A slow 'fast jet' - The Fairey Delta 2
I have set myself a challenge to fly an EDF with scale inlets and exhausts.
I did achieve this with my V-1 but in some respects it is a bit of a cheat as a pulse jet has an extremely inefficient cycle with no real 'compression' so not surprisingly it has an inlet more or less equal to the exhaust.
Apart from modern ducted fans any scale 'jet' is likely to have inlets significantly smaller that the exhaust - or is it?
The Fairey Delta 2 was certainly a fast jet.
An elegant simple delta it held the worlds speed record in 1956 at 1132mph in the hands of Fairey Aviation's test pilot Peter Twiss.
Always a research plane it had a relatively small exhaust set for high speed and simple inlets with no moveable body to make use of the supersonic shock wave. As a result the total inlet area is actually greater than the area of the jet pipe.
Of course the jet pipe is nevertheless pretty small so the task is to build a relatively big plane that will fly satisfactorily with a small EDF or in other words it will have to be light, very light.
It occurs to me that the most efficient use of an EDF, particularly an out runner type, is to mount it right at the back with no exhaust duct at all. In fact the nozzle becomes an 'annulus' of exactly the fan swept area and ensures the minimum loss of the fan exit velocity.
Scaling the FD2 for a 55mm EDF as the jet pipe the numbers look like this.
The FSA is 1760 sqmm (the out runner bell occupies nearly 25% of the duct!)
The area of the 55mm duct is 2376 sqmm.
The area of the inlets (and the duct upstream of the fan) is 3400 sqmm.
This suggests the inlet velocity (and the losses) will be only half that of the outlet.
At this scale the FD 2 will have a span of 32" and be 58" long and will weigh?
Now that is the question!
Excellent project. I started a plan for one years ago for rocket motor power, never got around to building it or finishing the plan but i do have a really good 3-view and the bones of a plan with former shapes drawn up. Maybe it's of some use, find attached.
That's a nice 3 view.
Fortunately the fuselage is truly circular with only the inlets and the actual cockpit area any different.
To keep weight to an absolute minimum this will be a Depron build but with even more attention to weight saving.
So far just a modest start. The tail section duct aft of the wing built in 2mm Depron.
As the duct tapers over this section from the 55mm ID diam of the EDF to the 69mm ID diam duct of the fuselage centre section it is formed with planks.
The completed duct with the EDF perched on the end.
The fuselage skin will terminate at the end of the EDF.
Looking forward to seeing this one take to the air.
It seems like a massive airframe to be flying on a little 55mm fan, but knowing how light you build I'm sure it will work just fine.
Subscribed :ws: You might want to move the EDF as far forwards as possible, reason being is that you will have to much weight at the rear of the jet, even if you get the cg correct, with the edf weight at the rear, your jet may want to stall at low speeds and go into a flat spin, I learned that the hard way when i built a edf jet and mounted the edf at the rear of the jet, it flew fine, but if i got into a stall, i could not recover, and it would just flat spin down, Just my 2 cents worth, Take care and have fun, Chellie
My Friend Steve is doing the maiden flight, I am Doing the video :)
You may well be right about a plane having a relatively large moment of inertia but I am confident that the very long fuselage and a fin well aft of the wing will provide sufficient aerodynamic resistance to keep the spin under control.
The rear mounted EDF is to keep the duct losses to a minimum as this is a rather small unit for the size of the plane.
The fuselage centre section duct on its rainwater downpipe 'mandrel'.
As this part of the duct is a constant diameter the Depron sheet is wrapped with a scarf joint rather than planked.
As the FD 2 has a relatively small wing, and a steep delta at that, the span at the CofG is just 17" so to save further weight it will have no spar as such and rely entirely on the Depron wing skin for strength with only Depron webs to take the shear forces! :eek:
The lower skin and shear webs.
Flat at this stage but each shear web is sized to create a scale (4% thick!) symmetrical wing section when the top skin is glued on 'free hand'.
The issue is how to joint a stressed skin wing to the fuselage which with its own inner an outer skins is also a stressed skin structure.
The wing will be attached along its entire root to both the fuselage inner and outer skins and the wing shear webs will match up exactly with the fuselage formers. This will ensure equal transfer of the loads with no stress concentrations.
A completed wing attached to the duct.
The mandrel is only removed when both wings are on and the top and bottom fuselage formers are added.
I’m pretty sure that it will make no difference to duct losses where you put the EDF. As far as duct losses are concerned it's the overall length (and diameter and shape) of the duct that makes a difference.
It might seem intuitive that the velocity exiting the EDF body is greater than that entering it and so losses are higher on the outlet, but providing the duct is the same diameter both sides the velocity is actually the same. It has to be when you think about it because the same volume goes in as comes out.
It's different on a 'real' turbojet because there is expansion in the combustion chamber, so much higher volume goes out vs. goes in, but with an EDF there is no combustion and no expansion.
But in the FD2 the inlet and exhaust ducts are not the same. The inlet and inlet duct are very nearly twice the fan swept area.
By placing the EDF at the back with the outrunner bell flush with the end of the duct the exhaust nozzle in effect becomes an annulus of exactly the FSA which is nearly 25% less than the area of a 55mm diam duct.
Well thats the theory!
ok, I think I'm with you now... By placing the fan at the rear the entire duct can be kept well above EDF diameter thus decreasing velocity and friction losses... Yes?
The wings and the fuselage formers fitted.
With the mandrel removed the rear section of the nose is inserted to form the inner walls if the inlets.
Part of the origianl duct is then cut away and the outer walls of the inlets created with planks.
With a complex shape and double curves creating the inlets is a slow process with much trial and error shaping and twisting of each plank before it is glued in.
With the duct complete the EDF unit could be tested to measure the thrust. The unit is claimed to give 18oz thrust on a 3s.
With it mounted vertically on the scales it delivered just under 16oz.
To my relief this is actually 2oz more than the same unit delivers front mounted in the V-1 which happens to have almost exactly the same duct length.
On this basis as long as the weight is kept down the FD 2 it should fly ok.
Gee, when you decide to build you really move along. Looks good.
The nose section (or rather the part with the cockpit) under construction.
Built vertically. It gets progressively stronger as the planking progresses but needs constant checks to make sure it remains true.
The cockpit lined with Depron to create the battery compartment.
It is sized to take an 1800mAh 3s.
The fuselage top decking is left off until all the electrics have been installed.
The 3 sections built so far 'stacked up' on the EDF unit to give an idea of what it will look like.
There is however another 10" of nose to be added! :eek:
The elevons under construction.
2mm Depron skins over Depron ribs.
They are huge adding significantly to the total wing area.
All the fuselage section joined.
I had originally intended to leave off the long and fragile nose for its maiden but I decided it might be better to have it on as a sacrificial 'shock absorber' to protect the rest of the structure.
Its easy enough to build another nose!
You have really captured the missile like quality of the real FD2. Cant wait for the maiden flight report!
It looks like the EDF is right at the exhaust. You don't need an exhaust tube? FSA and all that?
Yes the EDF is right at the back.
Yes there is no exhaust tube.
But as it is an out runner type EDF the nozzle (an annulus) is exactly the FSA so keeping the back pressure losses in the exhaust duct to a minimum.
However the area of the inlets and the inlet duct is nearly twice the FSA so the inlet velocity is significantly lower.
If I am right the overall the duct losses will be less (and the thrust higher) than having the fan say midway down the duct.
To save weight the long ESC to motor wires are solid with no silicon insulation as at the smaller sizes the silicon weighs almost as much as the conductor.
Although they look bare they are actually insulated with a polyurethane varnish that is good for 100+Volts.
At about this point it was coming obvious that with the 1800mAh 3s in the cockpit the CofG was too far forward.
To get it to 30% root chord the battery needed to be a good 4" further back which means behind the cockpit and with its own hatch.
So a bit of rework to remove the existing battery compartment, rebuild the formers and construct a fixed solid canopy, followed by reinforcing the formers behind the cockpit and constructing a new battery compartment.
Overall there is little or no weight penalty.
The fixed canopy.
It is structural and part of the fuselage skin.
The heavily finned 30A ESC mounted on top of the fuselage just behind the battery compartment.
With its fins protruding into the airflow to keep cool it is also well protected during belly landings and not too difficult to 'extract' if it proves necessary!
Thx for the explanation with the fsa. Beautiful plane & great work! I'll be watching the whole way.
The electrics installed.
I was originally going to use the DX6i elevon mixing function but it actually limits the servo travel to 50% with just elevator or aileron alone. The separate elevon 'mixer' on the other hand gives the full 100% and then takes the servos to 125% and 75% when both are applied. A better solution in my case.
The servo mounted.
Tiny 3.7g units lie flush with the skin lower surface. They will be covered with tape.
As the elevons are so big the deflection is limited to about 15 degrees each way, which means the horns are long. The linkage will have covers to protect them during a belly landing.
Really nice project, I have had the Fairey FD 2 on my possible projects list for some time, so will follow with great interest.
Good luck with the time management and thanks for sharing this wonderful project ....
With electrics installed and tested the skin can be completed.
The second prototype WG777 (it held the speed record) was originally bare aluminium but was later (and still is) painted royal blue overall.
So that what it will be painted. :)
Its very blue and it makes it look quite a bit smaller.
The first attempt at a maiden did not go too well as it proved almost uncontrollable so a very short (3 seconds!) wobbly flight that ended with the inevitable 'arrival'.
To my surprise the nose dis not break but just bent a bit however the shock split the fuselage almost completely all round at the air intake.
In addtion the battery broke free and ended up in the nose!
Whilst the repairs are under way I have time to consider what to do.
My gut feeling is the huge elevon is just to big completely upsetting the inherent stability of a delta.
The original has separate inboard elevators and outboard ailerons so I will divide the surface at the scale point and just use the outer section as an elevon.
The inner part is fixed with a small amount of reflex to aid stability.
In addition the battery can be moved forward an 1".
Hopefully the cumulative effect of these modifications will make it at least flyable! :concern:
Thank you Bluz for the complement, I Like these colors because they are easy to see way out there :)
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