The AFB (Amazing FleaBike)

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rtfm

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It will work better /simpler, splitting the front wing .
I thought of that, but the front wing is a single organic wing, with a central panel of 2.5m (approx 8 ft) and two outer panels of 1.25m (just over 4 ft), the outer two folding up and over the top (as per all Fleas). So I don't really have an option. I have had to beef up the aft section (which is fortunately very short) with internal bulkheads to counter any torsion.

Duncan
 

rtfm

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AFB front quarted 30 Dec 20.jpg
The five key bulkheads (in order to ensure a perfectly square fuselage) are: Firewall, Cowl floor, Seat, Seat back, and Turtledeck floor (not seen). With these bulkheads in place, the airframe aligns perfectly. Anyone who decides to build a FleaBike is going to have a really easy time of it.
 

rtfm

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Clamping airframe.jpeg
In this photo you can see the three pieces which constitute the one side of the airframe. A front section cut from a quarte-sheet of 19mm Hoop Pine ply, a rear section (using another quarter sheet) and a 50mm piece of Hoop Pine to bond the two halves together. Since this photo was taken, I have added a 9mm reinforcing doubler under the seat (bonded to the inside). Altogether, this makes for an extremely strong, yet surprisingly light airframe.
 

Protech Racing

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I've worked on 2 "Fleas". A 293 modded to spread the wings10in. This was the builders second Flea. Much better than the first that had the standard very close wings.
My Tandem shown above. As I looked at the Fleas and did the math, I found that the close wings made for pitch twitchiness and very unsafe conditions. Thus the 10 in or more wing spread . If you look at current builds, most have spread then out a little .
When analyzing the CG and fight performance, we determined that the rear wing is about 61-65% as efficient as the front wing . It carried around 32% of the total mass. The front wing is loaded around 67-70 percent of total for best performance. 67% being optimal for my plane and 70% for Jack's 293.
Model it before you fly it please. I still have my test model .
 

rtfm

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When analyzing the CG and fight performance, we determined that the rear wing is about 61-65% as efficient as the front wing . It carried around 32% of the total mass. The front wing is loaded around 67-70 percent of total for best performance. 67% being optimal for my plane and 70% for Jack's 293.
Model it before you fly it please. I still have my test model .
Interesting stats. Thank you. The weight carrying ability of the rear wing is, of course, dependent on its geographical proximity (both in the horisontal and vertical planes) to the front wing. I've determined that if the rear wing is 35% of front wing chord behind, and 16% of front wing chord below - it will experience almost zero down or upwash caused by the front wing. See my paper pp 10 - 18 (especially pg 15). So in my design, the rear wing carries 40% of the weight.

Regards,
Duncan
 

Protech Racing

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What sizes are your wings? Look at my separation. 40 in spread and 28 +- vertical . More than yours.
My front wing is 21 x 52in. rear wing is 16 x52 in. Both have the same 43015 foil.
I doubt that 40 % on the smaller rear wing will fly well and may well stall before the front wing, esp if you are going to be shaking the wing with controls.

Can any one X plane this please?
 
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rtfm

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Sorry - struggling to figure out your dimensions. Front wing 21 (ft?) x 52 inches? And do you mean your horisontal gap is 40 inches? Also, what does the "+- vertical" mean? Approximately?

My front wing has a span of 5m, and chord of 1.2m (16.4 ft / 41 inches). Rear wing is 4m/1m (13.1ft/39in) Horisontal gap is (as currently drawn) 72mm and vertical gap is 866mm, although the horisontal gap is provisional only. It has changed significantly over the course of the design. Remember, the sketches I've provided are working sketches only. It will only be finalised when the plane is built and I can measure the CG accurately - with both wings, engine, gear and pilot in place. In all probability the front wing will have to move forward quite a bit.

As the wind tunnel picture below shows quite clearly, the red circle is the sweet spot (probably a bit further forward, actually) - and that's what I'm aiming for as far as the leading edge of the rear wing is concerned. Ideally, I've calculated that the rear wing LE should be about 10% - 15% of the chord of the front wing behind the trailing edge of the front wing, and at least 35% below it at full deflection.

1610693416025.png
Oh - airfoil is the F5Fras15
 
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rtfm

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I always knew (of course) that I'd need a fuel tank, but I only started looking for one today. And I've run into a problem. Nothing I've found (in the affordable plastic variety) will fit inside my fuselage. It is only 360mm wide at the firewall, and then steadily decreases from there. However, my trusty 20 litre PVA jerry can fits just perfectly. Is it possible to use one of these? If so, how?
 

rotax618

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Evans Head Australia
The ICP Savannah uses polyethylene fuel tanks in the wings, they were never treated for porosity so there is always a strong petrol smell in the cabin.
A certified petrol (gasoline, dangerous goods) 20 litre container would be suitable if one fits, similar containers are used in marine applications (outboard motor fuel tanks) where fumes can lead to explosion, they are treated to minimise porosity.
Supply and vent outlet tails with flanges can be fitted from the inside with the retaining nut on the outside, the container acting as the gasket. I have seen quick release outboard fittings used so the tank can be removed and taken to the Gas Station for filling.
 

Will Aldridge

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Northern Utah
Well this outfit is in America but they cater largely to off road type vehicles so I'm guessing there is a similar company in Oz. But you could get a 12 inch diameter tank of probably whatever length you choose to sit vertically in your fuselage with the cap sticking out the top.


I had these guys make me a custom task for my kitfox and they welded in the bungs I specified where I wanted them. Also very lightweight for the capacity.
 

rtfm

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Hi,
My polyethylene 20litre container fits perfectly (albeit a little snug). I was worried about the supply and vent outlets, but if these can be fitted, then my problems are over. This is great news. Now all I have to do is to bond in some structure around it to keep it fixed in position. I'm sure I can get local help as far as fitting the supply and vent fittings are concerned. Thanks guys.

Duncan
 

Sockmonkey

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Is it just me, or does it seem that only the pilot's mass is at the 25% combined chord, and combined CG of pilot and engine is ahead of the fore wing. That would be normal in regular plane with a down-pushing tail, but this is a flea type. I know you're a smart guy so maybe I'm eyeballing it wrong?
 

rtfm

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I'm a "smart guy"? Thank you, but check with the long-time members here, and they will quickly disabuse you of that flattering notion... Ha ha.

The entire plane needs to be seen as a single entity. Essentially, the centre of lift needs to be behind the total CG. That's the fundamental truth of all powered flying machines. The only reason the Flea presents "problems" is that it has no tail. Other than that, no difference. The historical approach has been to use the "25% of combined chord" shortcut. But that's all it is. A convenient shortcut to a first approximation. The aerodynamic basis is simply to put the centre of lift behind the CG. So one needs to work out (1) the CG (by measurement or calculation), and (2) the centre of lift of the aircraft as a whole. Make sure the centre of lift is behind the CG and you're good to go. How much behind? My calculations suggest anything from 5% to 10% of the combined wing chord (i.e. from the front wing LE to the rear wing TE) will result in a nicely flying plane. Closer to 5%, the more "twitchy" and closer to the 10% the more docile.

The Fleabike is (currently) at 7% - but this may (and probably will) change as the build develops.

Hope this helps.

Regards,
Duncan
 

rtfm

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Good afternoon gentlemen,
It is currently pissing down with dogged persistence here. The pool is overflowing, and the grass doesn't know what has hit it. The shed is wonderfully waterproof, of course, so the plane is safe and sound.

I trial fitted the bulkheads today - and just as well I cut them from 3mm MDF, because none of them fit. It's very tricky with bulkheads which are at an angle, but I've made some adjustments, and tomorrow I'll cut a new batch and see how well they fit. When I'm happy, I'll cut into the beautiful 4mm Hoop Pine.

I have been fretting over what to use as a gas tank. I reckon 20 litres aught to do it, but there is precious little space inside this tiny airframe. I checked online, visited the buggy, and yachting sites, but nothing really. Some suggestions from the HBA community helped, and then I flashed on the fact that I have a 20 litre Poly petrol can. I tried it, and it fits like a glove in the forward area just behind the firewall. I had to change the cowl base bulkhead to accommodate it, but it looks like a very good option. I am guessing one can find feeder and breather fittings. At 7.5 litres/hr, that'll give me 2 hrs + reserve. More than enough. Oh - did I mention these petrol cans are cheap.

I also trial fitted the roof today. Every bulkhead (including the roof, seat, seat-back) helps square up the fuselage. I'm very happy so far. Now I just have to figure out what bulkheads I can bond in place that still allow me access to the control rods and bellcranks.

Finally, I cut the MDF test pieces for the cowl. Front and back semi-circles, over which the cowl will be fixed. These will need to be on hinges, because I need to get access to the gas tank.

That's it for today. I'll send some photos as soon as the Internet comes back on line.

Regards,
Duncan
P.S. I have a small but growing number of people who have expressed and interest in this little plane. I email them with updates (the above is today's update) and a regular basis. Since I don't always post updates on HBA, if you'd like to be included in my email list, please let me know (PM me with your email addy, and I'll include you).
 

Protech Racing

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You need to keep the pitch CL as far in front of the CG as you can . More is better . They crash when the CG and pitch pass each other, hence my strong recommendation to spread the wings , to stabilize the pitch moment arm and the CG relationship.
Yes the front wing will need to go foreword about 8-12 inches.
 

rtfm

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Hi Protech Racing,
"Pitch CL" = "Pitch centre of lift"? That is a term which (to me) makes no sense. If by "pitch CL" you mean simply the centre of lift, then I must strongly disagree with you. The CL must be BEHIND the CG. The CG must lead, otherwise the tail will want to swap ends with the nose. An arrow only flies true because the head is so much heavier than the tail. The CG "pulls" the plane.

In a "conventional" plane, the CG should be ahead of the CL by a margin of 5% to 10% (according to Raymer). It is no different for the Flea. The only difference is that in a conventional plane, one only has to worry about the CL of one wing. In a Flea, we have two wings to worry about, so we need to calculate their combined centre of lift. But the principle is the same.

If the CG is too far ahead of the centre of lift, the plane will be sluggish. It's the same for Fleas as for conventional planes. Too close, and the plane becomes too sensitive. CG and CL align = an unflyable sensitive plane. CG behind the CL = incontrollable and crash.

As for moving the front wing: no can do. It's position is completely at the mercy of the planes overall CG. Just for reference, however, many Fleas have flown successfully with zero or even negative horisontal wing gap (not that I intend to emulate this design choice):

1611003296944.png

The below photos tell an interesting tale. I have been doing some more approximations based on projected weights of components, and it looks likely that my rear wing can be mounted further aft, giving me a 38% horisontal gap "stretching" the wings as you suggest, while retaining a whopping 74% vertical gap. The photos below indicate perfectly smooth air in that region. This will give me a centre of lift of 644mm from the front wing LE, and a CG of 455mm - resulting in a margin of stability of 7%

1611003428495.png
 
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