The Hawker P1121 - again!

[Richard Sharman]

Simon, great progress, looking good !

 

Question: will there be a gap between the elevon and the fuselage? or is there some other way that the movement of the elevon won't be restricted by contact with the (fatter) fuselage at the point of the spar ?

 

I only mention this because in the analagous position on my Hawk the all moving elevator was restricted in movement because of unplanned contact, actually with the pushrod. Fortunately it only restricted the extreme of down elevator(stick forward) so was not too onerous.

[Simon Chaddock]

Richard

On the P1121 the tailplane was mounted slightly below the line of the fuselage in a bulged fairing rather like the Hawk has with its high mounted one.

This gives clearance for the nose of the tailplane to rise to +5 degrees. The reduced diameter of the jet pipe however allows the trailing edge to give -20.

 

This travel looks very modest particularly as I am using them as elevons as well so I taken the liberty to lower the tailplane a bit to allow +15. -25.

If necessary I can all ways put small cut outs in the fuselage skin to increase the travel further

The RH servo and elevon.

If I have done the geometry right the elevon horn will position the link from the servo arm to give the required differential travel - I hope!

[Simon Chaddock]

Its back to slow progress again!

This is the 4th rebuild of the servo mounting to get the desired travel.

The servo is mounted in line with the elevon so it has a short vertical link to the elevon horn.

All this will be completely covered by the fuselage outer skin.

The elevon in action.

The up and down travel is almost equal as on reflection a strong differential action in an elevon would introduce a positive nose up pitch with any 'aileron' input.

 

I suspect it may be prudent to add a small under fin to prevent the leading edge of the elevon from catching the ground when landing.

[Simon Chaddock]

Part of the day off house DIYing so some visible progress.

Both elevon servos are in and tested.

I decided that it would be much easier to achieve the desired CofG if the motor was positioned after all the RC gear has gone in. It will of course make the motor installation rather more difficult, but at least now the fuselage is in one piece.

It aint half long!

 

With the servos it weighs 5.9oz so still on course for an 8oz bare airframe.

[Simon Chaddock]

Thinking about actually launching the P1121 the lower fuselage was going to need some reinforcing around the CofG where is would be held so some additional closely spaced 'half' formers have been added to give additional support to the skin.

With the servos in place a start has also been made on the lower half skinning. This will stiffen up the structure somewhat to make the motor installation a bit easier.

 

Like the Skyray all the RC gear will be completely 'lost' inside the structure with just the battery cables running along the fuselage spine to the cockpit.

 

Edited By Simon Chaddock on 08/11/2011 18:12:30

[Simon Chaddock]

Laying laminate flooring so progress slow again but the lower fuselage skin is complete.

And the elegant Hawker fin added.

Getting there!

[Simon Chaddock]

Had a bit of trouble with the synthesised rx - it didn't work - so hopefully a replacement will arrive soon.

The 30A ESC has had a finned heat sink added.

The shrink wrap has been carefully cut away to expose the aluminium surface of the heat sink and the finned one is held down on top with nylon line.

As the ESC is mounted below the fuselage surface and Depron is such a good heat insulator putting the fins in the air stream will ensure that at least some of the heat gets away.

This type of installation works well on the Skyray.

 

[Simon Chaddock]

To quote the late Jimmy Saville "How's about this then!"

 

The ESC in the above picture did not work driving the motor on its test mount - just jerky oscillations.

Fortunately I had a spare and it worked ok.

Modified it to include the heat sink and it did not work - same result.

 

How on earth could adding a heat sink have any effect?

 

I cut the nylon line and removed the heat sink and it worked again.

Ah!

With the motor running slowly I picked up the ESC between finger and thumb and squeezed gently - the motor cut out!

 

It appears that some part of one of the 'internal' heat sinks is very close to touching something so the extra compression of the nylon line holding on the finned heat sink causes a short circuit. Obviously a design weakness as it occurred on both ESCs.

 

And just to make my day the prop was horribly out of balance as well.

 

 

 

[Simon Chaddock]

Having sorted the problem with the ESC (fewer heat sink tie downs!) the motor and prop can be tested in earnest.

The tiny prop has a rather small boss so the driver had to be turned down to the same diameter.

As the blades of the prop go virtually to the boss it will improve the performance if the motor is mounted in such a way that nothing is larger than the out runner bell. Small Depron fairings will be added to improve the air flow round the motor.

As the prop is full diameter it takes a bit more current than the slightly trimmed prop in the Skyray.

Not accurately measured yet but its close to 200W which if don't loose too much thrust from the duct should give the Hawker ample urge.

Edited By Simon Chaddock on 18/11/2011 23:24:48

[Simon Chaddock]

Progress slow again but with the motor installation I am having to make it up as I go along.

The motor "pod".

The mounting is all hard wood and picks up the four motor mounting screws. There is a small Depron fairing between the bell and the prop. I have left the original motor slots for cooling although with its whole casing directly in the slip stream they could probably be faired completely over.

The next problem is how and exactly where to fix this in the fuselage for the correct CofG.

Probably the easiest way will be actually cut the fuselage in two at the appropriate point to allow the motor to be accurately positioned in the duct.and the two halves then glued back together again.

We shall see.

[Simon Chaddock]

The first attempt to locate things to achieve the anticipated CofG.

With the CofG at bit ahead of the full size position and with the motor placed where the "area rule" has the fuselage at maximum diameter the battery ends up just behind the cockpit.

Not really a problem. It just means a small potion of the 'spine' will become a battery hatch and the cockpit canopy can be fixed.

I need to get the battery and motor positioned fairly accurately as the remaining RC bits (ESC and radio) are pretty light .

 

Now I can chop the fuselage in half and mount the motor!

 

[Richard Sharman]

You're very brave to contemplate cutting the fuselage ! But what must be, must be ? The area rule should cause the fuselage to have 2 wide stations - I assume you mean the rearward one after the wing trailing edge? Of course, the purpose of the area rule is to maintain a constant total cross section, but you're not going transonic (i hope!).

 

I'm a bit worried about your ESC -- it's obviously ok at the moment, but could vibration in flight cause the contact again, with possible loss of control, if only for moment ? I think I'd be inclined to try a different type of ESC, or possibly glue the heatsink on rather than tie it on. Just a thought.

Edited By Richard Sharman on 27/11/2011 16:40:30

[Simon Chaddock]

Richard

The only reason for placing the motor at the maximum rear area rule point is simply to provide the most rigid motor mount.

The motor overhang of an out runner on a 'minimum' mount with a close fitting ducted prop needs all the rigidity it can get.

 

Perhaps more important I have now finished my marathon 3 month 7 day a week house DIY so I can spend some serious time on the Hawker!

Edited By Simon Chaddock on 28/11/2011 02:24:02

[Simon Chaddock]

Well there it is - chopped in half!

Actually not that difficult in Depron.

Just before I soldered the long motor wires onto the motor I thought I would weigh them.

The motor, its duct mounting and prop complete weighs 1.8oz. The motor wires long enough to reach the ESC mounted close to the battery a whopping 1.4oz, that's at least 10% of the panes estimated total weight

I need a plan "B"!

[Richard Sharman]

Arrrrghh...! I've had the same problem - you need a fairly substantial grade of motor wire to cope with a serious current flow, but the length needed causes a weight penalty.

One possible suggestion: place the ESC as close to the motor as you get. Not only is this better from an electrical/interference point of view, but it means you only have 2 wires along the fuselage to the Rx (the DC circuit) and not 3 wires (the AC motor circuit). This might save a third of the wire weight ? And you might even be able to put the ESC in the fan exhaust for cooling ?and dispense with the heatsink ?

Edited By Richard Sharman on 28/11/2011 12:51:17

[Simon Chaddock]

Richard

Yes ESC nearer the motor could reduce the overall cable weight except the experts say it is the capacitors on the battery side of the ESC that can only handle a limited length due to induced voltages.

With such a long fuselage and with the duct occupying virtually the whole cross section of the central part there are bound to be long cable runs.

 

My proposed solution is solid polyurethane insulated wire. It is easily solderable.

20SWG wire is good for well over 20A, which all I need, before its temperature rises 20 Celsius above ambient and it is under 1/2 the weight the flexible insulated stuff although it wont be nice to install.

The full size Hawker had a small cooling air intake on the spine so I can make use of it for than very purpose!

Edited By Simon Chaddock on 28/11/2011 18:15:04

[Simon Chaddock]

The motor installed in the rear section of the fuselage.

A short video of a low power test run.

It had to be at low power (no more than 1/4) as until the fuselage is complete and fully skinned it is simply not strong enough to take the thrust!

It looks scary but those apparently bare copper wires are actually insulated.

[Erfolg]

I have noted your issues with cable lengths. I can see that you are concerned.

 

I have faced a similar if not identical issues with my Dornier.

 

 

There is a motor at the rear, as well as the front, the Lipo is situated right at the front. This has required long cables, as per the picture below.

 

 

The model has and does fly without any issues.

[Simon Chaddock]

Erfolg

That's some serious cabling!

 

With my super lightweight build I was more concerned about its weight rather than its length but of course one follows the other!

 

The ESC and the enamelled motor wires installed.

It has saved nearly 3/4oz.

Just the radio to go in then the skinning can be finished.

[Simon Chaddock]

In this state I felt confident enough to run at full power to check the thrust.

Quite easy to do with that long nose, just hold it nose down on some kitchen scales.

 

It produced 12oz thrust. Probably accurate to 1/2oz.

Exactly the same test with the Skyray (with an identical motor and prop) produces 14oz.

In free air the same motor prop combination produces 16oz.

Is such a discrepancy to be expected?

 

1. The duct length ahead of the prop (25") is almost exactly the same on both planes.

2. The actual inlet areas are comparable although on the P1121 it is dived into 4 rather than 2 for the Skyray.

3. The P1121 does have an additional 11" of duct aft of the prop whereas in the Skyray the prop is right at the back.

4. With a 'buried' prop the P1121 is noticeably quieter!

 

In terms of just duct length the 25" on the Skyray costs 2oz thrust so the 36" total on the P1121 would correspond to a 3oz reduction rather than the actual 4.

However I am fairly sure the duct aft of the prop has greater losses per unit length even though it is of the same cross section.

Ahead of the prop it is reasonable to assume a constant air velocity, normal to the duct, and more or less across the whole cross section. It would of course be slower at the duct wall boundary layer.

A prop however does not produce a constant velocity air stream over its full diameter nor is it normal to the direction of the duct so additional losses are to be expected.

 

I would be interesting (and it might prove necessary!) at some stage to move the P1121 prop right to the back to see the effect.

 

However 12oz is hopefully sufficient for a 16oz plane so it will be completed in its present form.

Edited By Simon Chaddock on 02/12/2011 15:23:16

[Erfolg]

How many amps are drawn?

 

As to differences in measured values, I have not got a clue.

 

It would be of interest if the average velocity of air were known, better still would be the volume moved in a time period. Again I have no ideas as to how to simply do it, without some equipment as made by Eagle tree et al.

 

What matters is how well the model will fly.

[Simon Chaddock]

Slowly ,slowly the fuselage is getting skinned.

As quite a bit of skin is being added well aft (only 1/2oz but that is still quite significant) I will leave the whole cockpit area until last so I can finally position the battery,

This does raise the question of where the CofG should really be.

With such a short tail moment I am very tempted to do what I did with the Skyray and build a simple half size 'profile' Depron 'chuckie' to get a better idea of its characteristics.

With a hand launch maiden you really don't want it to either a dive into the ground or go into an immediate stall.

However with the weather at the moment I wont be flying anything any time soon.

In fact I haven't even heard anything flying out of Manchester this morning!

[Olly P]

An interesting build Simon, nice subject, and well put together!

 

Mcr - you can't be far from me in Stockport then - we are on the flightpath!

 

Olly

[Erfolg]

I have seen some big jets on circuits out of Manchester, I suspect they are all climbing higher than I am used to seeing , as I have heard none low down. Certainly no ATP's or smaller jets.

I was thinking if it is a cross wind at Ringway, It could be a real challenge landing or taking of.

You are really chasing the weight down.

It should not be long now?

[Erfolg]

Just seen a big jet pass overhead, quite low for this part of the circuit, complete with all navigation and landing lights blasing and gear apparently down (which I have never seen before).