# Can an ultralight aircraft fly with a 18hp engine?

I have some questions - I'm planning on building single seat UL aircraft with this plan sketch (I have more), I drew this based on real plain and some research. Empty weight is between 50-70kg with engine

The power plant I will use this engine. 4 stroke, OHV, single cylinder, 440cc, power 18hp/3600 RPM, torque 3.0 kgm/2500 RPM (21.7 lb ft), ignition TCI, gross weight 37 Kg.

My questions:

• Is this engine enough to make it fly?
• Do I need belt reduction or gear reduction for a 1 meter propeller?
• What propeller size to fit with this engine?
• Do you see any problems with this design?

https://en.wikipedia.org/wiki/AmEagle_American_Eaglet

I found this on Wikipedia, it use a McCulloch MC101 15HP engine with 9000 RPM, that's make me doubt to use SFE 460 because the RPM far bellow. I can't find McCulloch MC101 here

• say plane and engine is 100kg and you are 100kg, that's around 2000N in total, say you have a lift/drag ratio of 10 then that's 200N of drag. WIth 13KW you are looking at max vorticity of 65 meter/second or 230km/h and that's pretty fast. So the direct answer to your question is yes if you do everything right. Commented Oct 28, 2017 at 17:19
• Everything right being you limit the total weight of the system to 200kg, you included, and make the wing to have lift to drag ratio above 10 and has enough lift to cruise around 100km/h, and efficiency of prop and transmission is above 50%. But the other question is, how do you take off? You need extra power to accelerate to take off speed within finite length of the run way, and you need more lift than gravity to accelerate vertically from the ground. That could more than double the requirement for power unless you have assisted launch or just use air drop. Commented Oct 28, 2017 at 17:30
• The Rutan Quickie would cruise at 185 kph with 18hp! Commented Oct 30, 2017 at 16:56
• The fuselage is too short. Make sure that the horizontal tail has more lever arm! Commented Oct 30, 2017 at 20:35
• Keep in mind that aviation engines, while expensive, have some important differences like safety compared with normal gasoline engines as explained here: aviation.stackexchange.com/questions/11527/… Commented Oct 30, 2017 at 21:39

Should very well be possible. The Wright Flyer had an empty weight of 274 kg, and an engine that developed 12 hp, so you're way ahead there.

As for the propeller, you want the tips not to exceed critical Mach number because of the increase in drag for no gain in thrust, so tip speed needs to stay below 340 m/s. At max torque RPM of 2,500, your engine is doing 262 rad/sec. The drawing shows a prop with diameter 1.2m, resulting in a tip speed of 262 * 0.6 = 157.2 m/s or Mach 0.46. A bit low, longer blades would be better. No need to gear down.

It may be a bit impractical to use a propeller longer than 1.2m, best to use the largest standard diameter propeller that can be accommodated. Of course, the engine will need to be able to reach 2,500 rpm, where its maximum torque is developed. Required torque and thrust go up with RPM$^2$, torque goes up with $R^3$, thrust only with $R^2$. Large propellers convert the horsepower into maximum thrust better, and this is the area where the Wright brothers made optimal use of the available horsepower, driving two 2m, slow turning propellers.

An "educated" guess at torque & thrust here. I would say you'd need the longest standard size propeller that is your craft can accommodate without hitting the ground during TO/landing, with the highest pitch available. If 1.2m is max diameter you may want to use a 4-bladed one.

• have you ever built an ultralight aircraft? Commented Nov 4, 2017 at 13:01
• No mate I have not, and have no interest in doing so. Commented Nov 4, 2017 at 15:46
• Did I read your post wrongly? I found in wikipedia regarding the AmEagle, the power plant is 50HP (37kW). Powerplant: 1 × Continental A50 air-cooled, horizontally opposed, four-cylinder piston engine, 50 hp (37 kW) Is this different aircraft? Commented Jan 9, 2019 at 11:57
• @AirCraftLover Please check the Wikipedia article for the Wright Flyer. Commented May 6, 2019 at 6:49

It's possible to arrive at a rough estimate by assuming that the L/D ratio is 9. Now, the empty mass is 70 kg. Let's add 45 kg for the engine, transmission and propeller. And we must not forget the mass of the pilot (85 kg) and 20 kg of fuel. It's a total mass of 220 kg. Let's round it to 250 kg in order to be on the safe side.

If we imagine the plane gliding at L/D = 9 and 25 m/s, the sink speed will be 25/9 = 2,78 m/s. As the mass of the plane is 250 kg, its weight is 2450 N. Thus, the 'gravitational power' involved in the glide is 2,78 x 2450 = 6805 W.

The plane, for straight and level flight, will need exactly that power. If we assume that the transmission and propeller losses are 50%, you'd need twice that power, that is, 13,6 kW. Besides, some extra power is needed, (in order to climb, for example...) so let's increase the figure by 30%. 13,6 x 1,3 = 17,68 kW = 24 hp. That's the engine power that, in theory, and according to this simplified estimation, would be needed for that plane...

• Op indicated 70kg with engine. Taking back the 45kg for the engine its 155. Rounding to 200kg, using your method I get 14.2 hp. Commented Oct 28, 2017 at 18:45
• with a 18hp engine definitely YES, specially designed planes flew with human power only. With that specifical 18hp engine probably yes but extensive adaptation may be needed and reliability of the thing may end up being sub-optimal. Commented Nov 2, 2017 at 8:20
• very clear and simple analysis, thanks for this! Commented Feb 17, 2020 at 7:41

Short and simple answer to your question is, YES, there are numerous ultralight AIRCRAFT that are flying with 18hp rated engines (and less - hp) and have been for years.

I understand from your questions and comments that you are completely new to this aspect of flying, and it is understandable that you will have questions; you would be foolish not to (have questions).

At the same time do you really want to fly or just kick tires and dream about it ?

Most of the people getting into this FACET of aviation will research the various offerings available and most will purchase maybe two or three plan sets to better understand what comprises a viable aircraft; I own ten PLANS for the following - Proven Aircraft: Bower's Headwind, Evans Vp-1 Volksplane, Sorrell SNS-2 Guppy, the Sky Pup or Mach None, and the Beaujon ENDURO, as well as the Chotia Woodhopper; also have plans for the Mitchell Wing, Easy Riser and Quicksilver (weightshift) gliders that were both built and flown, as gliders only. Oh yeah almost forgot my Bensen Gyro-Gider, a total blast !

I settled on building the SNS-2 Guppy with a 1972 750cc BMW engine since it is enclosed and I am too old to enjoy freezing my butt off in an open frame type ultralight. THE BMW engine was chosen because it is more than reliable and I own two of them, A No Brainer, plus I don't have to cut down a VW two get half as much HP as the BMW produces.

At this point in time I hope you have come to understand that the purpose of an ultralight is SIMPLY to get into the air, get out of the pattern and fully enjoy FLYING in a machine that truly permits you to be a part of the experience, same Reason people ride motorcycles instead of driving a car - to experience your surroundings to the fullest.

Forget all the mathmatic mumble-jumbo such as as nose/tail moment ratios and WING loading, etc., etc AND find out which ones have a GOOD support system still available.

Then and only then pick the one you believe will suit your interest, and build it - to the plans, If you want to fly it more than ONCE !

Best of Luck. LW

• I tend to build the aircraft and of course after it finish and tested, the flying experience is the reward of building it, but thanks for your response. i have bought a plan for Affordaplane and now i'm looking for suitable engine for it. I see one that might ok for it, but still not sure until i get the engine and test with the propeller. It's 2 stroke 150cc, 28HP. I know Affordaplane uses 35-40HP engine, but they have tested using rotax 277 28HP engine, i think i can use an engine that similar to rotax 277 power since my weight is pretty light and also try to reduce some other weight Commented Feb 23, 2019 at 20:35

Your drawing doesn't provide enough detail to give a definitive answer. The design looks similar to something like an Legal Eagle. Those generally use a half VW engine putting out about 40hp so I don't think that motor would be up to the task without building it up some. If you are married to that design the good news is there is a whole universe of parts and detailed instructions for these types of motors thanks to the popularity of gokart racing.

The wisest way to go would be to download plans for an ultralight which has already been extensively flown and had the bugs worked out. Or if you're dead set on doing your own thing download a few sets of plans for different gliders, then figure how to add a motor. I know the Goat glider has free plans and people have successfully added a motor to it.

I'm building a modified Bloop 4 ultralight right now. It's designed for an 25hp paramotor engine, so engine wise if you remove the governor from yours you should be there. You can check it out here: http://m-sandlin.info/bloop/bloop.htm

The same dude designed the Goat so you can grab those plans there too if you like?

I've found these pages helpful as I've built my own ultralight. The first one is for RC's but works equally well for paramotor/ultralights. Static Thrust Calc http://www.godolloairport.hu/calc/strc_eng/index.htm

If you do remove the governor from your motor you can play with different props/reduction drives here: Culver Propellers http://www.culverprops.com/pitchselection.htm

• i already change plan to another engine, 2 stroke and 28 HP. My problem with 2 stroke is just too much eating fuel, but yes the engine are pretty light and powerful Commented Mar 25, 2018 at 9:56

I would get away from Wright Brothers comparisons and listen to Peter. Lengthening the fuse will give you a much smoother feel with the elevator and more pitch and rudder control. Reminds me of the time when Richthofen recommended to same thing for the new Fokker D.VII.

As an added bonus, it will help keep your CG in the right place when you add a bigger engine. Although torque from overpowering would be dangerous, 18 hp may be a bit anemic, particularly into the wind. At lower power you have to go with lower wing loading. The wind will blow you around like a leaf. Also, lower flying speeds leave much less of a safety margin in the event of a gust or wind shear. More power can save you in these situations.

Research of other, proven aircraft of this plan form may show 30 - 50 hp may be more realistic. Good luck with your work.

• Thank you for the response. I've been thinking of another engine. It's two stroke, single cylinder 150cc. The company claim in produce 28HP, but many people "dyno test" the engine, they got 30+ HP. The problem is that motorcycle engine and water cooled, i'm still looking for more suitable engine, because this type of hobby is not really popular here nor even exist yet. The proper engine for aviation is out of my budget, something like snowmobile engine is what i looking for. The airplane design is not really a problem. I have some interest with Affordaplane and Legal Eagle. Commented Dec 25, 2018 at 20:35
• @Gifhary Syidhqa Hamim while gathering funds for your project, you may wish to continue to study plan forms. Unless you are into aerobatics, lengthening to fuselage will make it much easier to control. A matter of preference I guess. As far as engines, this is the heart of any powered aircraft. The one you pick needs to handle the prop load without overheating. The first thing you buy may be the prop. Then bench test your engine candidates and see what rpm you can get, and if possible, how much thrust. Then you can decide if this is the best engine for your plane. Commented Dec 25, 2018 at 23:27

Yes, a True Part 103 Ultralight and even some Small Kitplnaes can fly on 18hp. That's 18hp@3600rpm. That Engine uses a very Small 22mm/24mm Carb and a Restrictive Air Filter and Restrictive Exhaust with a Low 8.0-8.3cr. Put just a 32mm Carb Mukuni Flat Side Carb, with K&N Air Filter, and a Tuned Header Exhaust, and it will probably make around 25hp@3600rpm. If you add a Hi Rev Kit, disable Governor(Always for Plane use), disable Low Oil Sensor(Always for Plane use), and add a better CAM, it can make 35+hp@5000rpm. I would always use a Reduction Belt Drive.

• Your answer looks OK but why all the strange capitalisation of words that shouldn't be (unless you were writing in German)? Commented Dec 24, 2018 at 21:28
• That capitalisation doesn't even work in German. They rather combine the words ... like Flatsidecarb :) Commented Jan 7, 2019 at 14:42

Your question is "Can an ultralight aircraft fly with a 18hp engine?"

The answer is yes an ultralight can fly with that engine and you will not need a reduction drive with the correct sized prop designed to keep tip speed subsonic.

Check out the beaujon ultralights, the bloop motorfloater by Mike sandlin and the rutan quickie. Three different planforms and they all fly on hp in that range.

The beaujon enduro seems closest to yours and used a 16 hp engine and was heavier. Its Briggs and Stratton engine ran in the mid 3000s rpm range max output and lower than that at cruise. It appears to have the same power output profile as yours.

There's a wiki page for the enduro and the beaujon site sells plans and a manual on how to design your own ultralight.

Whether YOUR ultralight will fly who knows. Come back and post updates!

MTOW Weight in KG/10kg x 1kw = hp needed.

Example: MTOW 600 lbs = 272.2 kg / 10kg = 27.22 x 1kw = 27.2 kw = 36.5 hp.

So Plane Weight + Your Weight + Fuel Weight + any Bags weight = MTOW.

• Hi and welcome to the site. This rule of thumb you refer to would benefit from some sources or explanations regarding its reliability. As it stands it is clear that some aircraft break it, for example the Wright Flyer described in another answer. Commented Oct 18, 2019 at 11:08

I'd can say you can make it to fly, just attach an adjustable propeller in order to achieve the exact torque for a lift and constant speed.