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Thread: Minimum Horsepower

  1. #1
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    Minimum Horsepower

    How little horse power could you have in order to get an Ultralight in the air?
    Let me give you some basic numbers.
    Lets say, 100 pounds, with a wing surface area of 125 feet.
    Would something like 18hp be enough, or would you need more?

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    Registered User Autodidact's Avatar
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    Re: Minimum Horsepower

    You'd be hard pressed to design a UL with a 100 lb empty weight. Supposing it was possible (I don't think so), then gross weight might be 350 lb, that's about 20lb/hp. There are ultralights that weigh 220 lb empty (about 500 lb gross) and fly on 15-16 hp, which is about 33lb/hp. Look at the specs of different designs and devide their gross weight by their power and that will give you some statistical info to go by.
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    Re: Minimum Horsepower

    As Autodiadact said, a 100# empty weight isn't too realistic, unless it's a powered paraglider (PPG) or powered hang gliders (PHG). The 12HP Radne engine is often used for PHG and PPG, though PPGs more commonly use 15-28HP engines. The lightest "soaring trikes" (essentially a PHG with wheels) typically use 25-30HP.

    OTOH, the early Lazairs and Kolb Flyers used twin 9HP Chrysler engines, which is your 18HP, with about a 200# empty weight. Most were upgraded to larger engines, usually around 15HPx2, for performance and reliability reasons.

    Remember, more power means more climb rate, which is a significant safety factor.

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    Re: Minimum Horsepower

    I wasnt exactly thinking when I wrote this, so yes, 220 is much more reasonable. So Dana, then you wouldnt recommend an 18hp engine on an ultralight?

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    Re: Minimum Horsepower

    herb beaujon claims:
    16 hp on the bjenduro,
    15.5 hp on bebe,
    8 hp on flybike.

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    Registered User HumanPoweredDesigner's Avatar
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    Re: Minimum Horsepower

    No matter how light you make the ultralight, you still have the weight of the pilot, and need to divide that by power to get a climb rate, though climb angle is probably more important.

    The reason 100 pounds can be unrealistic is the engine itself weights a lot. I forget the numbers, but muffler and all you probably are looking at 2 pounds per hp, or maybe 3 pounds for a 4 cycle.

    As for the airframe, you can get pretty light if you use enough wires. That is the approach I plan to take. With compound struts you might even not need a king post. When I read the first reply that it is not realistic, my reaction was "what about a powered hang glider?". But even hang gliders can be heavy. I think the typical weight is 60 pounds. I bet I could make one a lot lighter than that without sacrificing strength or economy.

    To answer the hp question, there just are a lot of factors, and they can't be solved qualitatively. You have to get specific, know some numbers, and do some math in order to optimize it for what you want. We could tell you 8 hp is enough, but it might not work on your draggy airframe.

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    Moderator Dana's Avatar
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    Re: Minimum Horsepower

    Quote Originally Posted by alexkmmll View Post
    I wasnt exactly thinking when I wrote this, so yes, 220 is much more reasonable. So Dana, then you wouldnt recommend an 18hp engine on an ultralight?
    It depends on what you want. 18HP will fly a very light, slow ultralight, but that same light slow ultralight will be very limited in the winds you can fly in, and will also be somewhat delicate. For an ultralight to be more generally useful or versatile you'd more power.

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    Re: Minimum Horsepower

    Well, let's see.

    150 lbs aircraft, 150 lbs pilot. 100 sqft is enough with a normal profile.

    Assuming 1:12, we need just over 2 HP for horizontal flight. 5-8HP is probably enough for careful flight (you need to climb too). A typical 2-stroke gets about 5 HP/kg (2.2 HP/lbs), so we're talking 5 lbs including prop.

    The Diana II glider has 100 sqft and a wing weight of just under 200 lbs, though it can lift over 1000 lbs @ 160 kts. So I don't think it's unrealistic to have that same wing area for a weight of 100 lbs would it? Gravity control (displacement control?) is probably required, but flight below 10 HP is easily achieved.

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    Re: Minimum Horsepower

    There was a british made glider that had 8 hp engine on it. It had to be slinged into the air with a rubber. So apparently less than 10 hp is enough to stay airborne.

    http://en.wikipedia.org/wiki/Lympne_...ircraft_trials

    Also a 8,5 hp Harley Davidson powered plane took to the skies ( Karhu 1 = Bear 1 ); http://fi.wikipedia.org/wiki/Karhu_1
    Last edited by topspeed100; December 13th, 2010 at 06:32 AM.

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    Re: Minimum Horsepower

    A totally new angle into electric flying could solve the problem of intelligent humanpower input during a flight.

    I had a vision about 5 years ago about passengers actually redalling in their specially designed seats during a flight. A purpose built electrical powered passenger plane could use the human powered energy stored to the recharged batteries inflight thus making the flight more economical...plus loosin 2000 calories could make any customer happy too ?

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    Re: Minimum Horsepower

    The lowest I am aware of was the Minibat which could climb slowly with a 3.5 HP engine (at about 300 lbs gross). It didn't have enough thrust to roll for takeoff. You could get it airborne with a tow or with a big bungee catapult. However, the Minibat was a glider so its airframe was very clean. A structure with lots of struts and wires would need more power to overcome the drag.

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    Re: Minimum Horsepower


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    Re: Minimum Horsepower

    I think the desired speed pretty much define the hp output/need.

    DA-11 goes 155 mph at 18 hp Briggs & Stratton

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    Re: Minimum Horsepower

    Quote Originally Posted by topspeed100 View Post
    I think the desired speed pretty much define the hp output/need.

    DA-11 goes 155 mph at 18 hp Briggs & Stratton
    I think 155 was the redline speed, which is typically set by structural or flutter limits. The cruise speed was 125. If I recall correctly, the original Quickie got similar performance on a similarly rated engine. I think the Cri Cri may also fit into this category.

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    Re: Minimum Horsepower

    Right...albeit Cri Cri needs lot more power to obtain that speed.

    Colomban Cri-cri - Wikipedia, the free encyclopedia

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