Hi Charles,

It will depend on the engine and how you run it.  But, in the few I have tried, a 9" on the .25 and a 10-10 1/2 on the .40s.  Of course it still depends on how you run them.  HIgher rpm wet 2 runs will let you run a touch bigger prop.

BIG Bear
RNMM/AMM

Bill, Brett,

My apologies, I forgot to mention the application. 

I've talked about my interest in the F7F Tigercat. This would be for that model at 57" in span. At 57" in span, the prop, according to what would be scale, and I know this isn't practical, would be 14".

So you see my reason for the large three blade prop.

I have two MDS .25's for R/C (carbs) and two OS LA 40's CL.

Thanks,

Charles

are you intending to create a stunt airplane, carrier,  or a scale airplane, it makes a HUGE difference,,

Mark,

Yes, correct, I should have made that clear also.

I would prefer Stunt.

Thanks,

Charles

  It depends on the engine. For any of the current hot-shot 25s (25LA, for example) plan on using 10" props max. The engine wants to run upwards of 12000 rpm and you need to make the prop small enough to ensure that it can. It *could* spin more prop but that would just give up performance unnecessarily.
 Brett

Brett,

Thanks. Well, you didn't mention the MDS .25's so I guess I know what that means. ebay. 

Charles

You want diameter? Then whack a diesel head on the engine and it should be able to pull an extra inch in diameter without a qualm.

(And yes there are most certainly diesel conversion heads available for the OS Max 40 in America.)

I just want to see Charles holding on to a twin engined 40 doing the pattern!

Ya know Chris, you are a Diesel guy, and I'm sure you've had good luck with them, correct?

I do have stuff I could sell and the build is a way down the road. But I know absolutely nothing about Diesel engines. What do you do, just buy a head? Reliable?

Charles

Hi again Charles,
                            any 'suitable' donor engine can run a diesel conversion head on it (and that's all you need apart from Tygon fuel tubing), and by suitable I mean -
1. Its is strong enough to endure the rapid onset of ignition that kerosene/ether exhibits,
2. And to a lesser extent it does not have overly large gas passages that will slow down gas flow.

And the OS MAX 40 is one of the best engines to convert across 'if' you are simply looking for scale prop diameter on a scale looking model.

A diesel will typically pull around at least an extra inch of diameter as compared to your glow equivalent and at a lower rpm - all sounds very 'scale' does it not?

Reliable? Absolutely and there is a plethora of advice in the RC world about this very engine available on the net.

The conversion heads? Just Google 'Davis Diesel' and you will enter a world hitherto undreamed of.

I think that the words 'scale' and 'diesel' go very well together - others may think that the word 'diesel' only belongs next to 'trucks.'
We are all different I suppose.

Chris,

I went over there and looked around. Really interesting stuff.

Heads start at around 80.00. Ouch!

Those OS 40's of which you speak, do they have bolt on mufflers? I don't like straps.

I'm going to look into heads for my GMS 2000, .25's.

Charles

Hi mate,
             Price wise Davis is up there with the best of them though his product is very, very good but his is not the only product on the market - I got my conversion heads from A.J. Coholic in Canada at a cheaper rate.

The OS Max 40 conversion is bog standard excepting only that the head is changed, and yes the standard bolt on muffler still applies. (I hate strap-ons too, wait ....... that didn't sound quite right did it?)
One of the benefits with using a diesel in semi scale models is more a semi scale sound. Let me explain.

Because a kerosene based fuel completes its burn almost instantaneously the remainder of the down stroke's volume acts as an expansion chamber adjunct for muffling.
This is in opposition to a methanol based fuel that has a long burn rate that often extends well after the exhaust port opens.
In other words the sound tends to stay within the engine block with a diesel far more.

Another benefit is that a diesel will need far less airflow to cool it and thus suits cowled in situations more easily, and of course the a fore mentioned prop diameter size increase does paradoxically bring with it a lower tip speed because of lower operating rev range and thus far less prop noise.

Mate, instead of you hearing me prattle on about diesels why don't you head on over to RC Universe "Everything Diesel," on get the 'good oil' on what is going on.
There are modelers in that forum with far more experience than me and you may well pick up a second hand LA 40 head quite cheaply over there!

Talk soon, and now ........... back to glow motors.

Not that this is in the desired engine capacity range but anyway check this out -

http://www.rcvengines.com/rcv60sp.htm

THIS will swing a big prop! (16x12 is quoted.)