Important "discovery"

John Goddard Sun Jun 03, 2012 4:44 am

I'm sure most of the experts know this but......
Yet another of my not very scientific observations.
Yesterday whilst faffing around with the Peeweed
Bacaruda I switched from my usual 30% bitchesbrew to
Some slightly less hairy chested 20%.
Tbh there wasn't much difference in output
16.8k v 16k BUT whereas the 30 needled at
4 turns out the 20 only require 2.5 turns out.
So
1 this explains why my 010 always
Wanted 6 turns instead of the 4 in the manual.
2 why my engines drink motion lotion like
It's going out of fashion and explains the rather
Brief run times on the tanked engines.

Mentioned it to 'Cameraman' Glen who competed
Back in the day with planes on strings and
He said "of course, we always had to weigh up
Between balls out power and duration".

I don't think this has come up on here when
Explaining needle settings before.
Perhaps it's something to bear in mind.

Very Happy

Surfer_kris Sun Jun 03, 2012 5:49 am

Yes, this is in principle old-school stuff that is valid for any glow engine. Another thing to consider is that if you don't change the compression setting you will need to run the higher nitro fuel even richer to counter act the advancement of the ignition induced by the nitro. In other words, you can get back some of the fuel economy by adding head shims when running on the high nitro fuel.

Another thing that one can see when not altering the compression ratio, is that the engine will runner colder on the higher nitro fuel. Again this is a result of the richer setting needed on the higher nitro fuel.

John Goddard Sun Jun 03, 2012 8:11 am

Nicely Put Kris
I had been messing around on the PeeWeee with numbers
Of gaskets but didn't see huge differences and decided to stick
With a single gasket after advice from Matt/Bernie
Last week.
The overriding difference as you say was HEAT.
Perhaps those who run these may reconsider their
Thoughts on perhaps using 'cheaper' lower Nitro
Fuel cos if there's more heat the engine won't
Last as long. So there may be a false economy using
Lower.
Very Happy

**Cribbs74** Sun Jun 03, 2012 8:19 am

Well I'll be darned!

And here I thought I knew it all lol!

That's a nice bit of info John. Thanks for posting that.

dinsdale Sun Jun 03, 2012 8:41 am

cribbs74 wrote:Well I'll be darned!

And here I thought I knew it all

Well there ya go. Now you do Laughing

.

Surfer_kris Sun Jun 03, 2012 9:13 am

For those running diesel engines the connection between compression setting, needle setting and prop size become much more obvious as the compression setting is much easier to adjust. The best would be to have an adjustable compression setting on glow engines too, then this would be much clearer to everyone I think. The lesson is too never just settle on one specific number of head shims, as there is no way the engine designer can/could know which fuel and propsize each of us will want to use.

gcb Sun Jun 03, 2012 9:32 am

This fact becomes very important in events that use a timed engine run such as CL Stunt. Since these guys vary head compression and nitro content to get that "perfect" engine run, you will see them meter the fuel they put in the tank, or fill the tank and take a metered amount out.

In 1/2A reedie events where you are allowed to refill the tank and continue, it doesn't matter so much.

And, of course, us sport flyers don't give a rat's patoot how long it runs. :-)

George