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Mar 10, 2012, 08:24 PM
67,000rpm
Question

Hypothetical question - what happens if you run an EDF with the duct removed?


Does it still produce any thrust?

Does it produce more or less thrust than it did with the duct?

Will the motor revs change? If so will it under- or over-rev?

Will the noise change in volume or sound? How so?
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Mar 10, 2012, 08:35 PM
Registered User
Hi,

1 Yes.

2 less

3 under

4 yes, less



Ric
Mar 10, 2012, 08:53 PM
Registered User
without the duct it will be significantly less effective
Mar 10, 2012, 09:00 PM
67,000rpm
Thanks for the replies so far. Interesting that you say it will under-rev. I take it that implies that removing the duct puts the motor under more strain even though it spins the same fan. I'll take your word for it, but I'd really appreciate if you could explain why that happens?
Mar 10, 2012, 09:18 PM
ShopCzar
Endlesslag's Avatar
Then it's just an electric fan?
Mar 11, 2012, 08:56 AM
We want... Information!
Bruce Abbott's Avatar
Quote:
Originally Posted by LMF5000
I take it that implies that removing the duct puts the motor under more strain even though it spins the same fan.
Depends on the particular fan and ducting. Generally, using a restricted tailpipe reduces thrust and fan loading. Good inlet ducting increases static thrust and fan loading. Poor ducting may do the opposite.

But just saying 'increases' or 'reduces' is not very useful without knowing how much it changes. Here are some real-world numbers (Haoye 7 blade 64mm fan, 3400Kv outrunner, 2S Lipo):-

Bare fan (no ducting): 134W, 22650rpm, 267g
With tailpipe 95% fsa: 132W, 22753rpm, 250g
Inlet ring and tailpipe: 151W, 22130rpm, 390g
In GWS ME262 nacelle: 153W, 22105rpm, 329g

Note that these are all static tests. Though using an inlet ring boosts static thrust significantly, it has little effect on in-flight thrust. A restricted tailpipe increases exhaust velocity, increasing thrust at high airspeed.

If the fan is for a slow model (eg. glider, airliner) then use a fat inlet ring and a short tailpipe opened up to 100% fsa (fan swept area). For a fast jet you can have long ducting, but keep the mouth thin (for lower drag), maintain inlet ducting at 100% fsa throughout its length, and restrict the outlet to 85% fsa.
Apr 17, 2014, 05:05 PM
Registered User
LMF5000
If you are asking why a fan should be more efficient in a duct then when running in free air the answer is due to the duct wall increasing the effective aspect ratio of each fan blade.

Any propeller blade (or wing for that matter) has tip losses resulting from the higher pressure air under it 'spilling out' from the tip. Making the blade longer in relation to its width means the tip losses become a smaller proportion of the total lift generated.
In an EDF the blades are relatively short and wide so not very efficient as a propeller but by encasing the fan in a close fitting duct there is little air spillage over the blade tip increasing its efficiency but this has to be set against the losses from the air travelling over the duct.
I hope this helps
Apr 17, 2014, 05:31 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Quote:
Originally Posted by LMF5000
Thanks for the replies so far. Interesting that you say it will under-rev. I take it that implies that removing the duct puts the motor under more strain even though it spins the same fan. I'll take your word for it, but I'd really appreciate if you could explain why that happens?
What does rpm when you restrict your vacuumcleaner?

Vriendelijke groeten Ron
Apr 17, 2014, 09:40 PM
I am a nice guy! Really!
I have never put a tach on the vacuum cleaner. I believe when you plug the airflow on a vacuum motor the speed goes up. At least that is what it sounds like when I am sucking up water with the shop-vac and it fills up and the float plugs the intake. But that is not the same as taking the duct off of the fan.

In my paper mill experience, when the vacuum on a vacuum pump is increased, the load on the drive motor goes down, which seems counter intuitive, but what is happening is the airflow is restricted and the pump is moving less air, therefore it is doing less work, and the load goes down.
Apr 18, 2014, 03:38 PM
67,000rpm
Thanks for the reply Quorn. I think I understand now what happens (been 2 years since my original post) - by removing the duct you allow tip vortices to form, which increases the drag on the blades, thereby resulting in lower RPM. (Or at least this is the theory that makes sense).


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