The Hawker P1121 - again!

[Tony Bennett]

thats a nice build you have there.

 

glad to see you didn't give up.

[Simon Chaddock]

I almost wish I hadn't started this!

2 Views of the nose section under construction.

Unlike the centre section this only has an outer skin, apart from the chin duct, the internal ducting for the side cheat holes and the cockpit!

 

I have to say building vertically like this is.. er... a bit of a nightmare, terribly wobbly to start with but nevertheless it makes it much easier to incorporate 'adjustments' if the shape does not look quite right.

Light? Yes. I only hope it will be strong enough!

Edited By Simon Chaddock on 11/10/2011 23:00:42

[Erfolg]

Perhaps a carbon rod could be used to build the nose at least around.They are both light and strong.

 

One offs seem to cost about £5. In the past, if not at present "Robotbirds" were by far the keenest prices.

 

[Richard Sharman]

Posted by Simon Chaddock on 10/10/2011 00:14:15:

I have started to think seriously about the actual inlet geometry.....the inlet is only 65% of the PSA (prop swept area).........The inlet would now be about 75% of the PSA.....thus be trifurcated with a total a bit over 100% of the PSA.

 

Simon,

I think you will have to do this to get to a reasonable inlet area size. Less than 100% of the PSA does sound rather restrictive, unless the model is going to fly really fast. Alternatively,if it a very slow flying model and you have enough power you might want to increase the inlet even further.

 

PS I am shortly going to discuss the variation of inlet/outlet sizes in the "Ducted Fan Theory" forum thread which I humbly refer everyone to if they are interested.

 

PPS. Your vertical build is "courageous" as they used to say in Yes, Minister! Your're probably aware that some kits build fuselages like this in halves (left, right) over a plan, and they join the two half shells -- that's the way both the Hawker Hunter and Bae Hawk from WestWings do it. Good luck !

[Simon Chaddock]

Richard

For an EDF it will certainly be a slow flyer but then it is really a lightweight 'pusher' jet but with a small internal prop!

 

It will rather depend on the final motor position but I would like to get static thrust readings for:

1. The prop in free air

2. With just the constant area centre section

3. With the centre and tail sections i.e. The prop half way along the duct.

4. With the complete duct.

On the basis that at low flight speeds static thrust is a reasonable measure.

 

My concern about strength is not the flight loads but the thought of that long needle nose approaching the ground at any sort of an angle, but then perhaps it is a good thing if it does crumple as it might save the rest of the structure!

[Richard Sharman]

Simon,

yes, point taken - it is really rather more a "pusher prop" than a true EDF, but may be all the better for that! Certainly my pusher prop jets (hunter, mig, eurofighter) all fly really well on very low wattage.

 

How do you propose to measure static thrust - I assume you have a "rig" to mount the plane on, connected to a digitial wirgh scale, perhaps? Don't forget to measure watts at the same time as thrust (1 newton=3.5 oz = 102 grams) !

 

I agree your needle nose looks a trifle fragile - pin everything on a flat approach angle !

[Simon Chaddock]

Not much to show for a couple of hours work but this this photo shows how the concave underside to the fuselage slightly increased the actual inlet area and then takes the duct toward the profile in the fuselage centre section.

The inlet starts at the line. To form the inlet I will have to use a 'disposable' former to hold the inner planking.

The upper part of the duct will be fed from the side cheat holes.

[Erfolg]

It is interesting that propeller theory indicates that if tip losses can be eliminated, there would be a real increase in efficiency.

 

Practice indicates that this is generally not been achieved, notably with Fan trainer/liner where duct noise and losses were much higher than anticipated. I believe it was a similar story with the Optica

 

Yet if we look at by-pass jet engines, these do appear to have better performance than turbo props and pure jets. Could the aircraft operating speed be an issue here?

 

In the case of Steam Turbines, the loss problem is generally overcome by the use of spill rings, wear rings and recesses in which the blades run in. All designed to overcome or reduce the loss issue.

 

With respect to Simon's Supersonic Hunter, it looks really good to me. Being lazy I would have used a commercial fan unit. I still would be curious as to how a retro fit commercial unit would compare to the propeller approach. Just out of curiosity.

 

Will not be long now, remember to drop me a line, when you are going to fly it!

Edited By Erfolg on 14/10/2011 11:04:04

[Erfolg]

A link to a Scorpion powered version Mach 2 Hawker

[Simon Chaddock]

Erfolg

Airspeed is indeed the controlling factor.

Below 400 mph a prop is still the most efficient.

400 to 600 its a high bypass turbo fan.

Supersonic its more about the pressure recovery from the inlet than the type of turbojet.

I believe Concorde was by quite a bit the most fuel efficient mach2 plane built so far but even so it still gobbled a lot of fuel per passenger mile!

 

My theory book states that a correctly fitted fan blade in a duct has the effect of increasing the blade aspect ratio, so a stubby blade can be as efficient as a longer one. This suggests that a smaller diameter prop in a short duct can be as efficient as a larger prop in free air.

The question is how much smaller is 'smaller'?

I suspect not as much as we would like.

 

So back to the Hawker.

Compared to a typical EDF the blade on the prop I am using has a pretty high aspect ratio already (5:1) so the efficiency benefit from the duct increasing its aspect ratio further is not likely to be very significant.

 

As I see it the key factor in my low power concept is a high volume low velocity air stream that keeps down the duct losses. A by product of this approach is the 'neutral point' (where the dynamic pressure generated from the planes speed equals the duct losses) is also corresponding low.

 

Of course I can't prove any of this but so far it seems to have been born out in practise.

Now its back to the fuselage planking!

Edited By Simon Chaddock on 14/10/2011 23:19:58

[Erfolg]

Would or will you try a multi-blade propeller?

 

If my memory serves me well, steam turbines generally operate at about 3000-3600 rev/min.

 

When I first started working at AEI, the blades of the turbines which I saw were essentially parallel, perhaps with a slightly larger root in some instances. The machines in the test halls were from Parsons, Brown Boveri, Rolls Royce and our own (Merto Vics/BTH). They were not essentially different to the casual eye of a callow youth.

 

By the time I had left all new turbines had broader tips, according to PR, a consequence of detailed mathematical study by the Computer, which filled a large part of an office block. The claimed improvement in efficiency was about 2%. But on machines that were about 3% efficient, worth having.

 

It could be at low revs, the masking effects are not significant, with multi blade fans.

[Simon Chaddock]

Not much to show for an evening work but the chin inlet is more or less complete.

It is planked inside and out and matches up to the front former of the centre section.

The original had a central divider as the duct was bifurcated around the nose wheel. With no undercarriage it is not really an issue in my case but I will add it anyway to support the very vulnerable lower lip of the intake.

The cheat hole ducting and the cockpit lining have to go in before any more of the outer skin can be added.

 

If progress gets too slow I will make a start the tail section. As it has a circular constant section duct it should be relatively easy to build over a piece of plastic pipe.

[Simon Chaddock]

The RH cheat hole.

The opening is the size of the weapons bay.

Looking down the cheat duct.

Each cheat duct contributes about 15% to the total area.

How well it will work is anybodies guess!

[Erfolg]

How the inlets are joined will be a significant factor, with respect to how the duct system performs. Obvious, I know, yet important enough to re-remind ourselves.

You seem to be getting there. Not long now?

[Richard Sharman]

awesome building! This is a real education in Depron sculpture !

[Simon Chaddock]

 

The three parts of the duct come together with fine feather edges.

My big concern is the combined inlet surface area and the surface drag it will cause.

Not much I can do about it but the static thrust may well suffer.

 

A Depron sculpture?

I have an admission to make.

The picture was of the 'good' side of the fuselage. There is still nearly all the fiddly bits still to do on the other!

 

I do fear there is still some way to go.

I haven't even thought about the tail section with its elevon servos mounted within each all moving tailplane.

 

Then there is the CofG.

Realistically the battery has to go in the cockpit. The motor is the only thing heavy enough to act as a counter balance so it will have to be positioned when the plane is assembled - but the motor has to be 'built in' before the fuselage is fixed together!

A bit of 'chicken and egg' guess work needed.

The only saving grace is its enormously long fuselage. Just a few grams of lead should make a big trim difference.

 

But apart from that, and my 7 day-a-week job on the house for my daughter (hopefully 10 days should finish it), the build is going well.

 

Oh and by the way I am a grandfather (again!) today, boy, 7lbs 2oz.

[Simon Chaddock]

Now the nose section is alast finished, or rather as far as I can go at the moment, a view of the 'trifurcated' inlet.

Slightly squashed in this view but click on picture for the correct proportions.

This must now be joined onto the centre section, and then make a start on the tail.

Edited By Simon Chaddock on 24/10/2011 00:29:00

[Erfolg]

Looking good!

[Simon Chaddock]

Having joined the nose and centre sections together I wanted to get an idea of just how long the fuselage was really going to be.

However what immediately struck me was how incredibly short the tailplane moment is.

This might not be the nicest thing to fly!

 

[Richard Sharman]

Short tail moments are typical on most modern jets -- just take a side look at the Bae Hawk for example. Consequently, all-flying tailplanes are often used, but if the P1121 has one make sure you have a good pivot mechanism !

 

Otherwise - fantastic project !

[Simon Chaddock]

The P1121 did indeed have an all-flying tailplane (is 1956 a modern jet?) but in my quest for minimum weight they will be elevons.

Each has a span of 7.5ins.

To make matters worse the duct through the fuselage is so large that there is no room for any servos except in the nose which would result in a very long linkage.

To overcome this I intend to mount each servo within the tailplane ahead of the pivot axis to give a short direct link and a degree of mass balance.

The final issue will be selecting the pivot axis point so as to give as neutral as possible aerodynamic load to avoid overloading the tiny servo.

 

This could all go very wrong!

[Erfolg]

The intake looks really good.

 

The trouble with own designs is working out where all the gubins goes, in advance, then adapting as you get near.

 

It should fly well, the F16 in the club goes well and is broadly similar. If you come down it belongs to Peter.

[Simon Chaddock]

At last a bit of progress.

The tail section constructed on its sewer pipe 'plug'.

In this case the duct is a sheet of 2mm Depron wrapped round.

The duct narrows from 116mm (4.55in) diam to 110mm (4.3in) at the nozzle to better follow scale appearance. I am hoping this modest restriction will not have too much effect on the thrust.

The full fuselage.

Actually not quite full length as the nose cone and fin together add a further 6".

 

It looks like the complete airframe should be about 8oz so still on course for 16oz all up.

 

[Simon Chaddock]

When I laid a tailplane plan in position the tail moment looked wrong and in fact it was - the tail section was nearly 3cm too short - oops!

So an extension ring has been grafted in. You can only just see the join.

 

 

The tail planes under construction. The skin is 2mm Depron with a balsa/Depron/balsa sandwich spar. Like the wing it has a very thin symmetrical section.

Beneath is the glass fibre tube spar (fishing rod) around which each will rotate. The tailplane has significant dihedral.

It looks like there might just be room to fit the servos in the bottom of the fuselage which will make wiring them up a bit easier.

[Simon Chaddock]

Back to slow progress again.

An all moving lightweight elevon tailplane needs a bit of thought, or to put it another way, I am making it up as I go along!

The curved tailplane box spar (hardwood/balsa sandwich) with one of the glass fibre elevon spindle tubes fixed on the end.

The RH elevon mounted on the spindle.

The position of the fuselage restricts the elevon movement to about +10 -25 or just about the same as specified on the full size.

Each servo will be mounted in the bottom of the fuselage just ahead of the tailplane spar with a direct linkage (hopefully incorporating the required differential movement) to its elevon.

Using a mechanical differential linkage has the advantage that the full servo travel is retained so giving the tiny servo the maximum mechanical advantage.

The complete elevons, spar & servos assembly weighs just under 1oz (28g).