Hi Guys,

I haven't ever done anything with gassers, only nitro a long time ago and exclusively electrics recently. I have a project where I need long endurance, a couple of hours at least and electric is going to be far too expensive in batteries and weight to be able to do it.

This got me thinking that gas motors are pretty decent on fuel usage/energy density.

Does anyone know how much a 15 or 20cc motor (perhaps the RCG motors) consume per hour/minute at about 60% throttle (aka: cruise speed.) If not, how long can you fly for on a trainer style aircraft - and what is the size of your fuel tank?

Are there any down sides to having large, or multiple fuel tanks?

Thank you very much in advance!

I have a DLE 20 on my Pulse 125. I put nearly 8 hours on it last year during 58 flights; my average consumption was 20.8 mL (0.70 oz) per minute. At that rate, two hours' flight would require 2.5L (0.66 gal) of fuel. Of course, all that fuel has some weight to it - about 1.9 kg (4.2 lbs).

Of course, you might not be able to lift an airplane with all that fuel in it. Even if you could, the extra weight would affect the fuel economy.

I don't see the technical difficulty in building a plane to carry 4.2 lbs of fuel, just that it would be a physically large tank that most likely will end up in the fuselage. It will require a big wing and conservative construction. You should easily fly a plane 20 lbs (15 lb dry) on a DLE20 if you design it with a low enough wing loading. Your mileage may vary depending on how high you need to set the throttle to stay aloft of course.

Why we would want such large tanks is the question?

As for DLE 20 fuel consumption, 21 mL/min is not far from what I have seen (heard). I know that in a vq pilatus porter, loaded down to about 20 lbs with fuel - the DLE will burn about 1L/1.5h. That is at a throttle setting for cruising at a shallow climb (this is not first hand knowledge but from a friend that has this setup).

On the flip side, you can fly several hours with a 3m glider. I have a cheap bird of time that I converted to electric and it will ride thermals all day - but that is not so fun to be limited by the weather...

If this is for a UAV (you never said it was but I have a hunch.... lol), you will need some kind of IMU/autopilot. In my experience, they do not always play nice with vibrations generated from gas motors, and the tolerance to vibration depends on a ton of things (brand of autopilot and mounting mainly). The point is - if it is for a uav, and you do not have much experience with autopilots and the such - I would highly recommend learning with electric before stepping into gas... I may be totally wrong in this assumption but I thought i would throw it out there.

Chemnut - you're right, it is for a UAV. We have built about 20 over the years but need some more endurance for a special application. An electric 5060-380 outrunner (60-90 size) with 11600mah of 6s battery flies just fine and is about 2.2kg. The RCG 20CC should provide a bit more power than that from what I can see and is 1kg, so with two 800cc fuel tanks full it should end up a similar weight (I might be a bit generous with the specific gravity of fuel here).

On 11600mAh I see about an hour of mixed flying, going to petrol should get me an extra 45mins then @ 21ml/min - not as much as I hoped. If your friend is getting about 11ml/min of fuel usage that could be very good - almost 2.5hrs of flight. The airframe could easily handle another 500-750g of fuel, but it could be a bit difficult to package the tanks in - might need to design a new aircraft

ChillPhatCat, the aircraft I have in mind is only about 4.5kg all up. With a few changes we could go up several kilos without a problem. We dont need unlimited vertical, its agricultural and flying in open fields in a very flat trajectory.

I love gliders, I have quite a few. They are not very good for our purpose though.

4.5kg all up will probably hover on a DLE 20cc Going with a 15cc will probably suit your purpose better and allow you to carry less fuel.

Oh man at 4.5 kg you dont need a DLE 20. The EVO 10 gasser would probably fly it and it is really good on fuel. Then you run into another problem. The EVO 10 does not have a really strong pump in the carb, and needs exhaust pressure to keep it running well. That means that remote mounting of your tank will have a strong influence on its tuning. Also a large tank will change the tune of the engine as the fuel is consumed. You may be able to get around that by tapping into the crank and pressurizing the tank with crank pressure, then using a regulator at the carb.

As for power, I am pretty certain that the EVO10 is good for 800-820 watts of power when you prop it for around 11k rpm. If you know the power you need for your electric setup, or watch your telemetry/ logs, you will know if that will be enough.

I would look for smaller gas engines than the DLE20. A smaller engine will likely produce less vibes. Smaller engines without some kind of pump/regulator system of course have the fuel delivery issue as well.

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Thanks chemnut and ChillPhatCat. We were leaning towards the 15cc and a larger airframe - the info in this thread is leaning us further towards that. We need our systems to be very easy to use. Can you tune the mixture with an oxygen sensor or exhaust gas temp like you can on a car? If so we could do it with software rather than user input.

The 20cc consumption figures are also really nice to have as we're looking at building a sprayer with a 4m wingspan and 8m of sprayer boom + pump, chemicals, etc - obviously with a limited amount of pesticide you cant fly for very long, but it does allow for planning

Thank you again for sharing your knowledge, its much appreciated!

There are no fuel injectors on these engines, so it's not really something you can control with a computer. But, gas engines usually keep the tune very well, often you can go months without touching the needle valve. There are small mechanical pumps you can get that operate off of crank case pressure if fuel draw is a problem.

Well - there is one traditional R/C gasser that I know of that is fuel injected. Cloud Cap Technology distributes an injector system for the 3w 28i and they have it as an option in one (or several?) of the UAV's they sell. I believe the system was actually developed by Currawong Engineering, which claims to have developed fuel injection for small 2-stroke and 4-stroke gas and "heavy fuel" engines. I got really excited until I contacted them and found out that the whole system (ecu, injectors, pump, engine, etc...) would run about ohhh... $15-20k! So yeah.... no.... not on my budget...

Tuning can be a bit of an issue if you are wanting to make big changes in altitude, but flying under 1000 feet and crop dusting should not warrant a closed-loop system like what you are wanting. I have seen people fix a servo arm to the high-speed needle of 20-30cc gassers and tune by sound and temp (mostly cylinder head temp) with a knob on the transmitter. I would like to do this some day, but I just do not have a need for it right now because as Chill said, they pretty much stay more or less in tune.


Closed loop systems can be done with carbs as I understand it, but I never read far enough into it because I soon realized that I would never actually do it.

edit: nevermind, already answered

Hmm I guess I'm thinking tuning on a nitro - those are a pain in the....

Yup, $15-20k is way out of our budget.

I've done auto-tuning on nitro before with exhaust and head temp, it works pretty well - but they have servo horns on the needle.

Any idea of the fuel consumption of the 10cc? What size prop would you be running on one?

I cant remember what prop my friend used on his EVO. My power numbers that I posted above came from actual dyno data that I requested from the manufacturer because I was interested in using it for my own UAV project. As for fuel consumption numbers - I do not have specifics on that either. Sorry....

No problem, you've been more than helpful!

Do you have the dyno graph or numbers?