Originally Posted by madbomber

Do you have any idea how accurate your method of thrust measurement is? Seems to me like you would need to be really careful to make sure that the string holding the plane is attached at the same distance from the pivot point on the L as where the L contacts the plate of the scale. It really is a clever idea and one that I have never though of. I may have to rig something like that up for myself and see how it works.

Also, bird is looking great. Looking forward to seeing it in the air.

Originally Posted by brokenenglish

As MB suggests, I should think there must be some question of moment arm ratios in your setup. However, if you only want comparative results, i.e. one system is pulling a bit more, or a bit less than another, then your rig may work, although it may not be very sensitive to small differences, but then neither will the plane be...

Thanks both for your interest and comments. Though the the string and plate contact points are equidistant from the pivot point (13.5cm), I never expected to obtain an accurate reading of static thrust. However, in conjunction with a wattmeter, it does give an indication of the relative performance of different props. Also it was simple and quick to construct from bits I had. and easy to couple to almost any plane.

OK, so I didn't go the re-cycled Biro route but I still had to dig deep in my engineering skills to produce this spring strut. Bear in mind that for the last 35 years my mechaniking has been of the agricultural variety involving big lumps of iron and the primary tools a big 'ammer and a long lever.

When my son presented me with a box full of assorted RC car suspension bits, I was delighted to find a pair of mini dampers with alloy bodies. I started by removing the plastic washers that provided the damping effect and the connecting eye on the rod. The hardest bit was filing the end of the rod to fit into a piece of brass fuel tube used to join it to a length of 2mm wire with a generous dollop of solder. I cut off the eye at the top of the damper and drilled and tapped it to take the threaded bit of the 2mm rod with added 'Z' bend. It's locked in with a drop of epoxy. As I don't posses a 2mm tap, I tapered the end of a bit of threaded rod and cut a groove with my dremel cut-off disc, works fine into alloy or plastic.

With suitable card fairings, it should look OK.

BTW, the springs are from Biros!

Resourceful Mr M!!! When you say you removed the damping washers, Ipresume that they damp by friction? Don't know if it may be a bit less bouncy to land with a bit of damping......

I can always put the friction washers back and for more damping I can fill them with light oil. I'm also not sure if the Biro springs are stiff enough, I't'll be a case of seeing what it looks like when it's all assembled with the weight of the plane on them.

Almost there. Undercart and decoration done. I just have to do a bit more glazing, add the fairings to the U/C and struts, add a bit of paint here and there and then do some power tests with different props. There is one main problem! I need to extend the spring struts to give more ground clearance to the prop. I really like the 8 x 4 three blader but there's only about an inch of clearance. The two blade 9 x 4.5 is, theoretically the most efficient but even less clearance with that. I have run a 7 x 6 on the motor but I'm not sure if it'll have enough thrust which is why I have to do some tests.
I think it looks OK for my first attempt at a scale model.

Oohhh, I DO like that Mixer! What does it weigh now? Do post another photo once the glazing etc is finished so I can lift it to use in the Poll.

Good job!

Blinkie will be green with envy when he gets back from the antipodes!

Originally Posted by Sundancer

Oohhh, I DO like that Mixer! What does it weigh now? Do post another photo once the glazing etc is finished so I can lift it to use in the Poll.

Good job!

Blinkie will be green with envy when he gets back from the antipodes!

Looks good but will it fly? Your early estimate was spot on, 22oz all up. With those tiny but scale wheels I don't think it will take off from grass but who will I trust for that first hand launch! I'm prepared to fork out for a better motor if needed.

Originally Posted by mixer421

Looks good but will it fly? Your early estimate was spot on, 22oz all up. With those tiny but scale wheels I don't think it will take off from grass but who will I trust for that first hand launch! I'm prepared to fork out for a better motor if needed.

Drat, wish we were in the same country, I'd be happy to hand launch it for you - or fly it if you preferred!! Providing you have something in the region of 100 watts input it should be OK at that weight, and why wouldn't it fly - the original managed free-flight!

I think that looks a lot better than "OK" ! Its a little gem Fantastic Mixer

Al

Originally Posted by mixer421

I think it looks OK for my first attempt at a scale model.

Now that is looking GREAT!!!!

Re the prop clearance, although the current wheels arecorrect scale, you could consider having some 'flying' wheels that are larger diameter to give the clearance needed for the three blader.....would help with takeoffs from less than perfect surfaces too! I don't think that it would detract from the appearance much....

Good work, fella!!!!!

Originally Posted by Sundancer

!Blinkie will be green with envy when he gets back from the antipodes!

Blinkie's still here and he's feeling jealous - but WARM and jealous..... Return on Sunday and have been hankering to continue with the Kitten!

Mixer,

I would be inclined to lengthen the undercarriage legs a little. The model seem to be in danger of scraping it's bottom on the runway ( painful ) There looks to be a little more clearance on the full size.

Ray

Originally Posted by Colonel Blink

Blinkie's still here and he's feeling jealous - but WARM and jealous..... Return on Sunday and have been hankering to continue with the Kitten!

I suggest that you have your thermals in the hand baggage Colonel, could be quite a shock to the system when you land! Looking forward to seeing progress on the Kitten once you thaw out.

Thanks all for the kind comments and suggestions.
I'm going to extend the spring strut by about 1/2" and I like Col's suggestion about bigger 'flying' wheels. The antipodean warmth must have stimulated a few brain cells. Looking forward to seeing you back in harness and getting that Kitten finished.

Time to hook up my new wattmeter today and see how much ooomph I can extract from my 2830/14 on 2 fully charged cells. The Doc's peak efficiency chart shows that I should get around 200W in, 120out with the 7 x 6 and the 9 x 4.5.

I'm surprised at how little difference there is between the APC/E 2 blade 9 x 4.5 and the 3 blade, GWS 8 x 4.
Both at 7.6V under load, the APC took 120W in, at 13.5A and my thrust comparator showed 375gm.
The GWS took 105W in at13.1A with 370gm of thrust.
It's clear that the APC is better but I will have to jack up the U/C or it could be expensive on props!

Originally Posted by mixer421

Almost there. Undercart and decoration done. I just have to do a bit more glazing, add the fairings to the U/C and struts, add a bit of paint here and there and then do some power tests with different props. There is one main problem! I need to extend the spring struts to give more ground clearance to the prop. I really like the 8 x 4 three blader but there's only about an inch of clearance. The two blade 9 x 4.5 is, theoretically the most efficient but even less clearance with that. I have run a 7 x 6 on the motor but I'm not sure if it'll have enough thrust which is why I have to do some tests.
I think it looks OK for my first attempt at a scale model.

Now that's personality!! Very nice job indeed. One thought - a little more belly clearance if possible would be good. Maybe larger wheels which would also help with prop clearance. Maiden flight over tall, tall grass for insurance and peace of mind if at all possible. Again, beautiful job and looking forward to the flight report

John