Miles Sparrowjet

Now that appeals on several levels! Is it a scratchbuild from plans? Please say it is. I'd love to build something with a built in erect middle finger. And I like 50's British jets and I have a lurking interest in small EDFs!

Martin

Sorry Specialbuilder it was built from a 3 view (and lots of pictures) off the 'net'.
,
The fuselage formers were lifted from a Vampire drawing and sized to suit.

The cockpit section of the Venom was identical to the Vampire but was 'stretched' aft to accommodate the longer RR Ghost.
The scale of the whole thing was set so that the exhaust nozzle of the 40 mm EDF was the correct size for the Ghost tail pipe.

I first built a KK Vampire (balsa and tissue!) and put a 40 mm EDF in it.

Without proper wing root inlets and with cheat holes to get the air in it was aerodynamically poor and heavy for what it was. It just about flew but needed full throttle just to maintain level flight close to the stall!
I felt I could do better in 2mm Depron.

Thanks, Quorneng. What span is the blue model? Also, how did you make the barrel shape of the fuselage?

When you say it's twitchy, is that a product of the format or could the throws be a bit too much for it, especially as it's so light?

Thanks,
Martin

The Venom has a 32" span.
I only say it is twitchy when compared to anything of a reasonable weight.
It is very light at 179.5 g and that's with its 'big' 1400 mAh 2s battery. 20 g less with its original 950 mAh.
The fuselage, and the ducts within it, are planked onto the fuselage formers.
I did a brief build description on this site here which shows how it was built up.

It flies very well but it was a bit of a labour of love to build.

Thanks Quorneng, that's a lovely model. I may use up my one sheet of 2mm depron on something similar. maybe a Miles Sparrowjet!

Martin

Specialbuilder
A Miles Sparrowjet should work well (Clark YH wing section) although you would likely have to increase the size of the inlets and exhausts if you are planning on EDFs.

I chose the Venom specifically as its RR Ghost was in effect an 'over size' Goblin so it had a bigger tail pipe. The complex indirect inlet path of a centrifugal compressor and even more so for the Ghost's double sided one meant the wing root inlets were already quite a bit bigger than those for the equivalent axial turbo jet so to scale they were not too far out for an EDF.

Quorneng,
the Sparrowjet has very much "straight through" arrangements with the fans right at the front of the short, straight nacelles, so perhaps it would mean just slightly re-working the shapes of the forward parts of those and then leave me to play around with the shape and size of the exhaust tubes for best thrust.
The fuselage is also a big, long box, so should make housing the ESCs and batteries easy.
BTW, do you recommend using both BECs from the ESCs to double capacity for the Rx and servos? I don't see the point in cutting the positive of one BEC if you can double capacity, though NOT, of course, voltage.

Martin

I should perhaps say that I know nothing about EDFs, but I do fancy some nice old British jets.

Martin

If you don't mind making the inlet and exhaust ducts a bit larger it should be fine.
It might be worth doing some simple scaling from a front view to see what diameter the ducts are for a particular span.
Bear in mind the EDF ducts will have to go through the wing spar and do so at the point of maximum bending.

Just scaling off the screen I judge it would have about a 950 mm span with 40 mm EDFs kept within scale nacelles. The inlets and jet pipes would have to be a bit over size to handle the airflow.

It would certainly be an interesting subject.

I'm using 30mm EDFs, Q and reckon the wingspan should be around 720mm. Basically 1/12th scale.
The EDF I've found gives about 180 grams of thrust. Since I aim to keep it under 250 grams of weight like your Venom, that should be OK.
May have to wait a while until I can afford two of everything!

Martin

I had an idea that something like this will be both elegant and strong. This purely a try out. The long end would have Depron spar laminate fixed to it and the short end would go into the fuselage to a housing made of foam and aluminium. This baby with a 30mm hole scales at 7 grams and is very strong and thanks to having a rim turned over too, is stiff in the fore and aft direction.



Martin

Specialbuilder
Certainly robust. I assume it is aluminium.
7 g sounds light but I bet that is a significant % (1/3?) of the total weight of your 30 mm EDF.
In a sub 250 g plane you avoid metal if at all possible. Wood and plastic are strong enough.

I know I am probably guilty of showing off but this is the sort of thing I would use.

32 mm hole (to allow for duct wall thickness). Balsa? spars glued into it. It weighs 0.8 g.

I assume that is 3D printed, judging by the pattern on the surface. Whilst my son has a 3D printer, neither of us know how to create the necessary CAD files, so I have to work the old way.
After a clean up the aluminium piece weighed 6 grams. I can't see anything of 0.8g. being strong enough. And the wood element would have to be a butt joint into the V shapes, whereas mine would be effectively wrapped around by the turned rim.
I'm now wondering if the pattern on your's shows a very thin lay-up. In which case you must have made a mould to do it in? I admit that would be stronger.

Martin

Specialbuilder
Yes it was 3D printed but just a 'lash up' to show the principle involved.
The question on a sub 250 g plane tends to be not how strong it is but is it just strong enough. The closer you can get to this the lighter it becomes.

I don't know how or with what you intend to build your plane but there are huge advantages in not concentrating the loads into a particular element particularly if a large internal void is required.

My Sea Vixen had the same issue. Two ducts side by side down the middle of the fuselage and through where the spar would be.
The solution was to do away with the spar and simply carry the load bearing wing skin over and under the fuselage supported by multiple formers.
The spar less wing.

The load bearing centre section being skinned.

All Depron no wood, or anything else, anywhere.
As there is no concentration of load there was sufficient joint area to allow the wing to be simply glued on, carefully!

With two 40 mm EDFs and a 2200 mAh 2s it weighs 375 g but is still plenty strong enough to be aerobatic.

I look forward to your build log.

Quorneng, your lovely twin boomers have the kind of structure that suits the sparless wing. I'm not sure my far more conventionally shaped Miles Sparrowjet does, hence my concern to make a suitable arrangement for an annular spar section.
Maybe it can't be done at under 250 grams, being a twin engine.

Martin