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New EDF Mini Jets- Jet Provost & Folland Gnat

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Martin McIntosh26/03/2020 17:16:38
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3332 forum posts
1178 photos

Just had a look. Pilots are OK but no way am I paying £5.30 for shipping a few grams.

Barry Buxton27/03/2020 17:09:35
16 forum posts
5 photos

89f38ea6-839e-4ed8-b9cf-9fc48a9216d6.jpegThought I’d use the home time to see if I can get my gnat sorted. I bought a 3S FMS fan and a replacement 4S fan to try. I thought I’d test thrust before installing them so made up a rig and thrust tube as per the drawings.

the 3 S fan gave 540g thrust with the thrust tube and 620g without it, unfortunately it decided to shed its blades whilst at pretty much full speed, quite an interesting experience

i then put the 4S fan onto the rig and measured 750g with the thrust tube and 880g without it, does anyone know why having no thrust tube increases thrust, I thought the whole point of a thrust tube was to accelerate the air and create more thrust ? Any one any ideas ?

now got to try and perform surgery on the gnat and swap the fans over, it looking forward to that. It it’s the only way I’m going to see if I just have a low power fan in the plane at the moment, best thrust on a 4S with the original fan in the plane is ~470 g

Martin McIntosh27/03/2020 17:58:36
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3332 forum posts
1178 photos

Years ago I tried to make model by P.E. Norman, son of Marcus the DF guru, and contacted him about it. Evidently the intake/efflux ratio is critical and he sent me their experimental data which of course I do not have now. I never did sum up the courage to run my home made 5" fan on a K&B pylon 40 motor.

There is a YT video on here somewhere of a guy experimenting with efflux dia. v EDF dia. It would seem from that that Tony`s are the best compromise, in fact the efflux could not be any larger on the models and still keep them near scale. I have read that these are ballistic anyway, so why fit a 4s? You will reduce flight times and need to take a change of underwear.

Interesting that yours shed its blades. You were not running a 3s on 4s were you?

All of the ones I have were tested by just holding in my hand. All OK until the latest for my JP which burned my fingers. I must have distorted the case a bit because I could see where the blades had touched. Other than a scuff mark it seems fine now.

Not that there is any rush with flying banned but I have been waiting ages for the mouldings and some tissue covering, no doubt due to the shortage of staff/couriers at the moment. Can get no further until these arrive.

Barry Buxton27/03/2020 18:14:12
16 forum posts
5 photos

Hi Martin, yes it was definitely only a 3 S I was using and I’d successfully spun it up to full speed about a dozen times before it failed.

I went 4S as I saw quite a few people having issues with the provost, seems mainly solved in the end by losing weight and adding extra cheat holes so thought 4 S would be fine and I could always limit the throttle travel if it’s crazy fast.

I did see some of the articles on edf dia Vs eflux dia, not that I fully understand them all but it just seemed really strange that thrust increased by so much without a thrust tube, as if the tube was actually constricting airflow with maybe back pressure impacting the fan.

Tim Ballinger27/03/2020 18:47:26
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778 forum posts
283 photos

Barry,

it is now many years since I finished in the aircraft industry and jet propulsion in particular. So with the huge caveat that there is a huge scale effect involved which generally makes small scale less efficient....

First thing is that thrust is made up of change in momentum of the air flow and the difference in static pressure at exit multiplied by the exit area ( there is also a drag associated with intake flow but I will ignore that). If you push air down a convergent tube it will increase the speed and reduce the static pressure. There is an optimum balance to be had between the increased momentum and loss in static pressure. So one thing early to note is that engines generally have jet pipes and nozzles. The jet pipe is really just a means of getting from the engine to the nozzle. The longer it is the less efficient it is. What we really have in our small scale gnat is just a long jet pipe .
I have never undertaken measurements at this scale so I am just guessing that the jet pipe gives more losses than any marginal increase in momentum.
I guess the thing to remember is that all the input energy is in the fan , after that you simply exchange one form of energy for another. If it is an inefficient exchange you will loose out.

Just ramblings of an old engineer . At this scale You really need measurements to find the sweet spot.

Tim

Martin McIntosh27/03/2020 18:51:06
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3332 forum posts
1178 photos

Don`t know if you have looked at my Gnat but in the pics you will see a small restrictor added at the outlet. This was to give it a more jet like sound but you can actually hear the fan speed up as well. I have not tried to take any thrust measurements with or without this. Try sticking your finger up the back end and you will see what I mean.

Jack Petrie27/03/2020 22:33:45
22 forum posts
13 photos

I am currently building the Jet Provost and have a question about the build; when you cut the bottom sheeting to fit the wing, how do you fit the flat sheet back up the bottom of the dihedral wing, is it scraps of wood glued in to fit the shape after the top of the wing is glued to the fuselage?

thanks!
Jack

Martin McIntosh27/03/2020 23:47:15
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3332 forum posts
1178 photos

The build pics seem to show that piece left on when cutting the fus sides but it would not be practical. I am going to glue on the wing, add the scraps to fill in and only then sheet the bottom in one piece.

Geoff Gardiner28/03/2020 00:01:04
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534 forum posts
912 photos

I glued the bottom sheeting to the wing and then filled in the gaps after:

img_4655.jpg

img_4657.jpg

Jack Petrie28/03/2020 01:02:10
22 forum posts
13 photos

Okay thanks so I effectively glue down the sheeting at the trailing edge best I can and then fill in the gaps at the leading edge with strips of balsa?

Geoff Gardiner28/03/2020 09:30:36
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534 forum posts
912 photos

Yep, that's what I did.

Seems to have worked OK.

Make sure the edges are secure - the middle is cut out for the cheat hole:

img_4666.jpg

Barry Buxton31/03/2020 14:11:14
16 forum posts
5 photos

328436a2-3aa0-4832-ae31-7bad83b95bfb.jpegFinished the surgery on the gnat, I swapped the blades from one of the 4S fans to the 3S unit and installed that into the gnat. Getting 520g thrust and 50g lighter with using a 3S battery so hoping that when we are able to get out it’ll fly at last. In the meantime I’ve starting building the phantom

Lucas Hofman31/03/2020 14:14:39
666 forum posts
375 photos

Anybody used tape hinges instead of fuzzy one's? The stabilers/elevators/ailerons are thin, and it is then easier to bevel one side and fix them with tape.

Any opinions on whether this is good or not?

Lucas

Trevor Crook31/03/2020 14:38:36
943 forum posts
67 photos

I used film hinges on my Provost.

Martin McIntosh31/03/2020 17:26:01
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3332 forum posts
1178 photos

Mine is now nearly ready for spraying but have not yet made the launching handle. Is this really needed?

Tim Ballinger01/04/2020 09:39:29
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778 forum posts
283 photos

Barry,

Glad you seem to have sorted your power problem. I assume you are concluding that the original 4S fan was faulty?

Interesting that the blades are interchangeable, I had assumed there would have been a pitch change as well as a KV change between the motors but it does not seem so from your comments. Did you measure the max current as an insurance ?

Tim

Barry Buxton02/04/2020 13:57:41
16 forum posts
5 photos

TIm,

Yes, I'm assuming my original 4S unit had some sort of issue but no idea what, it doesn't look like there's any difference in the blades between the 3S and 4S versions, I'll have to wait for my 3S fan units to arrive to check it properly but the current draw seemed OK, 36A at full power

Lucas Hofman12/04/2020 20:18:20
666 forum posts
375 photos

Anybody who can tell me why one would glue the formers for the Gnat air intakes (12-14) to the inner fuselage instead of fixing them to the Intakes sides and the triangle lists? The latter would automatically ensure that they all line up. Am I missing something here?

Regards, Lucas

Edited By Lucas Hofman on 12/04/2020 20:51:41

i12fly12/04/2020 23:41:36
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632 forum posts
22 photos

Hi Lucas, I think that the intake formers are fixed to the fus sides so that when you fit the air intakes they line up for the wing position. On my build I fitted the wings in position loosely, then attached the air intakes, which you could do with the formers attached to the intakes. I drilled 2 holes in the sacrificial F12 so that it was easier to remove later.

I made the mistake of attaching the intake formers before planing and sanding and main fuselage -and kept knocking them off

Jack Petrie22/04/2020 12:39:31
22 forum posts
13 photos

What adhesive did you guys use to attach the intakes?

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