can i fly a very light aircraft with a small lawn mower engine?

I am trying to build an enclosed ultralight aircraft similar to the Cri-Cri made by michael colomban. I think i can make it around 100 or 150 pounds. I have an engine that is somewhere between five and ten horse power. i could possibly get another one of relatively the same amount of power. i have read about the DA-11 made by leeon davis that ran on an 18 hp engine, but is there a way to calculate weight vs. hp so i can be sure it will fly? (;

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silver544 years ago
Yes you can. The Gossamer Albatross flew on the power of a semi-professional bicyclist; approximately 1/5th of a horse power over an extended period of time. It was built by aeronautical engineers using exotic materials. So obviously it can not be that difficult. ( The last sentence was a joke ). To be serious there have been seemingly legitimate claims of sub 12 hp ultralight aircraft. Google -- Faa Far part 103 -- for the legalities of what is an Ultralight Aircraft in The U.S. If you google 8 hp ultralight aircraft you should find an Australian airplane that was capable of flying with a person in it when the weather was cool and dry; but evidently not when it was warm out. Another ultralight The Sky Pup was designed to fly on approximately 20hp; I have read a blog that it was flown on an 8hp briggs near Grand Junction Colorado -- elevation around 4500 feet. It was designed by a Cessna engineer. The Laizair is another ultraligt that had two engines but was said to be capable of level flight on one 8.5hp engine. It was designed by an aeronautical engineering student. Calculating weight vs horse power. 1 hp is 33000 foot pounds per minute. Do a little 4th grade math that works out to 1 pound of thrust at 375 miles per hour or 10 lbs thrust at 37.5 mph If you have a 400lb plane with a lift to drag ( L/D ) of say 9 to 1 at 37.5 mph divide 400 by 9 and you get 44.44 lbs thrust at 37.5 mph thats 4.44 hp to stay level. Assuming a propeller efficieny of 70 percent divide 4.44 by .7 you get 6.34 hp. shaft horse power to stay level. Remember this will not allow you to climb or turn both of which take additional power. Also remember that small engine manufactures generally exagerate their hp claims by 20 to 25 percent. Also remember to subtract 3 percent for every 1000 ft of elevation from your available sea level power. Also get a Kotch chart and figure you density altitude and correct for it. The above example would probably require a wing loading of less than 3lbs/sq. ft. And an aspect ratio of 8 to 10. If you are unfamiliar with wing loading, aspect ratio, L/D, mean affective cord, quarter cord, tail volume calculations, or Kotch charts: DO NOT ATTEMPT what you want to do. Try reading Daniel Raymers excellent book Simplfied Aircraft Design For Homebuilders, and a book by Jack Lambie called Composite Construction For Homebuilt Aircraft, also Ultralight Propulsion by Glenn Brinks, also Ultralight Airmanship by Jack Lambie.
Good Luck.

I have to say that yours is probably one of the best replies I've seen to a general aviation question, not just this one. You handled it simply and yet you also gave good examples and resources. So I'm saying a very sincerely well done sir very well done indeed


lr10cent2 years ago

BTW, I think silver54's comments are in the ballpark.

lr10cent2 years ago

There are ways to figure it out. They're somewhat complicated, but I think . If you don't have an engineering background, or want to do a WHOLE lot of studying (like years, but not necessarily a bad thing), I suggest you use an existing design. There are actually quite a few, but you probably need more power, depending on how heavy you are. Unless you're an experienced, disciplined pilot and a patient, careful builder, I recommend you avoid the Cri Cri! A better choice might be the Sky Pup. However, it may still be a bigger project than you want to take on. I have the plans, and, although I think it's simpler than most airplanes, there are a lot of fiddly little bits in there. People do hop up engines for kart racing and tractor pulls, so you might be able to use the sort of engine you have in mind, in a bigger size, but you'll probably have to do things to lighten it up. Or you can just pour money on the problem, as there is a 4 stroke, I forget the name, made for those powered parachute things. 4 strokes are generally considered to be more reliable than two strokes, although it's probably much easier to find a two stroke that will do the job.

There is an ultralight I know of that uses two lawn more engines called the "Home Depot Plane":

I don't know how good it is.

A lot of old homebuilt airplanes were designed for low power. The Pietenpol Sky Scout was meant for a model T engine, so it ought to do much better with a lighter engine that makes 20hp, or perhaps two of them, if you don't mind an ugly airplane. Like this:

http://tinyurl.com/n65569x (Lacey M-10 with twin VW)

For the Sky Scout, you'd probably have to bring the FAA into your life, but for the others, you could meet the ultralight requirements.

Another possible design that could possibly be built to the ultralight rules is the Heath Parasol. The Piet and the Heath are very old designs, which you can find out more about in the "Flying and Glider Manuals" reprinted by the EAA. And there are lots of more recent designs which might work for this, but I don't think you'll find anything really viable with less than 18 hp or so, unless you are small and light.

Anyway, if you're prepared to be really conscientious and put a lot of work into it, you can make this work, but I don't know of any designs you can purchase which will be as fast as the DA-11.

P.S. If you live on the salt flats you might be able to fly with just the engine you have, but you won't climb very fast and you will have to fly on light wind days only, cold ones if you want to get up high. Elsewhere, you might find yourself in the trees!

Without doing any research I'd say that a chain saw, or professional grade weed wacker motor(s) would stand a much better chance of being viable.

I'm considering building something similar seeing as my girlfriend moved about a 300 mi drive (140 mi or so as the crow flies) away.

If I ever get to the research phase of this I'll share my findings.

Lawnmower engine's power to weight ratio is all wrong, Q: how would a lawnmower engine perform at altitude?
rickharris6 years ago
I believe that running an aircraft on a standard unmodified engine would be illegal (at least here in the UK)

as a model it would be too heavy.

As a microlight not meet the FAA requirements for safety.

Building a reasonably large flyable aircraft weighing 150 - pounds is going to cost something in exotic materials - Wouldn't it be cheaper to buy an existing microlight.
Ultralights (as they're termed in the US) are almost completely unregulated in the US. Basically as long as it meets the definition of an ultralight (defined by weight, fuel capacity, and speed, mostly) and it's only operated in accordance with the rules defined for that definition, it's open season. Nothing has to be certified or registered, and the pilots don't even need any training. "Need" meaning a legal need, they still need training if they aren't going to kill themselves or someone else...

And you're quite right, the only way this is going to meet the 150lb weight requirement without extensive engineering, materials, and fabrication investment is if he bolts a pair of chainsaw engines to a hang glider (which is possible).
Things are much more regulated in the UK :-(
Yep. Ultralights have been left largely self-regulated here... the main thing preventing more accidents than there are is that the ultralight community knows that if they DO start to appear unsafe, they WILL be regulated. It's great for people who want a cheap way to get in the air, though it's terrifying for people like me ("real" pilots) who then get to share the skies with little flappy things that may or may not be controlled by someone who knows what they're doing.
elliotstewfus (author)  rickharris6 years ago
I dont know if it is illegal here in the us, but i think it isnt.
CameronSS6 years ago
The Cri-Cri runs off two 15hp engines, which appears to be about the bare minumum for any kind of performance. The Santos-Dumont Demoiselle from the turn of the century used a 35hp engine, so the minimums haven't changed much.

A small lawn mower engine will likely be far too heavy for the small amount of power it puts out to be very helpful, unless you start machining off excess bulk.

is there a way to calculate weight vs. hp so i can be sure it will fly? There is, but the equation also includes propeller properties, airplane shape (aerodynamics), weight, speed, etc. An estimate of "this much engine will power this much airplane" can best be obtained by looking at similar airplanes. Look at ultralights and motorgliders.

There isn't much of a kitplane presence here on Instructables, you'll probably get better information some place like the EAA Forums.
elliotstewfus (author)  CameronSS6 years ago
Well have you seen the da-11? It weighed about 175 pounds and had an 18 horse power engine. So i wonder if i could get a couple eight or nine horse power engines and power a plane that is somewhere around 100 pounds.
If you want to build a DA-11, get the plans and build a DA-11. You either need a lot more education or a lot more life insurance if you're going to design something of that caliber.