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The Norwegian Gnat Squad


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After doing a club build of 7 Microaces Scrappee's last autumn we have decided to prepare the summer season with building a handfull of Gnat's together.

Most work will be done during the Wednesday Club night, so progress will not be very quick. However, that does not matter since there usually is snow on the ground until May.

3 member (your scribe included) have signed up, and to make things easy we have ordered kits from Tony Nyhuis.

So let's Go!

Lucas

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We purchased (on Banggood) DHV56MG servo's. Digital, 5.6 gr, 1.2 kgcm, 0.10 sec. And a pleasant 6GBP price tag.

I have never used these before. They look good whn testing on on the bench after arrival: zero slack on the shaft, little play in the gear, nearly soundless and (as far as one can judge that) they seem to center consistently.

servotest.jpg

And the first one survived 500 cycles.

Lucas

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  • 2 months later...

To avoid local virus spreading our national assosiation has strongly advised all clubs to cancel any activities, so this will not be a club evening build. However, Morten has started. As a graphical designer his planes always look good:

20200318mortencolours.jpg

And his first steps. He is using a building board of 5 cm blue foam, which is both stiff, light and take pins very well:

20200318mortenfirststeps.jpg

Cheers

Lucas

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Some creativity in creating the thrust tube: Morten, one of the builders experienced that his tube material was to stiff to bend to the required diameter. He came up with the following solution:

mortenthrusttubebaking.jpg

150 degrees centigrade for 10 minutes. The result is this:

mortenbakedthrusttube.jpg

I wonder whether the ridges will lead to loss of thrust.

Regards,

Lucas

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Lucas, Glad you are getting going. Guarantee you will be exhilarated when you get the gnat in the air; I know I was.

Little concerned about the thrust tube. If it is that stiff how much extra does it weigh? Like you I would also be concerned about the ridges disturbing the airflow . You will know from reading the other build logs of the delicate thrust - mass balance with these mini jets. 1 oz is huge !

Seeing all the angst others were having getting their models away from the hand launch ( admittedly mainly the JP) I fine tuned my thrust by incrementally making the ‘Cheat’ intake larger until I achieved the spec thrust level.

Sure you know all this but I thought it worth emphasising.

Tim

Edited By Tim Ballinger on 22/03/2020 09:36:03

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Started at last. Covering the building board with a clean piece of film is always a milestone:

img_6764.jpg

I show my method of glueing wing sheeting together (this should after all provide some support for new builders). For most glue joints I use PVA (white glue), which remains elastic, but here aliphatic glue is used, because it sands better.

Step 1: fit the pieces together, sand a straight and perpendicular edge to each. Degrease and put a peace of tape over the joint.

img_6765.jpg

Fold open and apply glue

img_6766.jpg

Close the joint (tape down on the board). Weight the parts so they lay flat. Glue will oose out.

img_6767.jpg

and scrape away superfluous glue

img_6769.jpg

For thin sheets (i.e. when covering a wing) I tape the top side of the joint too. Here it is not needed.

Inspection by the master of the house

img_6772.jpg

Cheers, Lucas

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Profiling the wings tonight. I did not manage to mark the edges as described on the drawings. However, by putting the wing flat on the board beside a 1.5mm balsa flake one gets perfect lines straigh away:

img_6773.jpg

Using a plane was a little uncontrolled (the plane buried itself in the balsa). I bevelled the wings with a big sanding block with 120 paper.

Lucas

ps. anybody reading this, or is everyone already finished with his mini jet?

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Hi Lucas,

Glad to see you have started. You probably noted from earlier build logs that some of us found making the wing join much easier as you do the final assembly into the fuselage. It is a bit of a fiddle with the completed wing and you stand a chance of breaking balsa as you manoeuvre them in. Others seem to manage ok but I decided to join in situ after making sure the brace was a perfect fit of course.

Tim

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Lucas,

sounds as though it could be a bit marginal to me. My static tests gave me 42A , 600g thrust at 49000 rpm ( the rpm figures in the static test I showed in the build blog were out by a factor of two as I confused poles and pole pairs when calibrating the rpm sensor). Those figures were with a newly charged 40C battery. I’ve seen others measuring 38A but not sure of the battery status.

Tim

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I had expected more too. But this was with 4-5 year old 25C battery. But at least it showed me there is nothing wrong with the fan and I can glue it in the model tonight.

I wondered whether silicon may be better then hot glue since it will remain somewhat flexible. What do you think?

Regards, Lucas

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Lucas,

what is clear is that thrust to mass ratio is crucial with all the mini jets. The gnat is perhaps the leanest but still needs max thrust to get away from a hand launch in light winds. I spent some time testing with larger cheat intakes to get the figures I quoted. I only managed 2 sessions between flooded fields and virus lockdown but it flew away with virtually no dip.

Can not help much on the glue front except to say that a couple of folks have had the fan come loose when it belly flopped following a failed launch so make sure it is secure. I used multiple hot glue blobs .

Timz

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Posted by Tim Ballinger on 06/04/2020 09:09:08:........Those figures were with a newly charged 40C battery. I’ve seen others measuring 38A but not sure of the battery status.

Tim

38A? That might have been me, but I was using a 4s system to get 680gm thrust at 51,500 rpm.

Unfortunately the lock down means I have not flown it yet sad

Dick

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Trust tube in place. I do not remember who gave the tip of some pipe isolation material to press the thrust tube to F10. I did not have the correct diameter of pipe isolation foam, but a little sponge did the job just as well.

img_6800.jpg

Started with the nose block. This is going to be some sanding!

img_6801.jpg

Cheers, Lucas

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