Ducted fan theory and practice

[Simon Chaddock]

dirk

That looks very nice.

However my calculations suggest, assuming an EDF motor diameter of 35 mm, that the total exhaust area is only 68% of the fan swept area. The general rule is that maximum static thrust is achieved with an exhaust that is 85% of the FSA.

I expect it will still work but with a slightly reduced thrust.

[dirk tinck]

Hi Simon ,what if the motor diameter was 63 mm ? That's the size used in a schubeler HST98DIA (128MM).That reduces the fsa ,right ?

[dirk tinck]

I think that gives me 80.71 % right ?

[dirk tinck]

80.71 exhaust area x100 : 97.5 fsa =82.77% right ?

[Simon Chaddock]

The Schubeler web site give the FSA of the HST98DIA as 9800 sq mm.

Your two exhausts total 8144 sq mm giving a reduction to 83 % of the FSA so it is getting close to the recommended value however bear in mind that two small nozzles are more restrictive than one of the same total area.

In addition there are the losses due to the bifurcation of the duct when compared to a single duct of the same length but there is nothing you can do that. It is just a penalty of following a scale design.

I will follow with interest.

[Allen]

hello

One of Richard’s scopes – in post 1 – was “check the performance of the model and EDF unit” (against theory and practical experience)

 

Has anyone built and flown a suitable flying test bed?

 

Maybe something like an Alphajet pod-and-boom plane, (or the Ezefan)
Can hang a variety of fans on it
Ditto re entry and exhaust ducts
And compare actual flight with theory

 

Because, as nice as the models shown are, they are complex
Multiple inlets, curved tubes, obstructions, etc

 

This approach might be better suited to an ‘academic organisation’
Altho the hobby is full of keen and dedicated amateurs…

 

[Allen]

A question for Richard (or anyone who knows):

I have read (most of) the posts on this thread
I have also read thru Richard’s pdf ‘RS_EDF_theory_and_Practice_v5’

 

But I get lost at section 3.7.4 – adjustment to KS theory
Especially para 5 – the Hawk example
And especially - “take account of dv = ve – vi  we obtain a value of vi = 7.6 m/sec

 

How is vi obtained?

 

This is the static case, so vi = 0.  More detail please…

 

 

[Allen]

‘150 watts per pound of model’:
back in 2011, Richard commented this ‘rule of thumb’ is useful, but can’t be explained

I can’t explain it either, but a pdf on (propeller) park-flyer design lists 100 W/lb as the border between ‘fun’ and ‘insane’ performance

 

EDFs are often used on ‘fast jet’ types – there’s a clue
And the thrust per Watt from a comparable set up for an edf is less than a prop

 

So, ‘add a bit of Clarkson’ – more power!  Aka add 50%, and there’s 150 W/lb

 

[Simon Chaddock]

Allen

Watt for watt an EDF is about half as efficient as a prop as far as thrust is concerned. You nave to go to speeds in the order of 500 mph before the fan wins out.

Hence 150 W/lb in an EDF will give a good jet like performance but you need closer to 200 W/lb for 'unlimited vertical'.

The size of the plane makes a difference just as it does with a prop. The bigger it is the more efficient both it and the prop or fan are.

There are so many variables at play so these figures have to be taken a 'guide' based on experience rather than a specific figure.

 

Remember not all EDFs of the same fan diameter are of equal efficiency in converting Watts into thrust (g/W) and that figure varies significantly with the level of thrust being used.

The problem is most EDFs are placed in scale type airframes that have to fly fast to carry the battery weight so need a high thrust level which tends to 'gobble up' what battery capacity there is pretty fast. 

Place an EDF in an efficient glider type airframe where it can be throttled back to its most efficient power level and the glider could fly for at least twice as long as. However use a prop instead of an EDF and the glider would be able to fly for at least twice as long again. ?

 

It all comes down to what you are trying to achieve.

[eflightray]

150 W/lb ???

O.k. so it's not fast, but at least I can see what's flying by not looking for a coloured streak.

And it's flying on less than 60 W/lb, not every jet has to be a amps eater. ?

Ray

 

 

.

[Simon Chaddock]

eflightray

I do seem to remember that it is more of an EDP rather than a EDF but I agree built light and big enough even a fully ducted propulsion system needs much reduced power to fly. 

 

 

 

 

[Allen]

thanks Simon & Ray - I don't want to start a 'numbers war'

 

we fly all manner of craft, and that's just fixed wings

  the common thread - enjoyment

  so, peace and love everyone

 

me, I'm leaning toward light, small and slow

  the NZ authorities are likely adopt the (unproven) 250g limit

  plus old age, and my Scots ancestry (?)

 

EDF's are new to me

  so, can anyone please explain Section 3.7.4 - Richard Sharman's static calcs

 

thanks