Folland Gnat

[Simon Chaddock]

I have been following Tony Nijhuis's 'Mini Jet' series with great interest.

It is impressive how well the planes fly but being a pedantic so & so I wonder if it would be possible to use scale inlets and only scale inlets.

Keeping scale ducting is not too difficult when modelling high bypass turbo fans but a pure turbo jet, like the Orpheus in the Gnat, needs a inlet area little bigger than the tail pipe area.

For anything like maximum efficiency an EDF really requires an inlet area 1.2 times the fan swept area. If an over size jet pipe is required to accommodate the needs of the EDF a scale inlet is bound to become seriously restrictive.

In addition a trans sonic jet, particularly a relatively low powered one like the Gnat, has to sacrifice inlet area in order to achieve the desired maximum speed.

On the plus side the Gnat does have a fairly generous wing area for its overall dimensions.

Thus for a scale inlet to work reasonably well it appears the EDF has to be sized to match the jet pipe rather than what can be installed in the fuselage.

So the question. is a scale inlet and tail pipe Gnat possible, or more to the point, practical?

I don't know but I am thinking about it.

[paul d]

You can't "scale" air.......

[Simon Chaddock]

paul d

Agreed but then the strength of materials doesn't scale either.

A small scale structure made of the same materials is whole orders stronger hence a small plane can use materials in its structure, like balsa or foam, that would be quite useless in full size.

This advantage coupled with an endurance requirement measured in a minutes rather than an hour or so might enable a small scale EDF to fly with its jet pipe limited by the size of scale inlets.

[Simon Chaddock]

Using this, hopefully reasonably scale, 3 view I did some sums comparing the areas of the inlet against the jet pipe.

This picture seems to confirm that the jet pipe is very close to the diameter of the rear fuselage.

As I expected the total area of true scale inlets is only 0.8 that of the jet pipe area.

With scale inlets set to give an area 1.2 times the FSA of a 50 mm fan the wing span comes out to be 760 mm.

At that size the jet pipe would be 56 mm diameter however with an exhaust tube nozzle set to say 90% of the FSA it would be significantly smaller at only 43 mm diameter.

This suggest another possibility would be to mount the EDF right at the back with no separate exhaust tube. The 53 mm outside diameter of the EDF shroud would be much closer to the 55 mm of a scale jet pipe. Such a layout does have the advantage that the high air velocity duct is very short whereas the entire inlet operates at a significantly lower velocity (75%?) which should means lower overall duct losses than with a middle mounted EDF.

The downside of such a layout is the battery and ESC is likely to have to go well forward (the cockpit?) to achieve the CofG.

I just happen to have a 4 year old, but unused, 50 mm AEO EDF.

[Simon Chaddock]

Doing some research I was amazed to find out just how different the Gnat T1, as used by the Red Arrows, was from the F1 as used by the Indian Air Force.

The F1 had no ailerons but used the all moving tail plane as elevons to allow full span flaps on the wing.

So the T1 had a different wing with aileron and flaps, a fixed tail plane with elevators, a longer fuselage to allow two seats and a bigger fin.

The fuselage fuel tanks on the T1 were much smaller so requiring fixed under wing tanks to compensate.

Although all moving elevons would be an interesting challenge the conventional layout of the T1 would likely make a lighter model so that is what it will have to be.

The next experiment was to consider 3D printing the complete duct. If it was rigid enough this become a spine allowing the fuselage to be built around it.

An EDF that can be adequately fed by scale inlets is quite small compared to the size of the plane so the duct ends up a rather long and 'spindly' looking thing.

It is made up of 5 major parts glued together.

To ease the printing the each leg of the bifurcation has a semicircular cross section but combined they have the same area as the main duct..

It will require a short make up pieces to convert the semicircular duct into the shape of the scale inlet.

With the fan at the back the true exhaust area is 95% of the FSA.

All the inlet duct has an area 104% of the FSA.

These sort of figures should enable the EDF to operate fairly efficiently.

Some testing required.

[martin collins 1]

Interesting experiment, look forward to test flight results!

[Simon Chaddock]

Some initial results with scale type inlet bells mouths added.

.

With the duct in place the EDF generates 16.2 oz thrust drawing 22.2A and 212W using an over size 3s.

Interestingly the EDF alone, it has no bell mouth, only generate 15 oz thrust!

This is quite an acceptable result so it seems to be worth proceeding to prepare the full size plans.

Edited By Simon Chaddock on 08/01/2020 22:52:59

[Simon Chaddock]

The lines in the 3 views are very blurry at any sort of magnification so each line has to be redrawn just one pixel wide. A rather slow and tedious process but essential to allow the views to be 'blown up' as working plans.

 

There will be at least 17 formers in the fuselage so another tedious job is interpreting the cross sections required between each of the 6 given on the plan.

With a 50 mm EDF right at the back matching the diameter of a scale jet pipe the Gnat will have a wing span of 32" (813 mm).

Edited By Simon Chaddock on 11/01/2020 00:21:13

[john stones 1 - Moderator]

Interesting read Simon.

[Pete Collins]

That's quite a big airframe. It will have to be light, which means it will fly quite slowly. That will probably be scale, but not the way EDF models are usually set up. Will be interesting to see how it turns out.

[Simon Chaddock]

Pete

I have no problem with big and light. Indeed I rather prefer a 'near scale' speed.

As my first printed EDF duct was really just a 'proof of concept' prototype with the plans now printed out a new duct to the correct dimensions could be printed. As the duct cross sections ware unaltered it only requires a single print parameter on each part to be changed.

The new duct with the prototype for comparison.

Note the small Depron make up portions to alter the semi circular duct branch to the more complicated scale inlet shape.

As the majority of the fuselage will be constructed around the duct using it as a 'spine' the inlet branches were braced together for additional rigidity.

I still have sufficient Depron for a plane this size so hopefully it will indeed be 'light for its size'.

[Trevor Crook]

Quite an ambitious little project Simon, much respect. I too am a fan (sorry!) of models that have a low enough wing loading to fly at scale-like speed. Of course, this makes them more of a challenge to fly when it's windy/turbulent, and they need a lot of power to fly scale verticals such as loops, due to lack of momentum. I'm sure you know all this!

I never appreciated those differences with the HAL Ajeet. I worked for a company in the 80s that made the accident data recorder for the Ajeet, and didn't realise roll control was via tailerons.

Looking forward to hearing further progress.

[Simon Chaddock]

Trevor

It was pity that the HAL Ajeet was not more of a success with the Indian Air Force as by the time it was fully developed it was a pretty capable fighter/ground attack aircraft. With a 'wet' wing it carried more fuel than any Gnat and had 4 wing hard points.

The problem was the IAF had just got a deal with the USSR to supply Mig 21s which in the interceptor fighter role made the HAL Ajeet look pretty tame.

The printed duct supporting the 2 mm Depron fuselage formers.

With only 6 sections shown on the 3 view all the intermediate former shapes had to be 'interpreted'. A rather slow and tedious process. This section of the fuselage can now be planked 'as is'.

On the other hand the fuselage nose section will have to be built using the 'Half section over the plan' method and when complete simply glued on.

Structurally the wing is simple as it fits over the top of the duct but the tail plane is harder as it is fully bisected by the duct.

[Erfolg]

I would not be surprised that TN would get a bigger DF in there, somewhere in the fatter part of the Fuz. No doubt looking for much more power in.

Perhaps more importantly TN would be looking to build a model that would handle all weather. that is more wind, turbulent days. Also it would not surprise me that a higher top speed would be sought all in a package that is robust.

Another aspect that TN probably designs so that specialised equipment and techniques are not required.

[Simon Chaddock]

Erfolg

My target was to build a Gnat with scale inlets and exhaust not in competition with TN but to demonstrate a different approach. I was fully aware it would require some unusual techniques and built time to achieve.

I could certainly get a 55 or bigger fan in the fuselage but then the scale inlets would be very restrictive resulting in a portion of the extra battery power/weight being used to overcome the deficiency rather than create more thrust unless of course extra inlet area was provided.

With a light wing loading high speed was never a goal but a good performance and duration might still be achieved with a relatively modest fan.

Well that's the plan.

[Simon Chaddock]

With a start made on the underside planking, it is a tedious slow fitting process, the long EDF wires have to be installed whilst access is still possible..

Magnet wire is used as it saves the weight of the heavy silicon insulation used normal multi strand wire. With so many formers the solid wire is well supported.

A bit more planking and the tail planes can go on.

Each is a very light monocoque with 2 mm Depron skins over a pair of Depron shear webs. Each half is glued to the duct and will be supported by the fuselage skin. Although the 3.7 g servos are flush with the top skin the tail plane is so thin they do protrude 1.5 mm on the underside.

The elevators also have 2mm Depron skins and top tape full length hinges. Scale they have a remarkably small area considering the Gnat fighter used all moving tail plane.

The 32 AWG servo wires have to be extended up to the cockpit area before any further planking can take place.

So far so good.

Edited By Simon Chaddock on 18/01/2020 23:05:48

[Simon Chaddock]

The tail fin is mounted in the same way as the tail planes and glued directly to the duct.

The rear fuselage planking can now be completed to support the fin.

The next job is to build the nose section so the ESC can be mounted and once it is joined to the motor wires the EDF can be tested to make sure it still works.

With everything "built in" testing at every construction stage is essential.

[Simon Chaddock]

The nose section is built conventionally. Planking over formers but in foam.

Whit the planking complete it can be lifted from the plan.

And the othe half of the formers added.

After a lot more planking the complete nose is simply glued on.

As this is a fully stressed skin structure particular attention has to be paid to ensuring the skin to skin joint is good.

To save weight there is no cockpit glass.

Starting to look a little bit more like a Gnat.

[Simon Chaddock]

The wings are built in a similar way to the tail plane and fin, no ribs, and 3 shear webs but it does have a doubled tapered hardwood/Depron/hardwood box spar.

The Gnat has quite a generous wing area as shown by this view of the lower wing skins just resting on the fuselage.

It is quite close coupled, particularly given the size of the elevators. Obviously it was quite adequate for the full size but it might be a bit short of authority on a slow flying model.

[Dane Crosby]

The full size Gnat T1 had an all flying Tailplane. The tiny elevators were locked to the main moving tailplane. We could unlock them by means of a plunger following hydraulic failure but had much limited control. The all moving tail could be moved up by a DC motor in this case as a form of trimming.

HYD fail caused a complicated checklist to be carried out. All Gnat guys will remember the mnemonic STUPRECC. 🙂

[Simon Chaddock]

Dane

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!

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.

[Erfolg]

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 Chaddock]

Erfolg

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.

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.

[Erfolg]

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

[Erfolg]

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.