The AFB (Amazing FleaBike)

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,851
Location
KBJC
Good, Then you are familiar with "happy little clouds" and "happy little trees."

That's important for a pilot.
Happy little moment. Let's give him a shear friend over here to balance things out.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,851
Location
KBJC
Hi Duncan, I had hoped to get back to you on the front spar sooner--sorry about that. The front spar's looking good. Based on allowable stresses for spruce, the bending stress M.S.=1.48 and shear stress =1.28 (M.S.=0.25 would be plenty satisfactory for me). Looking forward to your test results.

1621125576444.png
1621125604786.png
1621125645413.png
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
Static load test - some help?
Hi. I have a few questions regarding static load testing of my wing.
  1. My rear wing is in two halves, each consisting of two panels. Each half is supported by struts at the hinge line. How do I static load test the wing? Obviously the wing needs to be upside down, and supported at the root and at the struts. I was thinking of a rectangular base with a gantry from which to suspend the struts. Is this a sensible arrangement? Does anyone have a sketch of a workable arrangement?
  2. I understand I would need to build both panels, in order to test the hinges also. Is it possible to test the inner panel only?
  3. My aim is 6G, which requires the rear wing half to withstand 360Kg. Would you recommend these panels be discarded and a new wing half built?
  4. Possibly a stupid question: Does the wing need to be fully skinned for the test, or is testing the spar alone sufficient?
  5. Once (hopefully) successfully tested, would this be sufficient to proceed building using the same method and materials for the remaining wing sections?
Regards,
Duncan
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
Sorry guys - some more questions:
The wing pivots on the 20% chord line, which is also where the struts attach - but the spar is at 40% chord. Do I:
(a) ignore the pivoting, fix the wing and just measure the ability of the spar to withstand the weight put on it? If so, where do I attach the struts? Ideally they need to be on the spar.
(b) Fix the wing pivot at zero degrees at the root, attach the struts to the 20% chord position and hope the wing skins can cope with the torsion?
Finally, do I apply the weight on the spar line? The 20% line? The 25% (centre of lift) line?
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
8,321
Location
Fort Walton Beach, Florida, USA
I would test the wing the way it's going to be on the plane. So I would install the strut fittings (pivots) and pushrod fitting(s) (pitch control) and suspend the whole wing upside down from those, then load the wing according the approximate center of pressure. If you test it to limit load (the max you expect the wing to see in service) than you can reuse it, if you test it to ultimate load that's a more difficult question and would require very careful inspection before deciding to keep the wing. If it shows no sign of any damage or deformation at ultimate load then it's probably overbuilt. That's my two cents, anyway.
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
OK, disaster at the RTFM back yard shed...
In building the Fleabike fuselage, I painted myself into a corner, so to speak.

First, not having a builder's manual to follow, I omitted to insert the captive nuts on the inside of the airframe before bonding the bulkheads in place and closing the fuselage top and bottom. Now I can't reach inside to position the captive nuts.

This is a major issue. And it doesn't help that I didn't seal the inside either.

I pondered this long and hard over many a bottle of Chardonnay, even considering grinding off the belly to allow me access.

In the end, I reached into my fast-emptying back pocket, bought a few more sheets of gaboon (oukume) ply, and cut a new airframe.

Bugger.

This time, the evolving builder's manual has been updated and triple-checked.

What this SNAFU rebuild DID do was give me the opportunity to update the design a little. Slightly narrower aft end, moving the rear wing above the deck so that the central section could be built as a single section, slightly more rakish belly shape (thereby increasing the cross section of the already identified weak area under the seat) and implementing single front wing masts.

Pictures when I get a chance.

I'm experiencing some issues with the cnc router which I have to attend to first.
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
2,226
Location
Flint, Mi, USA
That's just something you have to accept when making something for the first time because you're learning how to make it as you go. You're still allowed to swear though, so that's something.
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
After some issues with my CNC router (turned out to be a broken flex coupling) I finally got to cutting a new fuselage. Took most of the day, but I now have 30 freshly cut pieces - the total parts count for the fuselage - including seat, bulkheads, firewall and so on. There's not really a lot to it, to be honest.

I'm away for the weekend visiting a mate who lives a few hours away to do some flying, so no building this weekend.

1622802081804.png
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
Destructive testing? How? There's really not very much to test. It's just a (pretty) box to be honest. It's the wing that really needs to be tested.

And speaking of wings, I've been agonising over the basic design of the wing ever since you guys pointed out the inherent weakness of the shear web discontinuity and the 30mm wide foam ribs. So I've gone with 7mm wide ply ribs throughout.

What I think I'm going to do is to test the original wing to destruction and build a new wing. The original wing is rectangular, but the new wing layout will be like this:
1622841968425.png
Three panels, dihedrals of 1 deg, 2 deg and 4 deg. Root chord of 1000mm

The only thing worrying me is finding a happy compromise between wing area (I lose a significant amount of wing area by sweeping it forward with decreasing chord) and the ability of my router table to only cut 1200mm wide wood. I can easily cut ribs of 1200mm, but the limiting factor is the frame on which the wing panels rest - both to ensure straightness, and to get the dihedral right.
1622842771170.png

As you can see, the rib frames extend beyond the ribs themselves, so I'm limited to ribs of 1000mm.

QUESTION 1:
Referring to the plan view above, which is preferable? - straight leading edge, or straight trailing edge? Is this a matter of personal preference, or are there aerodynamic reasons to prefer one over the other?

QUESTION 2:
Fleas have traditionally had rather short wings. With my double-fold, I can increase span significantly (to recapture some wing area). Those of you with Flea experience: how will increasing the wing span affect flying experience?
 
Last edited:

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
8,990
Location
KWHP, Los Angeles CA, USA
Straight trailing edge is superior aerodynamically. Locating the lift where you want it, in relation to the CG and all that... may favor the other way around. Forward sweep may make your spars easier to build than swept leading edges. But purely from an aero efficiency, climb, and low speed/ stall behavior point of view, the straight TE is better.

BUT... please take a look at the idea of a constant chord wing with trapezoid tip sections, like the Wittman W10 Tailwind. You get the majority of the aero benefit of a tapered wing, with only a minimum of extra effort. It keeps your spars straight, 90% of the ribs the same, etc.
 

WonderousMountain

Well-Known Member
Joined
Apr 10, 2010
Messages
2,239
Location
Clatsop, Or
No, I like what you have here.

Count lift loss from planeform reduction @ 50%.
As in it was only lifting at half rate, Don't worry
About new rate. Anyhow, I drew up a modification.

As you like,
CK LuPii
IMG_20210604_151717_kindlephoto-3734618.jpg
 

Tiger Tim

Well-Known Member
Joined
Apr 26, 2013
Messages
3,765
Location
Thunder Bay
Destructive testing? How?
I dunno, maybe see how the skinny part under your butt reacts to torsion? Some around here have implied that a Flea-type sees a lot of torsion through the fuselage and honestly destructive testing sounds more fun than chucking it in the bin. Plus if you document it and it stands up to an enormous strain you can silence the naysayers while if it cracks early you can quietly improve the next version instead.
 
Top