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Thread: Engine Power to aircraft weight ratio?

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    Question Engine Power to aircraft weight ratio?

    I am a beginner and want to built an aircraft all of my own, i.e. from the scrap.

    My aim is to built a single-seater, fly-baby style aircraft, the overall empty weight (including assumed engine weight of 100 lbs)should not exceed 500 lbs.

    Now, the question is, to select an ENGINE. Is there any rule of thumb by which I can determine the engine power needed to fly a design of this weight?
    Assuming my design is optimized to use 70-80% of engine power as compared to what some 'good' designs do.

    Pls. assume other parameters to be similar to that of an ultralight.
    Eg. cruise speed - 30mph-40 or above

    Thanks in advance...
    Anshul

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    Moderator addaon's Avatar
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    Re: Engine Power to aircraft weight ratio?

    Check out Raymer's homebuilder book. It's cheap, straightforward, and will answer this question plus many, many others you haven't thought to ask yet.

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    Re: Engine Power to aircraft weight ratio?

    Check similar designs. The power to weight ratio needed depends mostly on the span loading and actual wingspan. For example, a normal airplane might be 14 lbs per HP. But a motorglider could be 30 lbs per HP. A human powered airplane is 250 lbs per HP.

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    Re: Engine Power to aircraft weight ratio?

    Thanks a lot Addaon and BBerson for the guidance..

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    Re: Engine Power to aircraft weight ratio?

    Hi,
    If you're serious about designing your own airplane - heed addaon's advice, andf invest in Raymer's Homebuilder's book. I did, and it was the start of a very rewarding design journey. I certainly couldn't have got this far without it.

    Duncan
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    http://rtfmaero.wordpress.com/

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    Re: Engine Power to aircraft weight ratio?

    hey... just a point came to my mind...
    I know i am missing something here .. but yet to discuss...


    Intuitively speaking... if the engine power required depends only on the design (e.g. wing span) ... in that case we should be able to fly any weight in sky with a given engine power available, just by modifying our design and wing span. But this seems to be unlikely.

    Pls. excuse me for this absurd-looking question ... but i am trying to get bigger picture of aircraft basics.
    thanks ..Anshul

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    Re: Engine Power to aircraft weight ratio?

    thanks rtfm ... i am going to order the same.

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    Re: Engine Power to aircraft weight ratio?

    Quote Originally Posted by flypassion View Post
    hey... just a point came to my mind...
    I know i am missing something here .. but yet to discuss...


    Intuitively speaking... if the engine power required depends only on the design (e.g. wing span) ... in that case we should be able to fly any weight in sky with a given engine power available, just by modifying our design and wing span. But this seems to be unlikely.

    Pls. excuse me for this absurd-looking question ... but i am trying to get bigger picture of aircraft basics.
    thanks ..Anshul
    The engine produces thrust which must equal drag at a steady speed, which can be calculated. On the ground you have rolling resistance to be overcome.
    The answers are in the books. You'll get it.
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    Re: Engine Power to aircraft weight ratio?

    Quote Originally Posted by flypassion View Post
    Intuitively speaking... if the engine power required depends only on the design (e.g. wing span) ... in that case we should be able to fly any weight in sky with a given engine power available, just by modifying our design and wing span. But this seems to be unlikely.
    It is true though. A 100 ft span, 2000 lbs glider can maintain horizontal flight on less than 3 hp...

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    Re: Engine Power to aircraft weight ratio?

    Quote Originally Posted by autoreply View Post
    It is true though. A 100 ft span, 2000 lbs glider can maintain horizontal flight on less than 3 hp...
    Now I'm curious, what's the lowest power to weight ratio ever recorded for sustained level flight? Surely at some point the drag will be too much and you could no longer increase wing span and area, wouldn't it?

    It sounds like an interesting record to try for.

    Here's a 3.5 hp aeroplane:
    Attached Thumbnails Attached Thumbnails Engine Power to aircraft weight ratio?-wren_eng.jpg   Engine Power to aircraft weight ratio?-wren.jpg  
    "ARE FLYING WINGS THE ANSWER?"

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    Re: Engine Power to aircraft weight ratio?

    Quote Originally Posted by Autodidact View Post
    Now I'm curious, what's the lowest power to weight ratio ever recorded for sustained level flight? Surely at some point the drag will be too much and you could no longer increase wing span and area, wouldn't it?

    It sounds like an interesting record to try for.

    Here's a 3.5 hp aeroplane:
    Since power is LDmax*speed*Mass, that's trying to get both speed and L/D max as low as possible. Mass is irrelevant since we're speaking about hp/lbs, though the lower mass (wing loading) yields lower speed and thus lower power

    The Diana glider has an empty weight (ready to fly) of 400 lbs. Including pilot and parachute that's 600 lbs. L/D max is 52, at a speed of 90 km/h or so.

    That yields a power required of 1300W, or a installed power of around 2HP. I doubt there's any realistic aircraft around that can fly on less power.

    If you're not concerned with reality, the best HPA's achieve a Pr of 400-500 Watt. That's something even Lance Armstrong can barely surpass...

    Of course that's pretty irrelevant since you still need considerable power (20 hp or so) to achieve a realistic climb value. To optimize for low power only is thus a pretty useless (and dangerous, low climb) business.

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    Re: Engine Power to aircraft weight ratio?

    Autoreply meant of course or power= speed*mass/ (CL/CD) = speed*mass*CD/CL.
    Higher L/D, less power needed and not the other way around.

    You can remove redundant variables, since
    mass = speed^2 * constants * CL
    speed = constants2 * sqrt (mass/CL)
    you get
    power is proportional to mass*CD/(CL^(3/2))

    This minimum level flight power optimizes for higher lift foils than maximizing CL/CD (which maximizes cruise range or minimizes energy per distance instead). The speed is whatever it is at that optimum.
    CL^(3/2) / CD is called the power factor and plottable for wing profiles in XFLR5...
    Norman likes this.

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    Re: Engine Power to aircraft weight ratio?

    Quote Originally Posted by Autodidact View Post
    Now I'm curious, what's the lowest power to weight ratio ever recorded for sustained level flight? Surely at some point the drag will be too much and you could no longer increase wing span and area, wouldn't it?

    It sounds like an interesting record to try for.
    I think the lowest power to weight ratios that you will find are for the human powered aircraft

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    Re: Engine Power to aircraft weight ratio?

    As far as record attempts go, I was thinking about an assisted launch craft and measuring only for sustained level flight. i.e., if what you guys are saying is true, you should be able to sustain level flight (in a big airship hangar?) of a 1000 lb aircraft with a small model airplane electric motor, like a 400!

    Of course, you can do anything with nothing as long as you take an infinite amount of time!

    On a practical note, it is interesting to think of a flyable craft powered by a one cylinder 3.5 hp industrial engine. Almost like I could afford it or something.
    "ARE FLYING WINGS THE ANSWER?"

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    Re: Engine Power to aircraft weight ratio?

    Keep in mind air moves. Thermals of 600 fpm up, with surrounding areas of 400 fpm down, are common even on calm days.

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