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Date: Sun, 17 Sep 2006 17:11:32 +0200
From: Donald Willard Garrett <nospam@avamail.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] ducted fan questions
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Russel,

I'm fascinated by the potential benefits of ducted fans, after all, 
isn't a high-bypass turbofan essentially a turboprop in a tube? So, does 
it really make much difference WHAT you turn it with? As it turns out, 
that's not the right question. All the GenAv duct attempts I could dig 
up wound up ditching the duct and going back to a prop, some after 
significant time and expense.

Inlet geometry is apparently far more critical than what goes inside it. 
There isn't much prior duct art for the speeds we fly (big bucks in 
turbine ducts); and the significant benefits only occur in the narrow 
speed range the duct is optimized for.

Turbines get around this by pumping enormous hp through a duct that is a 
dog at low speeds, popping up to vBestDuct as quickly as possible. You 
and I could hoist a 1700 hp turbine with one hand each, while an 
equivalent GenAv product approaches the max gross of your entire aircraft.

We can't throw hp at it, so the show stopper looks like dreamy cruise 
numbers with triple the takeoff roll, vs leaping into the air with 
cruise numbers like you forgot your speed brake out, or compromising by 
getting worse performance than a prop in both regimes.

...the bright idea of a variable geometry duct lurks somewhere on the 
other side of an engineering investment exceeding what went into the 
rest of the whole airplane...which isn't to say I've given up on the idea!

Donald Garrett     flyrotaryNOspam@avamail.net


Russell Duffy wrote:
> Greetings,
>  
> I'm still thinking of using the single rotor engine for a gyrocopter, 
> and started considering the possibility of using a ducted fan to 
> eliminate a couple problems.
>  
> Problem one is the lack of a proven redrive for the single rotor, and 
> problem two is height of most gyros.  Since the rotor has to clear the 
> pusher prop, the mast ends up being really tall, and the prop is 
> usually shortened as much as possible.   This means you end up with an 
> 8+ ft high gyro that won't fit in the garage, and a short 60" prop 
> which is far less than ideal for thrust. 
>  
> I know nothing about ducted fans, so I'm looking for a bit of a 
> reality check.  If I want to run direct drive blades at 7500 rpm, it 
> looks like I need to keep them around 32" in diameter to stay around 
> .9 mach.  This seems really short, but short is great as long as it 
> works.  
>  
> Can anyone venture a guess as to how long the fan unit would have to 
> be?  Would it end up being as heavy as a redrive, or is there weight 
> to be saved.
>  
> Thanks for any thoughts.  It may just be a wild and crazy idea, but I 
> had to ask.  
>  
> Cheers,
> Rusty (out of touch till evening)