Scrappy Doo.

Scrappy is coming along nicely. The idea is to make something out of the re-cycled parts from the crashed Cherokee & anything else I have lying around. I still haven't found a better way of covering foam than brown paper & wood glue, fairly light, nice & strong & paintable. So that's what it's getting.

Nice one! love a bit of up cycling, give polyester silk a go, laid up with wood glue it would be bomb proof.

I did think about trying polyester silk Paul. The downside is cost & you can't sand it. There may also be a weight penalty as I think it would soak up an awful lot of glue. I haven't ruled it out for future projects though.

Scrappy is all but finished & should be ready to maiden tomorrow, if the weather forecast is correct. The design is inspired by ultralight aircraft.
All though it isn't exactly ultralight from a model standpoint, it is at 41oz all up the lightest Iv'e built to date. As you can see it has triple 6mm carbon fibre booms, which took some time to get straight & parallel. The top two run the width of the wing inside the fuz. Reinforcing bars are in 3mm carbon & fixed in with JB Weld. The vertical stab, wing & horizontal stab are from the Cherokee, although the V/stab has been modified. 9 gram corona digital servos have been used all round. Motor is an 840kv brushless outrunner with new bearings, spinning an 11 X 7.5 prop & will be run on 2200 mah 3s. ESC 30 amp, which should be ample, as it's not going to be a rocket! There is no landing gear, just a simple skid on the bottom.

At 41 oz I would estimate you will have about 100 W/lb which should give a quite acceptable performance given its relatively clean air frame.
One of the big advantages of a belly lander.

Well as can be seen in " been flying", it doesn't ! well not at all well anyway. It is extremely pitch sensitive, especially when power is applied, even with the CG moved to 25% & at least 3 degrees of down thrust, maybe more. In the air it exhibits what I believe is called an unstable Phugoid, & I think I now know why. On closer inspection a few things stand out; 1. The H/stab was not parallel to the centre line & had a negative incidence. 2. It was at a height that put it directly in the wash from the wing. 3. And i'm guessing here, with the fuselage ending at the trailing edge of the wing, & only the booms to affect airflow between there & the tail, decalage would be even more critical?
I have stripped the fuselage down & will start again. I already have some ideas on how to remedy the problems. It can wait until the weather turns though.

crash
I wonder if it is the lack of area from the skeleton rear fuselage that is creating some of the pitch sensitivity. A covered fuselage will create a damping effect that is lost with skeleton. The only damping now comes from the fixed portion of the tail plane itself and that being a flat surface will have completely different characteristics to a 3 dimensional covered fuselage.
Just a thought.
It is certainly true that my thin boom planes are rather more twitchy in pitch and thus to CofG position than say a Super Cub with the same size wing.

The lack of fuselage damping from the skeleton rear fuselage in an Alaska 'trials' plane is probably an acceptable price to allow an extreme landing AofA to be achieved without the rear fuselage area contributing to the forces involved.

I think you could well be right Q. It is certainly something interesting to experiment with. I don't think the size & shape of the fuselage helped either. The Mk 2 will be somewhat narrower & more symmetrical in taper at the nose, what I don't want is it generating any lift. I plan on keeping the three boom layout, with the tail fixed. The two upper booms will fit into tubes fixed to the flat underside of the wing allowing the booms to be adjustable & removable. In theory this should keep the wing & H-stab incidences the same & raise the tail out of the wash. The lower boom will also be adjustable/removable. This should allow for some adjustment of the H-stab incidence. Convenient for transport too, as it can be broken down into three pieces. However it turns out, It's a fun experiment !

Scrappy MK2 is all but finished. I have been working on it in the evenings when it's cooled down a bit !
You may notice it is somewhat shorter as well as narrower than the MK1, there is a good reason for that. Somehow I managed to make first booms 100mm to long! :oops: I thought it looked a bit on the long side. :whistle: Well it's rectified now & looks more balanced. Other changes are; The tail is higher & parallel to the flat bottom of the wing, which has no added incidence. The booms can be adjusted for length if I need to alter the coupling. If I can get it flying properly, they will then be fixed in place & I'll paint the fuselage. It has also lost a bit of weight & now comes in at 40.2 oz. Maiden tomorrow, if I can get up early enough before the wind picks up.

crash
40.2 oz.
I hope you put some more 'lacky' bands on the wing before the maiden. When you are pulling g the wing must not 'give' at all on the bands or it will increase the incidence and you will suddenly pull even more g. :ohmy:

Er, that's a very good point, I'm glad you spotted it ! :oops: It is already to windy to maiden this morning, so I think I will make a few alterations & try this evening.

I made the mods today. When I actually sat & looked at it, I realised what a dumb idea it was to mount the booms on the wing. Not only would the elastic bands have to support stress on the wing (as Simon pointed out) but the booms & tail as well ! :ohmy: Then of course to remove the wing I would have to disconnect the rudder & elevator push rods. How dumb was that ! Rhetorical question, before you sharpen your knives So the tubes for the booms are now back on the fuselage & can still be adjusted, 4mm grub screws hold then in place. Once I am happy with the way it fly's I will probably fix them permanently, & it does fly ! More in Been Flying.

:woohoo: Measure twice, think about it, then measure again! :evil: [badass]

"lacking elevator authority" I see Simon has commented on CoG position, I agree, it's amazing how little elevator you need when the CoG is in the right place. The Multiplex Blizzard is recommended at 50% although I haven't had the guts to try it. My Fun Cub is sitting at about 36% and flies really well, but I find the recommended throws a bit too aggressive.
Having said that, couldn't you convert the tail to all flying? It looks to be a fairly easy fix and would save making another stabiliser.

Klippy. I hadn't planed on making a new stabiliser, just a wider elevator. I quite like the way it fly's, slightly nose heavy. If I move the CG back won't I loose some of the stability ? Or put another way, if I leave the CG where it is, won't a wider elevator give more control ? More force on the fulcrum, as it were. Although it is a bit of a test platform, what I don't want to do is make to many changes & screw the whole thing up ! :S