program trying to work out curves. Just build the find right down to the turtledeck base. Doh! So I built a flat fin and a flat rudder. (This plane is hardly going to break any speed records). They were extremely simple to build, and only took an hour or so. I cut the supporting structure on the router, cut the skins and bonded them together. Then had a cup of tea.

#### rtfm

##### Well-Known Member
<snip...> There is nothing special about how any stable flea derivative flys. When you reach the end of the fore stick control range you cannot go any faster. That is the fastest speed you can fly. It will not suddenly dive because of some underlying peculiarity of the flea configuration relating to rear wing overpowering the fore wing and relative incidence. Of course this assumes the flea is stable to begin with (some of the early fleas where not). Also there are some second order factors that are more relevant in flea type designs but this does nor preclude from the above principle.
The fact is - there IS something special about how Fleas fly. They have two lifting surfaces. There is no "sudden dive" as you describe it, but a gradual and inexorable nose down attitude caused by the rear wing lift. At 35kts, the front wing (with stick pulled back to a max pitch of 12 deg) will produce 174kg of lift, while the rear (set at (say) 6 deg, will generate 64kg of lift. At 50kts, however, the front wing (now at 5 deg) generates 160kg of lift, but the rear wing (still fixed at 6 deg) is generating 123kg of lift. At 60kts, the rear wing generates 190kg of lift, At 70kts, it generates 256kg of lift - more than the MAUW of the plane.
As the Flea speed increases, the rear wing can't but help produce increased lift - unlike the front wing whose incidence can be changed.

There comes a point at which the front wing incidence is decreased to the stops in order to keep the plane flying level.

Setting the rear wing at a lower incidence improves the situation, of course, at the expense of the lift that can be generated on takeoff and landing.

I know you mean well, but this last statement is incredibly condescending. My spreadsheets are available on my website.

Duncan

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#### BJC

##### Well-Known Member
HBA Supporter
Could you re-write that explanation using AoA rather than Incidence?

Thanks,

BJC

#### Protech Racing

##### Well-Known Member
Glue some foam to the LE of the tail stuff and shape some love into it.

#### Hot Wings

##### Grumpy Cynic
HBA Supporter
Log Member
They have two lifting surfaces.
Have you tried putting both of your surfaces into JavaFoil? JF lets you put in 2 surfaces and then take a look at the results. The streamlines can then be printed to share here to help us visualize your thoughts.

#### Sockmonkey

##### Well-Known Member
If the AOA of the fuselage decreases, then the AOA of a fixed rear wing would decrease along with it, so why wouldn't the amount of lift it creates also decrease and prevent tipping past a certain point?

#### ragflyer

##### Well-Known Member
At 35kts, the front wing (with stick pulled back to a max pitch of 12 deg) will produce 174kg of lift, while the rear (set at (say) 6 deg, will generate 64kg of lift. At 50kts, however, the front wing (now at 5 deg) generates 160kg of lift, but the rear wing (still fixed at 6 deg) is generating 123kg of lift. At 60kts, the rear wing generates 190kg of lift, At 70kts, it generates 256kg of lift - more than the MAUW of the plane.
As the Flea speed increases, the rear wing can't but help produce increased lift - unlike the front wing whose incidence can be changed.
Duncan
RTFM, You are confusing incidence with the actual AOA the fore and aft surfaces see. As BJC says think AOA not incidence. After all the lift generated by the surface is a function of AOA not incidence wrt fuselage. Ask yourself when the speed changes what will be the AOA of each of the surfaces. For that you will need to solve for equilibrium wrt to weigh and lift and also pitching moments.

#### Vigilant1

##### Well-Known Member
As BJC says think AOA not incidence. After all the lift generated by the surface is a function of AOA not incidence wrt fuselage.
Yes. The situation will be very dynamic, there's no invisible hand keeping the fuselage aligned with the relative wind so that the incidence of the front and rear wings will also be their AoA.

Compared to an RC model, a model in a "cartop wind tunnel" would be simpler to build, easier to control, impossible to crash, and, with a simple video camera, offer an easy-to-measure and study depiction of the AoA of each flying surface and the fuselage "deck angle" at various airspeeds. The RC model could be next.

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#### rtfm

##### Well-Known Member
The RC model is being built by a friend.

#### rtfm

##### Well-Known Member
Could you re-write that explanation using AoA rather than Incidence?

Thanks,

BJC
Hi BJC,
No - mainly because we're talking about the angle of the wings in relation to the airframe, not to the airflow.

Duncan

#### rtfm

##### Well-Known Member
Yes. The situation will be very dynamic, there's no invisible hand keeping the fuselage aligned with the relative wind so that the incidence of the front and rear wings will also be their AoA.

Compared to an RC model, a model in a "cartop wind tunnel" would be simpler to build, easier to control, impossible to crash, and, with a simple video camera, offer an easy-to-measure and study depiction of the AoA of each flying surface and the fuselage "deck angle" at various airspeeds. The RC model could be next.
Hi Vigilant,
Perhaps you missed my earlier post, saying that I am NOT going to implement dual pivoting wings in the first plane. It is going to be a bog-standard Flea. Once the FleaBike is airborne, and I have some significant airtime in it, I will start introducing the dual pivoting concept (if I still feel the need to do so).

In the meantime, a friend is building a quarter-scale version of the FleaBike with dual pivoting wings. So the above discussion is now largely academic.

Regards,
Duncan

#### Martin R.

##### Active Member
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. ...
... Why? Because two very keen customers were getting cold feet about the number of innovations I was proposing. ......
Hi Duncan,

I didn't know that you already have customers for your Amazing FleaBike although - beside the fuselage - it only exists on paper and has never flown. But why not? Boeing and Airbus do the same ;-)

But fun aside. Although the pivoting rear wing is now only an "academic question" in your thread, here some links which I wanted to show for a long time and which might still interest some people:

I think that on this picture you see the configuration "start/landing"

pouduciel montpezat_2004 look for "Milvius" . The first picture shows the "cruise-configuration" and the forth the command to change between the configurations.

Pou-Guide - Les poux Ã  "plan arriÃ¨re redresseur" look for "MILVIUS". The text mentions a minimal speed of 30 km/h!

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