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Member postings for David Mellor

Here is a list of all the postings David Mellor has made in our forums. Click on a thread name to jump to the thread.

Thread: The Ohmen
21/05/2018 14:53:06

Very nice indeed!

Thread: Anyone can build a model plane.
20/05/2018 13:31:29

Here's one I made earlier.

Cyano, of course, is a modern material. However, the same technique works using cellulose dope to lock the thread wraps into a nice collet. So the technique was available to the old guys back in the day. I wonder if it was ever used???

p1060311.jpg

20/05/2018 13:20:23

Posted by Michael Richardson 4 on 20/05/2018 13:08:14:

To hold the wheels in place I simply wrapped some copper wire tightly around the wire about 6 times for each collar, trimmed it with my scalpel then soldered the wire in place.

A lighter alternative is to wrap some thread around the wire about 20 times and add a drop of superglue (cyano). When dry, add the wheel and wrap another 20 turns and a drop (just one.... otherwise you'll glue the wheel in place) to stop the wheel coming off. Adds no measurable weight.

Thread: RM Aerobat
20/05/2018 09:00:44

Following with interest, Nigel.

I too have often wondered how different an airframe can be before it becomes something else.

One approach to answering the question is to use the 2-D planform as the "test". If it is the same planform, then the Neutral Points and CG ranges will be identical (these being independent of airfoil section, weight, power and construction details), so you could say the plane is "true" to design. So yours would be bang on by that test!

Dave

Thread: Were to buy lipo battery
19/05/2018 12:50:15

Posted by PatMc on 14/02/2018 20:43:56:

Hobbyking - specificaly : Zippy Lipo, Zippy Compact & Turnigy but not Turnigy Nano-tech.

That is exactly my experience too. I have around 90 lipos and the best performance has come from Zippy and Zippy compacts from HK. They are fantastic value batteries as long as you don't need more than 35C discharge rate.

Turnigy (ordinaries) from HK have been excellent too - as long as you don't need better than 25C.

Gens Ace from Giant Shark have been outstanding.

I have never bought or used Turnigy Nano-Tech because the small group I fly with all have them and have been very disappointed with them.

But...... as always in this game..... your mileage may vary! We are each of us giving our own personal experiences here and our collective opinions are not statistically meaningful. Better than nowt, though smiley

Thread: Anyone can build a model plane.
19/05/2018 09:14:51

Yes, the tissue hides the scorch marks.

On my Junior 60, though, I wanted a nice see-through doculam-only covering, so I had to be pretty careful to avoid burning the balsa underneath and producing those dark streaks.

I think it is very weird that you can burn (scorch) balsa through very thin doculam without melting the doculam.

18/05/2018 22:36:23

Good point, Pat. I've had the odd scorch mark on balsa under the doculam too. The trick seems to be to keep the iron moving so as not to build up too much heat in the balsa.

Edited By David Mellor on 18/05/2018 22:36:47

18/05/2018 10:48:43

I should have added that I use 38 micron doculam. I bought a 100 metre roll off e-bay a couple of years ago and I'm still using it.

Another option you have if 38 micron laminating film is available is to use wet-strength tissue and dope over the top of the doculam covering. This gives the best of both worlds - traditional tissue & dope covering which is puncture and tear resistant. I use it on slightly larger models:-

p1040447.jpg

18/05/2018 09:28:59

I've used doculam on a Junior 60 and it is very light and very tough.

Unlike other covering films it shrinks at very high temperature-settings on the iron. I shrink mine at 200 degrees C, but it would be a good idea to test first! It doesn't shrink much at all at lower temperatures....

No balsaloc is necessary, just iron straight on at low temperature, pulling it reasonably tight and wrinkle-free. Then when all attached, shrink on high heat for drum-tight lightweight covering.

p1060235.jpg

Thread: The joy of trees
17/05/2018 17:30:19

Very impressive..... as always, Soren.

Thread: Calculating Washout
17/05/2018 12:54:39

Things that fly enjoy complex relationships with the fluid through which they move. And that fluid itself (air) varies a fair bit from one day to the next and one place to another - especially at the scales we fly.

So.... it isn't really possible a lot of the time to change one thing without interacting with a host of other things. Sometimes the interactions are insignificant, other times they aren't.

Observations are fine. It is conclusions that are drawn from observations that we need to be careful about.

16/05/2018 16:57:29

Posted by Nigel R on 16/05/2018 15:55:33:

In my unqualified opinion, if the section is constant across the taper, and the taper is as mild as listed, then I would not bother, just build the thing all dead flat on the board and have done with it.

I would not bother putting washout in such a wing either. Why the modification? That seems to be a more potentially significant factor than washout.

Unless your design flight envelope is slow flight with high payload/wingloading, then I don't understand why you think washout is required on what sounds like a relatively high lift wing.

If it is important to you, perhaps you should calculate the plane's stall speed first?

15/05/2018 13:20:28

Not all wings require washout, it depends on type of plane, wing aspect ratio, wing loading and specified flight envelope.

Some designs cannot stall, even at AoA values of 45 degrees. However, these are specialised LAR designs.

Probably a "guesstimate" based on experience and comparison with similar models with similar flight envelopes will be better than a calculation because the existing models have at least been flight tested for you.  Jon's suggestion looks good from where I'm sitting.... as a general starting point, anyway.

 

Edited By David Mellor on 15/05/2018 13:22:40

Thread: Dead Lipo remedy?
12/05/2018 14:33:27
Posted by Cuban8 on 12/05/2018 09:05:56:

Since the widespread availability of Lipos and their common use in electric aeroplanes in what must be be some 15 years now, I've never seen a lipo spontaneously burst into flames by itself, unless it's been shorted out, had a screw driven through it (wing bolt) or wrecked in a crash.

Nobody in their right mind would charge a lipo in their house or car, because there are documented cases where batteries have caught fire - but these are very rare indeed compared to the number of batteries in use world wide. Be sensible and take care as with a multitude of other potentially hazardous household substances etc.

 

It is, of course, unwise to assume one's own personal experience is somehow representative of the world at large.  So I'm keen to point out I'm not doing that here!

 

But..... as mentioned in my earlier post (about the 14th one on page one), my FB narrowly avoided burning his entire house down from charging a lipo. As mentioned, he is an electrician and had some awareness of the risks.

 

He had left a lipo charging unattended on a kitchen worktop while he nipped out - despite that obviously being a bad idea.  He just got back in time to find that the battery had set fire to his kitchen units.  Perhaps he felt statistics were in his favour - he had charged his lipos hundreds of times over the years and never experienced a problem.

 

I have personally had one narrow squeak with a small lipo (a 750 mAh 3S) which I stupidly connected to the ESC in reverse . The ESC smoked and the battery puffed - neither burst in to flames, but both became very hot indeed and were plainly wrecked (and charred).

 

So.... I can attest to the fact that lipos can get very hot indeed and burn under some circumstances.  Both events were caused by carelessness/stupidity.  No human being is immune from making mistakes...........

 

Edited By David Mellor on 12/05/2018 14:41:51

10/05/2018 14:57:37

This is probably a SOTBO, but if you intend to be "creative" with using a charger on a dead lipo, it is perhaps best to not wander off too far while the patient is under treatment.

An FB left a lipo on charge while he nipped out for half an hour. He got back just in time to be able to get the fire under control. It did, however, burn down half his kitchen and stink the whole house out for months. His job? Electrician.

Thread: PU Glue removal
08/05/2018 15:05:58

Too much information......?

Have you noticed that PU glue used on balsa or ply in a model always stays light straw coloured? Yet on human skin it tends to darken and even turn black. Why is that?

Well, it seems PU darkens in contact with metal ions and goes very dark in contact with iron ions in particular. On skin, it seems it may be darkening in contact with traces of haemoglobin from some of your near-surface capillaries.

08/05/2018 14:48:52

Don't use solvents (thinners etc) as these (a) don't dissolve PU which has cured and (b) they enter the blood stream via the skin and damage your liver.

PU cures by chemical reaction with water. The cells of your skin provide sufficient water on contact to react, so even if you wipe it off straight away, some PU will already have bonded.

I find the best (as in "least worst" treatment is to scrape off any PU that hasn't fully cured with a Stanley knife blade - used at right angles to the skin surface a bit like a cabinet scraper. Then let the residue cure (an hour or so) before abrading the skin back with coarse sandpaper. It does come off, but it takes a fine layer of dead skin cells with it, so you can't use the same technique on the same bit of skin for a week or two!

As The Wright Stuff says, wears off all by itself after a week or two simply because we all shed our skin cells..........

Thread: horns
07/05/2018 15:18:40

HK sell 0.6mm thick glass epoxy sheet, Macgregor sell 0.8mm glass epoxy sheet. Both cut well with scissors and make very good horns. For larger models simply make two identical horn shapes, rough up with sandpaper, epoxy together then drill the link-hole.

Thread: How large scale electric can you go?
28/04/2018 12:44:23

There are no limits as far as the technology is concerned. You can make them as large as you like, though you will need to respect the appropriate legislation for really large craft that can carry a human......

Thread: Prop adaptors: How tight is just tight enough?
25/04/2018 11:24:51

As Bob says, not every collet type prop adapter is as well made as it should be. I've had an off-centre one too and it was useless.

Donkey motor shafts are very smooth, and the temptation is to sand it slightly. I've never found it necessary, though a touch with very fine wet and dry (used damp) may help.

Just be aware that if you abrade it dry some of the ferrous powder from the abrasion may get attracted to the magnets in the motor can..... you don't want tiny bits of steel in your motor........especially on a Donkey because they use oiled bronze bushes for bearings.

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