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Author Topic: Comet P38 Lightning - BUILD  (Read 4811 times)
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Rich Moore
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« Reply #25 on: March 01, 2019, 02:32:43 PM »

I have started. And what better place to start than wheels...

Normally I make solid balsa wheels, but thought I'd try vac forming them instead. I have vacuum formed the wheels in halves using a thin styrene type plastic. The plan is to assemble each half around a balsa disc with an aluminium bush. I'm not sure they'll be lighter than balsa, so I'll probably end up making a second set. I want them to be light-weight, because I want to see a P38 fly around the hall without it's bits dangling, which means retraction.

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danmellor
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« Reply #26 on: March 01, 2019, 03:28:04 PM »

Rich: You don't do things by halves, do you!!

Hope to see this in action.

Dan.
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Pete Fardell
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« Reply #27 on: March 01, 2019, 03:36:26 PM »

Twin rubber, 30" span indoor scale model with retracting undercarriage built and flying in about seven weeks. O-kaaaaay....



EDIT: Sorry- you've actually got a tad over EIGHT weeks. You'll be fine then.
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Rich Moore
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« Reply #28 on: March 01, 2019, 03:54:22 PM »

Here are a couple of pics of wheels so far - half unpainted and balsa hub, a spring, half painted.

I ordered some little springs. They are supposed to be used for coupling trains together, but I think this is a much better use for them.

If the wheels go well, it'll motivate me for the rest of it. The retracts won't be elegant - they'll be simple. I want to use the fact that the forward rake of the wheels when UC is down will hold them down until the model lifts. They'll then spring into the nacelles in a horribly fast way. I have an idea to slow the movement down, which is essentially a small dash-pot (dampener device) made from a small plastic tube and a balsa cylinder (like a mini syringe), but this is probably a step too far in the available time-frame.
The main thing is that I can (just) make the wheels fit behind the motor peg.... Also, getting the wheels up quick negates worrying about variable trim - 'wheels down' trim transitioning to 'wheels up' trim - I could do without the worry.
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Re: Comet P38 Lightning - BUILD
Re: Comet P38 Lightning - BUILD
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Squirrelnet
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« Reply #29 on: March 01, 2019, 04:05:33 PM »

Impressive build Rich.  Looking forward to seeing this one come together
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« Reply #30 on: March 01, 2019, 04:37:28 PM »

That old maxim, K.I.S.S. gloriously pushed aside.  Smiley

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Monz
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« Reply #31 on: March 01, 2019, 05:00:55 PM »

Can't wait to see your retracts (and the P38) come together.
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Jack Plane
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« Reply #32 on: March 02, 2019, 02:11:59 AM »

If you want to get a job done, give it to a busy man...  Grin
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abl
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« Reply #33 on: March 02, 2019, 03:08:46 AM »

Best of luck with it, Rich.
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Graham Banham
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« Reply #34 on: March 02, 2019, 03:26:33 AM »

Sounds ambitious Rich, but exciting too!

How are you going to get the gear down again for landing, or aren’t you? A belly landing will hammer your flight scores, but i understand you want to see it in all its sleekness in flight.

A rapid shift from gear down to up doesn’t necessarily alleviate all the potential trim variations. In the distant past when Richard C built his rubber Dauntless with retracting gear i believe he had to rig some sort of change in the elevator position via a pushrod as the gear came up (and down), as the trim was affected by the drag of the undercarriage, and the absence of drag when retracted. More than likely the disturbance in the airflow under the wing as it retracted and extended played a part too.

Mind you, if your intention is to have the gear flick up into the wells as it lifts this may not be so much of a problem

Good luck though!
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Pete Fardell
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« Reply #35 on: March 02, 2019, 05:59:43 AM »

As well as the retracts and general ambition of this, one thing that really interests me is the practicality of flying a scale twin rubber model indoors? I’m trying to remember but I can’t think of any examples I’ve seen; co2 and electric multis, but not rubber. Are they far trickier to get circling consistently?
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Monz
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« Reply #36 on: March 02, 2019, 06:41:41 AM »

As well as the retracts and general ambition of this, one thing that really interests me is the practicality of flying a scale twin rubber model indoors? I’m trying to remember but I can’t think of any examples I’ve seen; co2 and electric multis, but not rubber. Are they far trickier to get circling consistently?

There was Tim's twin Mustang and Peter's JU88.

https://www.youtube.com/watch?v=76_3b0JO57M

https://www.youtube.com/watch?v=1p07mnRZUjs

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billdennis747
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« Reply #37 on: March 02, 2019, 07:22:38 AM »

I remember Mike Hetherington's enormous Mosquito. The undercarriage twanged up immediately it left the runway and there was no visible effect on trim. Ivan Taylor has experimented but gave up on wheels coming down because of the trim change
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Rich Moore
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« Reply #38 on: March 02, 2019, 09:18:38 AM »

However much I'd like to extend the gear for a beautiful landing, I am indeed planning on a belly landing. The idea is that a quick retraction as soon as the model lifts will negate the need for fiddly trim adjustments mid-flight. Having the ability to lock the gear down would enable at least one flight with wheels down for the whole flight, so I won't necessarily have to lose marks. I am not kidding myself that this is a contender for a podium finish, so I'm not really bothered about losing 'landing marks' anyway.

Pics coming later of the wheels, which have now been painted. They look quite nice but rear wheels are looking like they'll be about 1.9-2.0g each - is this heavy?
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Rich Moore
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« Reply #39 on: March 02, 2019, 09:20:40 AM »

Quote
one thing that really interests me is the practicality of flying a scale twin rubber model indoors

This bothers me more than anything. Keeping the sprung loaded wheels down whilst holding two props still, without crushing anything and then releasing in a controlled manner? Good luck to me with that.
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Rich Moore
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« Reply #40 on: March 02, 2019, 09:24:06 AM »

Quote
That old maxim, K.I.S.S. gloriously pushed aside


K.I.S.S my arse! When have I ever taken notice of such ridiculous advise?
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Rich Moore
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« Reply #41 on: March 02, 2019, 10:46:38 AM »

Wheels done. They are big and fat (rear wheels are about 42mm diameter, front wheel 32mm). 5.5 grams for all three.
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Graham Banham
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« Reply #42 on: March 02, 2019, 11:19:17 AM »

Comparable to balsa i’d say for that diameter and chunkiness. So at two differing moments distant from the CG you’re going to need to compensate for an approx 6g plus gear leg weight generated rearward shift in the CG right at takeoff as all three swing backwards into the wells. Should be able to do this by balancing and trimming initially gear up, but potentially problems rotating to take off as with the gear down the CG will effectively be too far forward, which may in turn result in it accelerating too fast on the ground then zooming on lift off right at the point when the CG shifts back....

Still, nothing like a challenge, and i can’t wait for all this theory to be proven wrong and to see it work like a charm! At worst you’ll have to lock the gear down and still have a spectacular model  Grin
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Rich Moore
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« Reply #43 on: March 02, 2019, 11:41:16 AM »

Thanks. Yes, that all makes sense. I was thinking of putting some ballast at the top of the front strut, above the pivot so it moves forwards, which can compensate for this shift in cg due to uc retracting.
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Pete Fardell
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« Reply #44 on: March 02, 2019, 11:49:24 AM »

The wheels look great!
Regarding indoor rubber twins... thanks Monz-  I can't quite believe I forgot Tim's recent twin Mustang (sorry Tim!)
There is also Jiri Dolezel's Vickers 432. Here's Tim's film of it at Nijmegen 2017: https://www.youtube.com/watch?v=Zpf_GN9hAuM&feature=youtu.be

Also, possibly of interest, is that Vincent Merlijn has been building a Comet Lightning and sharing pics on facebook lately (and hopefully he doesn't mind me showing one here). It does look like a great model, although I think this is for outdoor (and without retracts!)
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« Reply #45 on: March 02, 2019, 01:02:27 PM »

Quote
K.I.S.S my arse! When have I ever taken notice of such ridiculous advise?

....this is why I enjoy watching what you do so much  Smiley
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Rich Moore
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« Reply #46 on: March 02, 2019, 01:08:47 PM »

This should be a good one. There are a lot of parts to cut out...
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« Reply #47 on: March 02, 2019, 01:23:04 PM »

Since I first saw Richard's Dauntless, I have mused about retracts. I've also watched The Dambusters ... a lot. Like many, I liked the converging light thing. Not at all sure of the mechanics of such a thing BUT , could such a thing be rigged with an IR beam and an IR sensitive switch? Each convergence would rotate the UC through the cycle?
OK, so I'm a dreamer  Roll Eyes
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Rich Moore
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« Reply #48 on: March 02, 2019, 01:43:27 PM »

Perhaps. You could use also use ultrasonic sensors to activate at a certain distance off the floor (echo). Not sure on the legality of such wizardry,  but it isn't remote control...
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« Reply #49 on: March 02, 2019, 03:01:00 PM »

Hi Rich,

Love the idea of this, watching with interest!  Grin

Could the gear be let down due to the wind down of the rubber somehow?  I mean it won’t land until the motor torque leans out, so thus it could be the trigger for the gear to drop?

Just an idea, in awe of the ambition and effort to make this work...

Cheers.

Andrew
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