Hello all,

I’m relatively new to gas engines and thus would like to learn how to properly size a gas engine to a plane. Let’s say you build a plane from scratch what would be the starting & ending point for sizing an engine? Is there a “Rule of Thumb” for how many cc’s the engine must be per pound of the total plane weight? Heck for that matter are there any “Rules of Thumb” when sizing gas engines. Just for arguments sake lets say you have a 92” wingspan plane that weighs 22 lbs. How would you go about sizing an engine to fly either scale, aerobatic or 3-D?


Rick

Here is a link that will give you a good idea. You can get an idea of what size engine to use for the weight of the plane

http://www.rcshowcase.com/html/chart.html

The manufacturer of a plane will spec a required engines size and many even will state a brand. That being said, some engines are lighter than others of the same size. Lighter isn't always better just like bigger isn't. But as they say "too much power is just about right". Find the balance between power and weight. For example I have converted many Echo engines up to 50cc which is the basis of the BME 50 brand also. Plenty of power for the 17-1/2# Wildhare Extra it was in, plenty meaning a rocket going verticle. I am converting a 72cc Husqvarna now that will weigh about the same when done. Don't "need" the extra power in that 28% plane but.... There lies another potential issue as you go bigger with singles and that is vibration. Can the lighter airframes being produced now handle the extra shock and shake of a larger displacement albeit lighter engine? Your call. That Wildhare plane I had I flew every weekend for almost 3 years and I just sold it. Excellent condition but with a bigger engine up front it may not have lasted that long. Hope this helps. Good luck.

Hi,

I think there is better idea, and a lot more freedom for you:

1. Airplane Wings Load is usually the "thing" all refer to. Some fly at 20g/sq.dm, some go to
120g/sq.dm. depending on many things, for example if you want to "battle" cross wind then
it is better you go to 100 or more g/sq.dm. If you want to fly slow 3D you better try to stick
around 30-40g/sq.dm. The surface of the other parts is usually not taken into account in first
glance.

2. Engine power is the next to consider. Take a look at the Propeller Calcualtor of Pe Reivers
which is an Excel file at (it will start downloading as soon as you click next link):

http://www.mvvs.nl/prop-power-calculator.xls

and there you will find the Thurst of your engine and the Speed of you plane and many
other uselful parameters too.

It is a very realistic way to go if you want to be on the path of do-it-yourself.

Of course there are some book at www.Towerhobbies.com you might be willing
to read about this.

Cheers,
Nick

Thanks guys, this does help some. IH82, that link you provided tells me that the weight of the plane is the most important step in sizing the right engine to the plane. That is according to rcshowcase.

Tim22, I understand were your comming from when you say the mfg will state what size engine. However what if your scratch building and don't have that info either because the plans are very old and/or your the designer.

The things that I think play a role in sizing the engine to a plane are 1) weight 2) wingspan 3) wing area 4) style of flying. What % of those 4 factor into engine selection I really don't know (and I guess is what I want to learn). Just as an example a small gas engine can fly a large cub but if you take that engine off and stick it on a geebee that weighs the same it will never get off the ground. There has got to be a way to figure out "X weight + X wingspan + X wingarea + X flying style= (ballpark) gas engine cc size" . I may be 100% wrong in my approach to this.


Thank you NikolayTT some very usefull info in your post above. hehe I think I'm going to start telling people "Yeah I can fly it's everything else about airplanes I need to learn".

Yes, it feels better if you depend less on the comercialisation in the RC because if you buy ready planes with the
reccomendations there, it is almost impossible to guess all "twists of the mind" of the Sales & Marketing. In fact
as a "joke", the World divided the people into Men, Women, and Others, i.e. in that third group I believe are placed
those Sales & Marketing at least... Still, the best planes come from RC-guru and are very expensive because those
guys defend their name, while the rich RC companies, seems to me, are never rich enough to be honest enough,
they just change the management team and every thing is "OK"... if the income grows nicely up.
Good luck and post your experience sometime; would be nice to know what is the trade-off in frame-vs-engine you
have made.
Cheers,
Nick

In the old days it was about 1 cubic inch per 10 pounds of plane weight. 3D changed all that so now it hits closer to 1.60 to 1.75 cubic inches per 10 pounds. Floater type planes with very large high lift wing areas can still use the old formula just fine.

My 'rule of thumb' is twice the cc's as the weight of the airplane for comfortable sport flying.

I have a 92" airplane that weighs 22# and am flying it with a 58 cc engine. The airplane was flown with a G 38 before it was handed down. The same 58 cc engine was used to fly a 35#, 96" span stick.

Bill

For my style of flying,,,,In the 23-27lbs range (approx. 30oz/sq WL) I like 100cc++, in the 14-18lbs range I would go with a strong 50cc+++. I like them light and powerfully similar to foamies, or at least 2+/1 thrust to weight. That is what the left stick and throttle curves are for

Unfortunately you almost have to kill the engine to attain a decent landing speed...[8D]

That only happens when your firewall loosens up and re-adjusts your idle trim settings