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Folland Gnat

Scale inlets and no cheat holes?

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Simon Chaddock26/01/2020 23:27:25
5571 forum posts
2931 photos


Thanks for that it certainly makes sense.

I was confused by a comment made by HAL industries that their Ajeet had an all moving tail plane designed by them so I assumed the UK T1 just had elevators. The Ajeet did not have elevators but apparently did have a much redesigned hydraulic system presumably to give some redundancy so the 'emergency' elevators were not necessary.

Now I am faced with what to do! smile o

An all moving tail plane would require quite a change at the tail end so perhaps the simplest 'fix' would be to simply extend the elevators to the full tail plane span. Not much of an area increase but it would be better than nothing. wink 2

Erfolg03/02/2020 13:02:30
11556 forum posts
1271 photos

Substituting an all flying tailplane seems to be something that at the point you have reached, something that is not worth doing.

Probably the biggest issue is that of insuring the weight does not go up, Also given your very small duct (with its already less than ideal velocity profile, in that slower velocity with a larger diameter is more efficient), putting a rod across it, would be much less than ideal.

Who can tell when its flying, or a casual glance? But the again, I am a Philistine, in that I would have a much larger inlets, bigger ducts everywhere, even on a limited powered fan.

Simon Chaddock03/02/2020 16:13:01
5571 forum posts
2931 photos


The duct and inlets have a constant cross section area of 1964 sq mm.

The fan has an FSA of 1618 sq mm.

The duct and inlets thus have an area that is 1.22 times the FSA which is actually not a bad figure.

I do accept that bifurcating the duct does increase the losses but it is relatively short compared to the total duct length.

With the fan right at the back gives an exhaust nozzle area of 1548 sq mm which at 95% of the FSA is a bit larger than the optimum 85% required for maximum thrust but gain would only be about 2%.

New full span elevators are now installed.

Full span elev

The servo positions are unchanged.

Of more concern is obtaining a suitable CofG with the weight of the EDF in the tail rather than the thrust that can be achieved from it. wink 2

Erfolg03/02/2020 16:37:02
11556 forum posts
1271 photos

From the perspective of duct design, without resorting to text books, we both know that duct losses are related to cross sectional area and velocity. Taking a drinking straw as an extreme example. The boundary layer represents a greater proportion of the velocity gradient. It also self evident that the greater the length of the duct, in proportion to the diameter, the higher the losses, in moving the same cubic volume.

Again at a practical level all changes to a duct come with a penalty, be it an expansion, contraction, the fan itself and so on.

About 40 years ago I was involved in the design of a air curtain. It did surprise me how much energy that these type of systems consume, although the objective was not thrust, rather control of materials and volumes. The main difference is that everything was calculated, to predict performance.

Edited By Erfolg on 03/02/2020 16:37:45

Erfolg03/02/2020 17:24:06
11556 forum posts
1271 photos

I am not suggesting the model will not fly. Just casting my memory back to the Jetex era. These models flew on virtually no thrust, as long as the model was light, had enough wing area (wing loading).

Even my Fairy Delta could be made to fly. Although briefly, as long as it was not windy.

Simon Chaddock08/02/2020 11:25:51
5571 forum posts
2931 photos

The next important bit is mounting the 30 A ESC with its heat sink flush with fuselage skin just ahead of the left intake so it should have significant airflow over its surface.


I did the same thing with my Concorde although there I removed the shrink wrap to improve the heat transfer. I may have to do the same on this.

The ESC has 2 mm bullet connectors so is, by my standards, fairly easy to remove just in case.wink 2

ESC3As the wing is a one piece structure stressed skin structure it can be adequately retained by four M4 nylon bolts close to the leading and trailing edges. Fourbolts

They use 3D printed fittings.

A removable wing was chosen for ease of storage and transport as there is nothing under the wing that requires access. Of course there is a slight weight penalty compared to simply gluing the wing on but it is only a few grams.

With the ESC in place a 'nose on the scales' test showed a thrust of 11.75 oz (333 g). I am happy with that as the slightly shorter prototype duct only gave 1/4 of an oz more and its inlets were in 'free air' rather being alongside the cockpit wall in the plane.

So far so good.

Dad_flyer08/02/2020 18:37:24
201 forum posts
245 photos

This is an amazing build. thumbs up

Stephen Jones09/02/2020 12:32:02
2762 forum posts
1598 photos
Posted by Dad_flyer on 08/02/2020 18:37:24:

This is an amazing build. thumbs up

Yes i second that. Nice Simon.

Steve.thumbs up

Simon Chaddock09/02/2020 20:53:29
5571 forum posts
2931 photos

Thanks for the kind words but at the moment I fear the pictures rather mask the actual quality.

The one piece top and bottom skins on the flying surfaces are good enough but the planked fuselage does not give the same quality of finish.

With the fuselage completed above the wing the basic construction is complete.


Before any painting the fuselage including the EDF and ESC weighs just under 8 oz (225 g). The wing is 2.5 oz (71 g). The 1500 mAh 3 s is 3.85 oz (109 g). I am hoping for close to 15 oz when painted and ready to go. wink 2

Trevor Crook09/02/2020 21:07:24
904 forum posts
67 photos

That's light for its size! Should be a good "proof of concept" machine so don't beat yourself up too much about the finish. You'll never see it when it's cruising around the sky on a calm summer's day.

Simon Chaddock11/02/2020 10:37:03
5571 forum posts
2931 photos

I decided I really didn't like the finish of the planked fuselage so it comes down to the age old "paint, sand, fill, sand, repeat" process. In this case the first 'paint' acts as a filler/sealer which hardens the surface and make meaningful but still very light sanding even possible.

It will add a bit of weight but fortunately its only the fuselage that needs any treatment.

Paint 1

It seems to require 3 'cycles' to get a reasonable surface. The sanding is slow but at least it removes about 50% of the previous layer. wink 2

It is Scola 'scarlet red' artists water based acrylic so its easy to wash out the brush but like any emulsion it is very water proof when dry.. Almost a 'gel' it has an vast amount of colour in it but is best applied 'as is' and well brushed in. Adding even a small amount of water tends to make it turn into 'droplets' on the Depron surface.

We shall see.

Simon Chaddock15/02/2020 10:15:09
5571 forum posts
2931 photos

After much painting and sanding along with a light coat of white on the flying surfaces the Gnat is complete.


A Lemon stab rx (borrowed from my Ballerina) is fitted on the cockpit floor.


The intention is that this rx will be 'temporary' just for the initial testing until a safe CofG is found.

With a 1500 3s in the nose it has ended up slightly heavier than I wanted at just over 15 oz (425 g) but it should still manage happily enough with its 11.5 oz of available thrust.

As storm Dennis is about to hit and the field is already very soggy it could be a while before the true effects of using true scale inlets and exhaust can be tested.

Edited By Simon Chaddock on 15/02/2020 10:16:37

McG 696915/02/2020 11:14:21
2895 forum posts
1097 photos

... really impressive as usual, Simon.



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