The Fleabike

[duncan_rtfm]

Hi,

After much deliberation, and not a few false starts, I decided to build a Flying Flea.  Not one of the usual HM-varieties, but a brand new design, which I'm calling the "Fleabike" because the pilot sits a-straddle the airframe, just like he would in a bike.  Of course, there are other designs using this concept, like the Aeromax, the Airbike and so on.  But this one is a Flea.

 

The airframe consists of CNC-cut Gaboon ply with a 3mm Gaboon skin.  Having a cnc-router helps, of course.

 

These are all the airframe pieces.

 

When all put together, I'm hoping it will look quite attractive. 

The plane is very small, but no smaller than the HM293.  Construction is very simple.  Just line up the correct parts, add epoxy and set aside.

The skins are jigsawed together (my router can only handle half a sheet) and clamped to the airframe.  And before someone asks, the odd-shaped pieces in the photo below is the seat doubler, to address the potential weakness at the point under the pilot's bum.

 

Next job is to bond the firewall and the tailstock, so that the two sides can stand upright.  Then invert so that the two bellies can be bonded.  The bellies are crucial to getting the fuselage square.

 

Once back upright, the addition of the final bulkheads completes the construction of the fuselage.  There are 12 bulkheads in all (Firewall and tailstock of course, the two bellies, six "external" bulkheads (cowl base, two control panels, seat, seat back and turtledeck), and then four internal bulkheads.  But they all fit together quite easily, and with each successive bulkhead, the squareness of the fuselage is enhanced, as is the stiffness of the fuselage, which is essentially a pretty-shaped box.

 

I've actually completed the fuselage already (five days of building, and about 20 hrs of actual construction) the rest of the time was spent waiting for epoxy to cure, and on the computer finalising the wing design. However, I can't find a way to rotate the images on this forum.  If someone can help me out with this, I'll post the remaining pictures.  The forum seems only to like landscape oriented photos, and most of mine are in portrait.  That's a pity.

 

I start on the wings tomorrow.

 

Duncan

 

 

 

 

[Marty_d]

Good luck Duncan!  Looking good.

 

What sort of engine are you considering?

 

 

[duncan_rtfm]

Hi,

Using a Generac 990cc prepared by Valley Engineering.  50hp, 53kg

 

Duncan

[cscotthendry]

Hi Duncan;

I'm probably telling you something you already know, but are you keeping test pieces of your glue ups?
A long while ago, I was building a glued wooden framed kit and every time I did a glue up, I glued two scraps together with that batch of glue. Back then, I was building under SAAA rules as I don't think RA AUS (AUF) was in existence yet. The SAAA inspectors wanted to see the test pieces and see how they broke, ie the wood should fail before the glue.

[duncan_rtfm]

Hi Scott,

This is something I didn't think of doing.  Excellent idea.  Fortunately, I have plenty of pieces from two "failed" attempts at building the fuselage, forcing me to return to the CAD package, redesign pieces and start again.  I'm using West System BTW.

[duncan_rtfm]

Hi,

Please excuse the side-ways pictures, but until I figure out how to rotate pictures, you will have to stand on your head to see them properly.

Here is the airframe bonded to the skin.

 

I mounted both sides upside-down in their cradles, so that I could fit the belly.  As you can see, I favour a concave aft section.

With the assistance of my trusty collection of bricks, I fitted the front half of the belly, applied lots of West System epoxy and waited for it to cure.

 

Followed by the aft section

 

Back on it's feet as it were, I trial fitted the outer bulkheads (Cowl base, both control panels, seat seat back and turtledeck.

 

Total construction time so far: 16 hours  This does not include all the time sitting round waiting for epoxy to cure, or sitting at the laptop re-drawing pieces or actually cutting the pieces.

[kgwilson]

Nice job. You managed to get the first photo in the first post to display in portrait so just do the same or if all else fails take future pics in landscape. 

 

I uploaded a portrait photo from my computer & it worked OK. If you are uploading from a phone or other portable device it may be that device rotating the pic not Recflying.

Edited June 28, 2021 by kgwilson

[duncan_rtfm]

Mmmm  OK, I'll give it a go.  Not sure how I got the first photo in portrait...

[duncan_rtfm]

I cut all the ribs for the rear wing today.  Every rib is a different size, because the wing is tapered.  Next job: cut the jigs for the wing panels (tomorrow's job).

 

[cscotthendry]

37 minutes ago, duncan_rtfm said:

I cut all the ribs for the rear wing today.  Every rib is a different size, because the wing is tapered.  Next job: cut the jigs for the wing panels (tomorrow's job).

 

That's just gorgeous. I love seeing wooden airplane components. I think it's the most elegant use of wood possible.

AND, it looks like you're doing a brilliant job cutting the parts out.

[cscotthendry]

9 hours ago, duncan_rtfm said:

Hi Scott,

This is something I didn't think of doing.  Excellent idea.  Fortunately, I have plenty of pieces from two "failed" attempts at building the fuselage, forcing me to return to the CAD package, redesign pieces and start again.  I'm using West System BTW.

Duncan:

From memory I was using a glue called T80 or something like that. It was epoxy. I didn't finish the build and sold the kit. Back then I didn't believe in using gloves and respirators and I've often wondered if I had completed the kit whether I would have become allergic to the glue.

 

The first pic is the kit I was building and the second is how I was going to paint it.

 

[duncan_rtfm]

Beautiful plane, and STUNNING colours!  The glue is T88.  It's great stuff, but is very thick.   Mixed, it is like cold condensed milk.  So it is great for bonding smaller pieces - you can't paint the stuff on or use a roller.  Strictly tongue-depressor use only.  I have quite a lot of it, and started using it - only to find that the parts I bonded almost fell apart.  It has a shelf life of about 6 months.  Mine was at least four years old.  And it is bloody expensive, to boot.

 

So I am sticking to West System, which paints on very easy, and a roller is perfect for larger surfaces.  My West System stuff is brand new.

 

Another epoxy I like, because it is half-way between T88 and West System in viscosity is Bote Cote from Boatcraft Pacific.  But no good epoxy is cheap.  Even crap epoxy is expensive.  Ha ha.

 

Regards,

Duncan

[kasper]

Agree on the epoxies.  I’ve built using t88, west system and boat cote. 
 

I’ve always done post cure heat treating and the only one that’s a not to use again is boat cote.  The issue is the low thermal softening point even after post cure treatment.

 

I used it to build a white fabric wing and the measured temp inside the wing was too close to the thermal point of the glue for my liking. 
 

west system is my favourite - it can be thickened with west system fiox and makes a great glue and it’s cost is half that of t88.  
 

my flea is going together with west on the wing and t88 on the fuselage just because that’s what I had when I built the fuselage. 
 

wish I had done routed ply ribs - I was over ply discs and the kit of sticks to make them … but they are beautiful to look at and weigh very little.  

[duncan_rtfm]

That's interesting about the thermal softening point of Bote Cote.  I wonder if I can return mine?  😀

[kasper]

For marine use it’s fine … build some nice wooden floats for your flea … then you can be the first splashy flea in oz

 

it should stay nice n cool in water.   
 

note - seriously I found that the white fabric wing was only 15deg c below the critical temp when the aircraft was in 32degc day full sun.  In reality it wears ok as when flying it cooled down but I’d never have painted that wing any colour as that would have probably hit the critical temp and the whole thing goes a bit soft and droopy. 
 

changed over to west and the test pieces are brilliant and it is not just cheaper but easier to use than t88

[duncan_rtfm]

Hi,

I haven't opened my Bote Cote yet - so I'm actually going to ask if I can return it (only arrived a few weeks ago).  Thank you very much for the info.

 

Regards,

Duncan

[duncan_rtfm]

Today I cut the rib "cradle" jig. The ribs rest in this cradle perfectly spaced and aligned. The spar caps (19mm x 19mm Hoop Pine) are then slid into the recesses in the ribs. There is a second cradle jig, so that I can turn the ribs upside-down and bond in the top spar caps.

 

In the photo below, you can see:

1. The "Pivot" mini-spar. This is set at 22.5% of the chord and has a 9mm shear web. This is where the wing actually attaches to the masts, and pivots on it.
2. There are two spar caps at 40% chord (thickest part of the wing). You will notice a gap between them - this is to accommodate the 4mm shear web. I was going to use 3mm, but since I had a sheet of 4mm Gaboon ply, I again erred on the side of caution, and used this instead.
3. I was going to use a single spar, but decided to use two, as you can see.

I think this wing is going to be as strong as a brick SH** house. I'm aiming for 6G (though in a Flea, this is never going to happen).

One of the nice things about a Flea wing is that is can be built in sections. My wing has three sections: the inner section (what you can see here), a middle section (which will fold upwards as per standard Flea doctrine, and a third outer panel, which will fold downwards. This gives me 5.75m span (on the rear wing) with an aspect ratio of almost 7 which is more than almost all Fleas.

[kasper]

Design concern - your pivot spar is not your lift spar therefore you are expecting the full lift forces to be passed from the lift spar to the pivot spar through the ply ribs - I would reconsider that and use the same full span spar as both your lift and pivot as you are then not passing significant loads through the ribs

 

Design comment - I am assuming that your pic omits the verticals on the ribs that will make each of the ply spar webs complete framed panels.

Design comment - while its a hassle its not a big hassle to cut your ply spar webs on the 45deg - especially as they are small panels and you have a cnc you can use to cut them.  that 45deg face ply orientation is significantly stronger that running the face grains along the spar. 

Design comment - having effectively a five piece wing does make the folded span really narrow but you are adding quite a lot of weight in hinges and pins.  Why do you want it so narrow?  if you are folding it to then road trailer it with wings on consider that the trailer loads on the fittings for a heaver wing up on thin struts will require additional support fixings which are a bugger to design nicely if you do not 'lock' the airframe undercarriage springing and just have trailer springing - a few people found to their cost that a trailered SV2 vampire with the supports on the wings managed to damage both the wing bolt points and the tail boom attach plates - both of which are a bugger to get to and replace.

 

Operating comment - you definitely CAN achieve 6G  in a flea ... you can also achieve over 100kns in a single seat flea with 40hp ... I did both when test flying a flea in and around all the corners of the envelope.

[duncan_rtfm]

Mmmm  You have a point...

You said: "Design concern - your pivot spar is not your lift spar therefore you are expecting the full lift forces to be passed from the lift spar to the pivot spar through the ply ribs - I would reconsider that and use the same full span spar as both your lift and pivot as you are then not passing significant loads through the ribs"

I can't  use the main spar as the pivot spar - the main spar is at 40% chord, while the pivot spar is at 22.5% (i.e. ahead of the CP).  I'll have to think about this.  Good catch.

 

Duncan

Edited June 30, 2021 by duncan_rtfm

[kasper]

Well that issue of getting the pivot on the cp or there about a is the reason fleas NEVER have the spar way back at 40%

 

the spars end up front on the cp and the rear spar way forward at around 66%.

 

flea pivot wings never need strength in the rear of the wing - there are no control surfaces and no twist forces from ailerons - but every Pou flying has a single lift/pivot spar at/just in front of the cp

 

hate to say to you if it were me I’d use the ribs you have for the rear fixed wing as you can use the lift spar as the fixing spar but start again and redo the design and ribs for the front wing with the lift spar forward. 

Edited June 30, 2021 by kasper

[duncan_rtfm]

Hi,

I'm afraid you're right.  I spent most of last night and then again at sparrows this morning working on the wing.  Two spars.  Main spar now at 22% chord, rear spar remains where it was.  Checked out the HM360 and HM293 plans I have to verify pivot placement.  I am very please you picked this up.  I am now waiting for new router bits to arrive by express courier so that I don't lose too much time, and then I'll cut new ribs (fortunately, I had only cut the ribs for the inside panel, and I have extra ply in the shop, so no big deal as far as that's concerned.

 

And speaking of time - while I'm waiting for the bits to arrive, I'll go and re-check the wing.  Again.

 

Duncan

[kasper]

Well if you are reconsidering the wing  ...

All semi modern pous I have flown (14,290,293,360,1000,1100 plus non-Mignet pou layout) have an issue with the trim on the front wing.

 

They nearly all have nicely put very low cp movement sections on the wing so up to stall the stick load is linear to angle of attack/load with a significant increase in load at the stall as the cp moves.

BUT nearly all have put the pivot point in front of the cp and have a constant pull load on the stick.

I have flown with bungee trim to restrict the stick load in the mid range of the flight envelope and also with a fixed trim tab on the front wing to achieve the same unloading of the stick

BUT if you are looking at redesign and redo on the front wing consider exactly where you put that pivot point to minimise the trim load.

 

Look at the graphs for your chosen section (or change your section choice) to put your pivot at a  point that will be minimally in front of the most forward point the cp is likely to be at in normal operations - usually occuring when at minimum AofA. 

You will then find the pull force in flight much less of an annoyance than it otherwise would be.

 

eg the HNM290FB has a pull force that is distracting for longer than 1 hours flight

 

I think you will find the pull force likely to be less than ideal at 22% of chord pivot.

 

To work out the expected pull force you will need to work through the entire airframe calcs as its one of the last numbers that pops out after you have worked through lift splits fore/aft and the sectional AofA for the selected airspeed.

 

And as a reminder - the pou layout is a cannard and the rear wing is at a significant apparent AofA reduction due to downwash from the front wing so the actual load on the rear wing is amazingly low so you generally have to estimate a massive differential between fore/aft wing loadings/area.

 

Have fun

[duncan_rtfm]

Great point, and it's great to learn from your experience.  However, this is something I have already considered.  The pivot will not be fixed, but will have an extension bolted to the main spar, something like this, allowing for ground-adjustable pivot point of 22%, 23% and 24% chord.  This will allow me to sort out the optimal pivot position during testing, and permit pilots to choose a setting to their own preference.

Edited July 1, 2021 by duncan_rtfm

[kasper]

just like all the trike out there - variable hang points.

 

Just be grateful I did not suggest an electrically operated hang point movement for the front wing - very nice on the trikes I've flown with that but it is a bit overkill for a single seat flea.

[duncan_rtfm]

Hi,

Nice to know my idea isn't way out there.  Just seems sensible.  But please keep a watchful eye on my build and save me from further SNAFUs

 

No building today - router bits didn't arrive.  Bugger.

 

Regards,

Duncan

Edited July 1, 2021 by duncan_rtfm