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Old Jan 30, 2008, 10:27 AM
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Ducted Fan Performance Questions

I am a mechanical engineering student and am building (with a 14 person team) a vertical take-off and landing R/C aircraft with a 6ft wingspan.

We have decided to go with glow ducted fans since we think the vertical thrust we will initially need is around 10-15lbs.

I am in charge of choosing the engine and fan setup to provide this lift (through some duct work and gyro-chip controlled butterfly valves).

Initially I have chosen the OS-91 VRDF engine with a Dynamax fan.

My problem is that I cannot seem to find any performance data for this fan/setup. I know only that it is capable of providing up to 15lbf thrust with minimal duct losses.

My question to yall is if anyone knows any technical specifications of the Dynamax fan or another similar (5.25in ) fan. The type of specifications I am looking for are: a fan performance curve (head v. flowrate, power v. flowrate), fan efficiency, pressure drop created by the fan, mass flow rate of the fan, maximum back pressure before fan stall.

I tried calling the manufacturers of the fans but they don't seem to have any information about it. Ideally a fan performance curve would be all that I require, or the characteristics of the fan's "sweet spot."

ANY help that ANYONE can give is much appreciated. I will be working on this project until May (when it should be completed) and will be more than happy to keep you guys updated and provide any engineering support I can.
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Old Feb 01, 2008, 03:30 PM
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Joined May 2004
157 Posts
Here's an old Excel file I have for the Dynamax and Ramtec fans. You'll notice that they only produce about 10lbs thrust static (on a good day). This file refers to swept inlet areas of about 80% to 85%. I think that Trim actually recommended a 95% swept inlet area. You'll have to experiment to get optimum inlet & exhaust ducting.

This data is based on no nitro fuel, so you might do a little better with 10% or 15% glow fuel. Be sure to add castor oil to get to 22%-23% total oil content by volume. Or buy DF fuel from somebody like Red Max or Wildcat.

Bob Violet also had performance curves for his fan that were published in an issue of their Inlet magazine back in the late 80's. If you call them, you'd probably have to remind them of their existance. Maybe then they'd dig a copy out and send it to you. I don't think I have those any more.

If you need max static thrust, you'd probably really want a Byron 6" fan. But there's no data out there that I know of. It would also need the large head version of the OS engine an the Byron pipe.

Performance on these fans is heavily dependant on the right tuned pipe, proper engine cooling and the right needle setting. I liked the BVM pressure pipe on the OS engines, but Jet Model Product (Dynamax) also has a good pipe system.

Be sure to use an inflight adjustable needle valve. Lean DF engines die fast and OS DF engines aren't in large supply anymore.

An alternative engine is the BVM .96 with the OS adapter for RamTec/Dynamx. BVM has turned over production & sales of this engine to Jerry Nelson. You would likely get more thrust from these fans with this engine. It would be the only other engine I would recommend. BVM can give you more info.

Frankly, buying a used turbine, such as a Wren, is a much better choice. Unless you have experience at properly setting up the fan with tuned pipe, getting the exhaust o-rings right, aligning the pipe correctly, breaking in the engine, getting the needle right, etal., it can be an expensive experience and a real mess. You will undoubtedly need more than one engine to get the project done. The turbine will give you the benefit much greater thrust, no mess, and no critical tuning although the initial cost is higher.

Good luck
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Old Feb 02, 2008, 09:49 PM
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Hubbardston
Joined Oct 2006
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Thud Driver is right DF engines can be alot of work to get adjusted right and get the pipe working the way it should they take some experiance to get tuned in right and when they do run good theirs still a good chance at burning out the motor their high performance and the os 91 I believe it has no warrenty on it if it goes I don't know what your budget is but you would be much better with 2 JetCat P70's plus it you use 2 df engines your going to have to get them running exactly the same for it work hope this helps
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Old Feb 05, 2008, 11:48 AM
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Many thanks, I will keep you all posted.
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