boundary layer thickness - Page 2 - RC Groups
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Oct 01, 2012, 08:15 PM
B for Bruce
BMatthews's Avatar
Frank, I'm suggesting that the "safe" way to get the maximum speed is to go with the lower side fan location. If the fan is "eating" a trailing turbulence off the upper side forward hump then you will see some reduction in the maximum thrust you can get.

Kevin said that with 3.5Kw it really won't matter. And there is no doubt that with that sort of power if the fan is decent then it WILL be fast. But if you want that last little bit then it pays to stack the deck in your favour rather than against yourself. And with the fan up top and lacking the sort of CFD software and resources to make up fans that can "eat" the turbulent layer and actually produce a higher thrust I would suggest it's safer to mount the EDF on the bottom where it can digest a cleaner supply of air.

There is also the issue of the mouth of the inlet side on a fan unit. A "raw" fan unit will have to swallow a fair amount of turbulent air coming off the edge of the duct. A fan that has a flared "venturi like" mouth will tend to see a smoother flow of air and produce more thrust. If you're after a high top speed it's not only about the current draw from the pack. You need to feed the fan with the best airflow you can that is as smooth as you can get. And that means no slower and turbulent boundry layer and no possible trailing turbulence off a hump that is upstream from the fan inlet. It's fairly safe to say that the difference in making it to 200 and being "stuck" at 180 could well be the side that you decide to mount the fan.
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Oct 02, 2012, 10:40 AM
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richard hanson's Avatar
How the -------- do you see these little buggers well enough to fly em at 200mph?
The kerosene powered models get awful small -very quickly at 200---
Oct 03, 2012, 01:55 PM
Registered User
frank40's Avatar
Hi again

Well Bruce you are probably right, when trying to go real fast it is better to stick with the safe way. Anyway Claus is making a new reinforced model of his super sling jet..... Here the nacelle is placed on the on top of the model in clean air, maybe next time we can go past 25% of throttle. But thanks for all your inputs to both of you.

richard hanson, How to see a small model at high speed, Jonas my test pilot is an excellent pilot and this project is just to see how far we can push the envelope.
Oct 31, 2012, 07:50 PM
mlp666's Avatar

Just stumbled across this thread and while reading thought SR-71.

If you have just the one motor why not the drone that was intended to be launched at Mach 3.

Just my 2c.

Nov 23, 2012, 01:37 AM
mithrandir's Avatar
200 mph at sea level is a completely different flight regime than mach 3.5 at 75,000 feet...

a high aspect ratio wing is the hot ticket for small models going around 200 mph.....

like a FAI pylon racer wing.....

A long duct, inlet or outlet, is plagued with a total pressure loss due to thickening boundary layer....

prolly only a few percent if care is taken with the design....

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