How does one size a gas engine to a plane when only glow engine sizes are recommended. Example : a 160 sized glow engine is recommended but what size gas could be used. A friend told me that if you multiplied the cc' s of a gas engine times 3, that would be the comparable size glow engine that it could be used. Is this a fairly accurate way of comparing the two different engines ?

The expected finished weight of the aircraft is a better method of determining what you will need. There is no "rule of thumb" anymore since the advent of 3D. It's all about thrust to weight ratio. How much thrust that a given size engine provides will determine what you can do. A thrust to weight ratio of 1.5 to 2-1 is great for 3D. A 3/4 to 1-1 thrust to weight ratio is great for warbird type flying. Then you come into planes with extremely light wing loadings and it all changes again, permitting smaller engines than either the 3D-aerobatic types or the warbirds. Sometimes it can get pretty confusing even for those with a lot of time in this stuff.

I guess you're talking a 160 two stroke glow vs. the gas equivalent. Somehow the 3 to 1 ratio doesn't seem reasonable to me. A DA50 (50X3=150) develops 5 HP, will swing a 22X8 or 10 prop typically, and is recommended for a 14 to 18lb plane. Will a 160 glow engine do as much? I don't know because I never had a big two stroke glow engine (the biggest I ever owned was an OS 1.08). I owned two 300 Saito twins and they were sweet engines, in fact, I still own one which is on the shelf (want to buy it?) The 300 Saito will fly a plane about the same size as a DA50 but not as forcefully, in my opinion. It does not have as much power but it is a heck of a lot smoother. It also will burn several times the fuel at several times the cost per gallon.

I basically echo what Silver said..

One piece of advice.. when you make that change dont try putting gas on planes that are 72" wing span or most 1/4 scale planes. The wingloading just makes it go from flying to dropping in a click or two. When I started with gas ( Iwanted to really badly) I spent time and money trying to get too small of a plane fly with a gas engine. You need to find a very light engine to fly small planes on gas.

There are smaller planes that will fly well with a 25cc engine like the Brillelli www.brillelli.com but for 3d you need more power so it becomes a weight issue again. Try 27+% planes... or even the Great Plane Patty Wagstaff Extrta 300 will fly well on gas... it still floats well on landings and flys great with a 40 or 50cc.

Thumbs,
My rule of thumb isthe gas engine should have 150% of the displacement of the recomended glow engine. This is because the average gas engine has 2/3rds the power of a typical glow engine. It is just like the glow 2 stroke to 4 stroke ratio. You put a .90 4Cycle in a .60 2 cycle plane. 3D is a different story and Surfer is correct about the weight issue. Wing loading has to be in the correct range for the size of the plane.

Dave

In the old days of gas it was something like 1 cubic inch for each 10 pounds of plane but they flew like bricks. You really had to watch the energy in the turns. Thankfully much better engines and ignitions became available!!

Dave,

Thanks for that rule of thumb. Works for me and I like it. I haven't flown glow in so long now I don't know if size references even matter anymore for me.

I also agree with Daves math.

Glow puts out more HP--pound for pound. But a gas engine has got TORQUE, and plenty of it.

I think glow engines are stronger--but only to a point. If you keep them wrapped up and keep a lighter load on them--they will keep going. But, if you slow it down by putting a big load on the prop (vertical), it falls on it's face.

If you get into the BTU of the fuel and all that crazy math, it seams like gas puts out more BTU than the glow, so you'd think gas would do more work. But, the glow engine is buring twice or three times the fuel of the gasser, so even though it's fuel has a lower BTU output--you get more power because your burning so much more of it in the same timespan.

I think........ [sm=spinnyeyes.gif] [sm=stupid.gif]

I used to fly the big glow engines. ST3000, ST4500, Moki 2.10, Moki 1.80, MDS 2.18--etc.... I had most of them at one point or another. They just don't have the pull. They just don't have the grunt that a gasser has. I could always tell when a glow engine was running out of power. Pull vertical and just go up. Even a Moki 2.10 will run out of grunt eventually and start to slow down. It's not running out of fuel. It's got a Cline regulator on it. It's just feeling the load of carrying all that weight straight up and the engine can't handle it. The load catches up with it and the engine starts to loose torque. You can hear it.

Not so with a gasser. Point it up and hammer the throttle. It just goes and goes and goes. The plane may start to slide back down if you don't have enough engine--but that gas engine will just keep on gruntin'. It won't sag or slow down. The prop will turbulate and the plane may slide back, but the engine just keeps on going.

Pull a big HUGE loop with a big glow engine. You'll hear it starting to groan just before you get to the top of the loop. Right when there is maximum load on the prop--you'll hear it groaning and slowing down. No torque.

Pull a big huge loop with your gasser. It won't quit. It won't groan. Listen too it. Tilt your head out and point your ear at it. No groaning. No slowing down. It's like the Energizer bunny. It just keeps going and going and going. GRUNT!! Thats what I love about gassers. So much torque.

But you'll still need a bigger gas engine to do the same work on a plane because the gasser doesn't burn near as much fuel and doesn't have the same overall BTU output.

To replace a 1.60 glow engine--I'd probably want a 35cc -- 40cc gasser. It's a bit heavier, but it's gonna pull the plane around like a ragdoll compared to the glow engine.

Gas powered engines do not compare well as the size goes down. You mentioned a 1.6 glow so lets compare it to the 1.6 mvvs. This is easy because of the hundreds of posts about this engine. At the end of the day the 1.6gas+tuned pipe+high rpm= almost as good as glow. The same 1.6 gas with muffler and low rpm has horrible power to weight. All of the glow engines larger than 2cu. in. are old and heavy designs and don't have the same power:weight advantage over gas. Glow engines are not too hard to handle in smaller sizes but could you imagine a new guy trying to start a 100cc glow engine, or a wealthy weenie having to wipe dirty slime from his plane. These two reasons are enough to ensure that larger engines will always be gas powered.

Maybe not ALL I guess race planes don't count
GT80 gas, 150 mph, 9000 rpm, stock
GT80 glow, 190 mph, 12.000 rpm, modified
DA 150s are running glow, turning 10,000 rpm, modified, 200 mph

Rcpilet,
I dont understand why your big glow engines where running out of torque. Where they overheating? A pump or regulator does not guarantee rich setting and sufficient fuel supply. I know some guys running fuels without nitro or very low % are having more trouble with the needle settings and overheating problems. I usually grossly overprop all my glow engines and use quiet pipes for two strokes as we have strict noise restrictions here.
My ST G3250 and a APC 20x12 @7600 would pull my plane vertical all the way to the moon if my transmitter was strong enough. That was just an example because all my glow engines will pull vertical until the tank is empty. Pattern guys are doing loops bigger than a fotboll stadium all the time with the 23-26cc twostrokes loaded down to 7-7500rpm at peak.

They just do.

If you don't know what I'm talking about--and you've never listened to a glow engine run out of torque--then there's nothing more I can say to describe the sound and feeling to you. They just DO.

You have to just listen to it. You can tilt your head towards the plane and listen to a glow engine slow down and loose power on top of a big IMAC loop or a long vertical upline. I've heard all of them do it and guys will swear, "Listen to that engine--it's pulling like a freight train!!" No it ain't. It's falling all over it's face every time you pull back on the stick and put a heavy load on it.

I don't want to argue or get in a fight. You say they run good and have plenty of torque, and I hate 'em because I don't think they have enough consistant torque to keep swinging a big piece of lumber. If your happy with the big glow engines and they put out enough torque for you--them I'm happy for you. Go fly and have a ball. I don't like them and I quit using them about 4yrs ago. Thats just my opinion. Your not wrong and I'm not right. I just think that glow engines run out of torque. If you've never noticed it, and your happy--then great.

I just prefer gas engines. Once you get a taste of a gasser--you'll never want to touch a glow engine again. I still use them, but only .75 size engines on little 3D planes. Everything else is gas.

Thanks for your reply, I have had a few gassers in the 50cc-100cc range and I liked some of them better than others. I sold my last two 60cc & 80cc gas engines for a month ago because I dont build planes bigger than 80" wingspann anymore, I think.
Any glow or gas engine that is not getting enough fuel or cooling air is gonna run out of torque.

45cc engine...was running petrol...waste of time.
converted to alcohol
running waay better
was originally rated at 4.25hp @7500rpm
now has well over 5hp at the same rpm rating

starts and runs reliably flight after flight

mmmmmm....a da150 on glow and alcohol......sweeet!!!!!

cheers
craig

Rcpilet, I do not want to get in a fight either. But, do your .75glow on little 3D planes run out of torque? What do they weigh?

What I am getting at is if the plane is properly powered and tuned (correct prop and mixture settings, cooling) so there is no heat build or leaning up under load, than it does not matter if it is glow 2 or 4 stroke or gas or electric. They all will go straight up without running out of power.

Around sea level with right fuel and prop, pitts ect.....

With a Moki 2.1 on pitts plane should weigh about 14 pounds or less
With OS 1.6 plane should weight about 12 pounds or less
With OS 1.08 plane should weight about 10 pounds or less
With YS110 plane should weight about 10 pounds or less
With OS .91FX plane should weigh about 8 pounds or less
With SA100 plane should weight about 7.5 pounds or less
With a ST75 plane should weight about 6.5 pounds or less
With OS46FX plane should weigh about 5 pounds or less
With OS25FX plane should weigh about 3 pounds or less

With 50cc gass/pitts plane should weigh about 18 pounds or less
With 40cc gass plane should weigh about 14 pounds or less
With 26cc 1.6 gass/pitts plane should weigh about 10 pounds or less

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Put another way...

Power your plane so it can blast out of a hover. The greatest load you can put on the motor is static (unless you are in a back slide). If you have your motor tuned so that you can run wide open for 5 min on the ground without the motor losing RPM and the motor has fuel delivery such that it is not going lean or running hot going vertical, what makes you think that it will lose RPM (or power) while the plane is going straight up?

My Carl Goldberg Extreme will go strait up forever with my Como .51 ABC. The .51 was built by Supertiger in Italy.