Can somebody help please? I have just been given a large model of a Cap aircraft approx. 80 inch wingspan,fibreglass fuse,foam wings. I was wondering what size engine would be needed and if there is a conversion available for some of the small Ryobi engines.Any help would be greatly appreciated.

Ian,


Converting an agricultural gas engine was once the accepted solution.

It no longer is, since the intended engines have magneto ignition, which is heavy.


What you need is a purpose built aero-model engine in the 24-36 cc displacement bracket, with an electronic ignition system.

There are many engines out there that fit the bill.

Thanks for the reply mate.This size aircraft is all new to me but it was to good pass up.The fuse will need a little tlc but every thing is there.Thanks again for the info.

Cap 232's like power for best performance. Best power range for an 80 size cap would be at least a 40cc engine. Hobby people have just come out with a very reasonable priced Thor 45cc with electronic ignition. This is a new produce so there is no reviews availible yet.

Though 'converted' engines can be made to work, and several do so quite well, those in the size range you would need are usually very heavy due to the method of ignition they use.

As DarZeelon advised, your best option is to use a purpose built R/C engine. Not knowing the weight of your airframe, but simply based on it's wingspan I would suggest an engine in the 35-40cc range.

Karol

Poulan Pro 46cc engine will fly that airplane perfectly. For an 80" Cap 232--I would recommend closer to 50cc size engine. The worse thing you can do to an aerobatic plane is starve it for power. No bigger sin than building an anemic aerobat.

Conversion engines work just fine. I know, because I've actually DONE it. I've done TWO Poulan 54cc with electronic ignition. Yes, I did that motor twice. It runs that good. I've done a Craftsman 38cc. I've done about 3 or 4 Ryobi 31cc engines. I've done a Makita 45cc engine.

You can put electronic ignition on the engine to save weight and make for easier starting. It will still be hundreds of dollars cheaper than any commercially available engine. Or, you can run it on the magneto.

Some commercially available engines that also run on magneto ignition:
Zenoah
Quadra
Evolution

Thousands of guys put these "overweight" magneto engines on aerobatic airplanes all the time. It works fine.

Go over to the conversion engines forum and start reading. Those guys have done all kinds of aerobatic planes with conversion engines.

With all due respect, I have to disagree with Dar as well. Sure, commercially available RC gas engines are growing in numbers. That's great! It simply means the demand for gassers is growing, and that includes conversion engines. The RC gas engines marketplace is big enough for all sorts.

Not all converion engines use magneto. Some of us prefer to use electronic ignition. In fact, for many conversion engines, it is actually easier to go EI because there are commercially available EI conversion kits so there is little or no machining required.

The Poulan 46cc is a great example of such a conversion. Conversted to CH Ignition, the engine weighs less than 3lbs and will swing a 20x10 or 22x8 with ease. If you can keep the 80" CAP to less than 15lbs, the Poulan 46 will fly it very well.

The Ryobi 31cc is probably the grandaddy of conversion engines, arguably the one that started the conversion craze. Very easy to convert. The old style single ring version wasn't very powerful, but the later dual-ring is pretty strong. Still, it is not enough juice for your CAP.

people always try to say purpose built engines work best but if you go to http://www.bcmaengines.com/ which has converted engines you will see that its not always true. the 46 poulan with EI would work beautiful on that aircraft. i am working on converting homelite 25cc engines for use on a 72 inch 120 cap. just something i want to do for fun. i am not a 3d pilot and wont go for competitions so why bother with an expensive purpose built.

edit: in this hobby i have learned there are alot of naysayers that are aparently experts and have done everything. but most havent tried conversions for themselves or will ever try based on what they were told by someone else.

A wise man once told me ...figure two times the weight of your bird and thats about how many ccs your gonna need to fly it. Good rule of thumb! Conversions are Great And have fun doing them! Besides, a lot of the gasser engine companies are selling their conversions. Save some dollars and have some fun to boot! Good luck!

I am getting conflicting info on what size engine to use and its probably due to my lack of info I've posted. The aircraft has a wing area of 6.4 sq ft ,total weight of 231 oz including 40 oz for the engine . This would give a wing loading of 36 oz/sq ft ,if my calculations are correct.I was thinking of fitting a Super Tigre 3000 as they are reasonably priced at $350 Aussie dollars.It is possible to convert petrol "weed wackers" and would be cheaper to run but i'am worried about interference from the ignition system and I think the total cost of conversion would not be much under the cost of the Super Tigre.

Are you sure that the wing area you quoted is correct, as it does seem a bit low for a model of that size and weight.

I recently started a post on this forum on 'Thrust to weight ratio' for sport / aerobatic models in this size range. Use the search feature and see the comments made, which I found to be most informative.

Karol