>> Has anybody ever come across a paper calculating the (bio-) Isp or total
>> impulse of insect or a bird wing propulsion systems? The Isp of an
>> aircraft engine is indeed huge but I forgot the numbers.

> The power to weight needed for bird flight is generally over ~60W/kg. If
> you assume that the fuel for this is fat, you get an 'apparent ISP' of

About now the Godwits leave NZ and fly to Siberia - often non stop and 
direct.
Have a look at a map and be utterly impressed.

They leave NZ "clinically obese" and arrive 5 or 6 days later, much much 
thinner.
Anything that can support itself aerodynamically for 5 days while flying 1/3 
of the way around the world with a mass ratio of about 2 will have an 
awesomely impressive Isp :-)

I rather doubt that Godwits need 60 W / kg to do this.
Lets see -
A typical hydrocarbon will give around 10 kWh/kg
So half a kg will give 10,000 Wh/kg x 50% / 60 w = about 80 hours or about 3 
days for 50% weight loss.
Hmm. Given imprecision in actual energy available from the fuel and other 
facytors that original figure appears not too bad. Probably more like 20 or 
30 W / kg for this admittedly rather niche performer. Or Godwit fat is 
especially highly hydrogenated and has suitably higher energy per kg :-).




        Russell


>> Has anybody ever come across a paper calculating the (bio-) Isp or total
>> impulse of insect or a bird wing propulsion systems? The Isp of an
>> aircraft engine is indeed huge but I forgot the numbers.

> The power to weight needed for bird flight is generally over ~60W/kg. If
> you assume that the fuel for this is fat, you get an 'apparent ISP' of
> over half a million seconds. Since these birds don't support themselves
> through direct thrust you might want to divide that by the L/D of the
> bird which would reduce that value to maybe less than a tenth. Small
> insects cheat a bit by exploiting viscosity but they do usually support
> themselves more nearly directly by thrust. Their 'ISP' is probably in
> the hundred thousand seconds range.
>
> Nick
>
>>
>> This question relates to a paper at present in New Scientist claiming
>> nanomotor arrays on chips to have larger Isp than even ion engines.
>>
>>
>>
>> jd
> _______________________________________________
> aRocket@exrocketry.net
> http://exrocketry.net/mailman/listinfo/arocket 

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