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? (;

David Hoskins9 months ago

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.

silver542 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.
Lawnmower engine's power to weight ratio is all wrong, Q: how would a lawnmower engine perform at altitude?
rickharris3 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)  rickharris3 years ago
I dont know if it is illegal here in the us, but i think it isnt.
CameronSS3 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)  CameronSS3 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.