Prop for my KK Auster Arrow

[Andrew D]

Hi all,

I built this KK Auster Arrow as a potential Kit scale entry.  I did all of the normal stuff to keep it light, including making the tail bits as light as possible and moving the peg a bay forwards.  However the model is very nose heavy.  No worries I thought, I will add some tail weight.  However I had to add a lot, and the model flies like the pilot has had one too many!  I think that the tail weight is acting as a lovely pendulum and affecting the directional stability.  So I think the answer is a lighter prop.  I have always built my models using commercially available plastic items.  So the question is is there a "gold standard" instruction document for making your own?

[Nigel_M]

There were a whole set of docs on the last HPA with which you could thoroughly disappear down a rabbit hole designing your own props. I have a copy of several if you can't get hold of them; I can email them directly.

Which would I recommend? Sorry, can't remember. That was several unwanted and distracting domestic 'projects' ago. I'll scan the folder and email a couple tonight if no-one beats me to it.

[Andrew D]

Yes please Nigel...

[Jmk89]

Here is a link to my copy of Ron Warring's book on airscrews, which has a lot of practical and theoretical knowledge on the subject

[Andrew D]

Thanks JMK

[Nigel_M]

An alternative which Andy Hewitt prefers is to scrape the plastic prop. Apparently it is possible to reduce the plastic to 'very thin' - no, I've no idea what measurement! - but they become translucent and much much lighter. If you're not after the ultimate duration performance from a bespoke duration design, which is going to be considerable effort to set up with jigs and so forth, and quite a major rabbit warren, I'm assured a worthwhile weight reduction can be achieved by scraping.

The greater problem I have is ensuring the wire hole is straight to the hub axis. I find drilling and bushing leads to a brittle hub. Does anyone have any tips for what to use for the bush and how to avoid weakening the hub? I'm thinking the waste sprue from plastic kits might be suitable, not that I have any to hand. I think it would be glueable to the prop plastic?

[Nigel_M]

Nice looking model, by the way   

[TheLurker]

...I'm thinking the waste sprue from plastic kits might be suitable...

I don't know about other manufacturers, but the current Airfix polymer formulation seems very soft to me and I suspect would wear very, very quickly.

I'd second your suggestion of scraping & sanding down the prop.

Nice looking model...

+1

[g_kandylakis]

I prefer to glue aluminium or brass tubing, CAed to the plastic hub.

And yes, scrapping works, it is mainly a matter of patience getting it as light as possible.

[Andrew D]

Thanks guys.  I have read the articles that Nigel sent me and understand them.  Hmmm.  It's something that I would like to do, but it needs a bit of a run up and the time and investment to make all of the necessary bits, bobs and fixtures as Nigel says.  I did think about scraping, but I didn't think that I would remove enough weight.  I do scrape these props - but to balance them only.  I might try it first, I can also hollow out the nose block a bit and see what happens.

[Andrew D]

I managed to spend some time making a compete mess by scraping the prop as thin as I dare and then sanding it with wet n dry to improve its aesthetic.  I resisted the temptation to paint it...

It is now pretty flexible and floppy!  So we will see how that goes.

I managed to remove some of the nose block at the rear and used a prop shaft with a smaller loop to save a smidge more.

This work reduced the total mass from 5.8g to 4g. In theory it is still not enough as the blu tak tail weight was 1.2g with a moment arm ratio of over 2 to the C of G.  Having said that the model was behaving very badly at the end, so it was hard to tell what was what.

If this doesn't work I will make a new nose block and use a smaller button of the type I normally use.  The larger ones that I used were simply more akin to the old KK type and I thought little to nothing about its heft...

It's Alfreton a week on Sunday so I will see how it goes.

Andrew

[SP250]

Careful Andrew; Alfreton is this coming Sunday 4th Jan, not in a week and a half.  Also it's the Velodrome meeting on the same day.

John M

[Andrew D]

Careful Andrew; Alfreton is this coming Sunday 4th Jan, not in a week and a half.  Also it's the Velodrome meeting on the same day.

John M

Yes, thanks John. I realised a couple of hours back.  It will be the Feb session then, as this Sunday clashes with a meeting with 30,000 others at Pride Park Stadium...