Gas to Electric Equivalent

donaldj

Junior Member
Hi Folks!

Long time FliteTest fan, first time poster. I'm doing my first build! Yay! I've always wanted to build a nice balsa plane. I hope I haven't jumped in at the deep end. This is the plane I'm building.

I should be fine on the actual construction part, but I'm struggling with the power plant. The 1970s original is a freeflying gas powered plane. I'm planning on turning into an electric RC. The original calls for a .09 to .12 OS Engine. I'm really not sure how to go about converting engine displacement to electric units.

Also, if anyone has any other tips on converting gas to electric, I'd love to hear some tips.
 

pgerts

Old age member
Mentor
The biplane calls for a 1,5 to 2 cc glow. It is a nice biplane and you wont get to much trouble converting it to electric.
Your biggest concern is to make the battery compartment easy to access to replace the battery.
I would have looked for an electric enginge with about 200 W power and 50-70 gram weight, a 20-25 A speed controller and some 3 cell lipo batteries 900-1200 mAh. You can build the plane slightly lighter with electric engine, but the battery and electric motor is probably more in wieght than the small OS enginge plus tank.
Try to make the speed controller cooler as a part of the underside of the plane. Epoxi it making the esc a part of the fuselage.
 

quorneng

Master member
There are some issues in converting any FF plane to RC and particularly to electric.

The biggest is weight. By the time you have added everything (including the modifications to incorporate the moveable surfaces) you will almost certainly be flying it at a higher all up weight than was intended for FF. Whilst the plane is likely to be adequately strong enough in flight you will find the added inertia will make it rather more 'delicate' when in contact with the ground!

A good method of sizing an electric set up is to "guestimate" the planes total weight. If you look at say the HobbyKing web site the weight of every component is given and the maximum power (watts) that each motor is capable of.
So starting with pgerts suggested components add every thing else you can think of to the bare airframe weight (I assume that given on the plan).
The 'rule of thumb' is measured in Watts/lb.
50W/lb for a sedate performance
100W/lb for sports performance
150W/lb for aerobatics
200+W/lb for vertical performance

For each set up you also have to determine the likely electric load (amps) that the motor will draw which depends on the battery voltage and the size of the prop. Typical current data for each motor is usually given as well.

It is a bit of a chicken and egg process that may take a few 'iterations' but the object is to arrive at a motor/prop/ESC/battery combination that can easily withstand the full power current, provide sufficient Watts/lb for the required level of performance along with a meaningful battery duration. The higher the performance required the harder it is to get a workable solution but as you are building 'scale' the specification should not be too critical.

Perhaps you might like to list on here what you intend use for comments before you actually buy anything.
 
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donaldj

Junior Member
Thanks for all the tips folks! I've only tonight just put No. 11 blade to balsa, so it will be a while before I actually get to the electric stage of the build.

I'm shocked to see how light gas engines are compared to an electric powerplant! I'll definitely look into beefing up the fuselage around the motor. The plans already call for bass- and plywood in the engine bay, so I might be able to just roll with that.

Quorneng, thanks for the rule of thumb! I'll have to keep that in mind. I honestly, however, have no clue how much this thing will weigh in the end. I still haven't figured out how to skin it, whether I go monokote or silk-and-dope or somewhere in the middle. It will probably come down to what my local hobby shops have on hand. I'll keep you folks posted!
 

quorneng

Master member
donalj
Just remember there is a big difference between the effect of weight and vibration.
An 09 might use a 7x4 prop. This motor would turn it at about the same speed on a 3s LiPo.
The difference is the electric motor could be bolted directly onto a 3mm light ply bulkhead that itself was glued directly to the balsa frame work of the fuselage.
Such a set up would not last long with even a tiny 09 pounding away!
 

HawkMan

Senior Member
A lot of RC Hobby stores sell convertion kits. wich lists what motor they replace.

HK makes it fairly easy with their G series motors. the motors are named for what size nitro motor they replace. the G60 replaces 60 size motors for example. Then you just need to match it with a suitable EC(listed) and prop(usually also listed but that may also depend on the plane) and battery.
 

willsonman

Builder Extraordinare
Mentor
Having done several conversions now and building from plans not intended for RC I'll give a few of my observations that may prove helpful.

First: You are building a biplane. They are inherently draggy so you are not going to want to go for speed from say a 12x8 prop (1.5:1 ratio). You need gobs of thrust for these WWI era planes so you are more after a prop that has a higher ratio, like 2:1. So a 12x6 prop will be better or better yet a 12x4. I would select your prop FIRST and plan your motor from there. To plan the motor you need to figure out how "scale" you want the plane to fly. I shoot for the lowest possible RPM, which involves lower cell counts and higher Kv motors. My Top Flite Texan is a 60-size ARF similar in size and weight to the Jug in the above video. However, I am only using a 4-cell battery over a 6-cell. The lower voltage allows me to use a slightly higher Kv motor on a larger-than-intended prop for that motor, a 15x10 (fighter and WWII so I need the speed). A good example of this is my O1 Tummelisa. I am using a 16x6 wood prop , which seems HUGE) on a 25 size motor. Knowing this would pull a lot of current I opted for a 60A ESC and a 3-cell battery. While it seems overkill at first, the plane flies exceptionally well at just 1/2 throttle. Any more and it just looks wrong but I have the gusto to get out of trouble. I selected a 4000mAh pack to give ballast (in addition to 8oz. of lead) in the nose... which leads me to my next point.

Second: This plane is short-nose coupled. You will need to build the tail VERY light so that you do not need an excessive amount of ballast at the nose. This is a problem for nearly all WWI era aircraft.

Third: The plans are for Free Flight (FF). These include a lifting tail (airfoiled) that are set at an incidence intended for slow-speed and to maintain a slightly positive attitude. DO NOT be surprised to see your nose go WAY high on maiden. See the maiden of my SE5a.

Fourth: Take-off and landing are ALWAYS fun (knee-knocking). Take-offs are best in the following way: slightly raise the throttle just to get the prop spinning at the minimum RPM. Apply full-back elevator and then quickly advance the throttle to 3/4 or better. Depending on how you power it, be ready on the rudder/aileron to counter any torque the motor will put into the airframe. Your take-off should be short. The very moment you see the plane lift off the ground drastically reduce the elevator and push forward to level out to maintain airspeed. Do not focus on gaining altitude... focus on airspeed. Landing: These are much harder to do well. Reduce throttle to at least 1/3. You WANT the thrust over the tail planes to give authority. make a long approach and ease into the landing and the moment you touch down, avoid the tendency to reduce throttle but apply full-back elevator and let it roll. let the plane slow itself down. NEVER go to full-back throttle until you have stopped rolling. Try to maintain level roll attitude using RUDDER, not aileron. A helpful prayer thrown in sometimes works but do not be surprised if you nose-over--so overbuild the vertical post of your rudder... plan for it to take a beating because of this.

Fifth: The current plan for your motor mount is sound. Typically and IC (internal combustion) plan is more than adequate for an electric setup. The vibration is not an issue and the usual build-ups handle the torque-ups from these motors just fine.

For reference, my electric conversions:
Top Flite AT-6 Texan conversion
Giant P-47 Conversion
SE5a build
O1 Tummelisa
 

donaldj

Junior Member
Wilsonman! What an excellent post! I'm sure to bookmark this one to look back to in the future.

On the prop/motor question, I've found some engine specs for old OS engines, which are the engines specified in the plans. An OS 09 recommends an 8x3 at 13,500rpm, and the OS 10 an 8x4 or 9x4. Similar ratios you were mentioning, but scaled down. Should I perhaps wait till the build is complete, weigh the plane and see how much power I need from the weight?

Ideally, this plane would fly like scale. I'm not going for speed or acrobatics. I'd be happy with a safe take off and landing, some fun dives and maybe an aileron roll if I'm feeling especially adventurous.

My knees will seriously be knocking when I maiden this plane. So much time will have been invested!
 

willsonman

Builder Extraordinare
Mentor
Yes, I would try to get a better idea of weight if you are unsure of what to expect. I'll keep an eye here if you have further questions down the road.