What size glow engine would give near a 2:1 thrust ratio for a 2 or 2.5 lb airplane, 48" semi symmetrical and hopefully reach at least 70 mph ?

Also what size tank would allow at least a 20 minute flight ?
If it stresses you to tell me, then don't cause any suffering to yourself.

I have a plane similar size but a little heavier and my OS 25 FX pulls it pretty fast. I have never measured the speed. I have a 4 oz tank and it is too small for that engine lasts maybe 4 minutes.

I think you want the lightest engine you can find that will spin a 10x6 prop at around 12,000 rpm. That would give you about 5 lbs of thrust with enough speed potential to reach your goal.

That OS 25FX might do it with a mousse can pipe. The old Webra 32 or 36 would be better. Can you afford a Jett? There are several possibilities in the .25 to .36 size range that might get you there at reasonable weight.

If engine and muffler weigh around 11 ozs, then you've got fuel tank, motor mount, and radio gear...say around 18 oz. total, and then you build an airframe that weighs 20 oz with wheels and covering.

Seems possible if you're experienced at building light; what are you trying to do?

I have an Electric J scratch build, a bit too much for a 3S 2200 and 1200kv APC 9x6 SF. I was figuring I might give a small glow engine a try on it. It's already so light it can thermal, so some more weight might even help get the thing down out of the air. It was an early aerobatic trainer, and a good one.

That size plane usually uses a .25 size engine. But you have a choice of engines to choose from from .25, .28, .30, .32, 36, and even a .40 in that engine size range. You would likely want a .32 to .40 size engine. The NV-Engines AX40 would work well. It is very light for a .40 engine and would give you the maximum in power to weight ratio. But the NV-Engines (Norvel) AX25 would work well too. The ASP .28, .32, or .36 would work, the Thundertiger .36 pro is another one, Enya has the SS .25, SS .30, and .35X engines and they would work fine as well. The small case "C" frame Fox .40 engine is another choice. If you can find one the Webra .32 is one choice too.

I was figuring somewhere between .25 - .30. I also think they would be gas sippers and the fuel cost low.

I doubt you will get 20 mins out of a plane that size, (at least at full throttle)

the Os 32 would be your best bet for power! and an 8oz tank if you can stuff it in the plane!

Well if you really want a engine with low fuel consumption then you need to go with a NV-Engines GX40 gas engine that uses a glow plug. It is a .40 size engine, very light in weight and uses gasoline. Fuel consumption is pretty spectacular in comparison to a glow engine that uses methanol for its fuel. The engines work really well too.

I can make wing tanks if I need to. It does have a narrow width fuselage, similar to a Pulse XT but mine is a high wing.
These glow fuel thoughts got me craving some more Jack Daniels BBQ sauce on a juicy tender steak. Hmmmmmmm.

A .15 would be plenty just to fly around, any .25 will make it quite peppy... Past .25 is just excessive, but that's not a bad thing. .

If you want 70 mph and 2:1 thrust to weight I'd suggest a BB .46 running a 10x6 prop (10x7 will easily get you over 80). You'd need at least a 14 oz tank to get 20 mins and that would require good throttle management.... An 8 oz tank would give you a solid 10 mins of spirited flying, but not flat out.

Can the airframe handle a glow engine? Electric planes are often built too light for glow and must be modified.

If you want to put fuel tanks in the wings you'll need a pump. Much easier to forget about the 20 minute flight time requirement.

I could fill it at the wing tank, if I had a capped pipe sticking out. But what after it empties ?
I'm sure the airframe could take a small fuel engine, but I don't know about the hs-81's.

Going to extra fuel tanks with a small .25 is going to be iffy. Usually the engines are already challenged for good fuel draw, now adding extra fuel line length with wing tanks can add more problems that one might want to deal with. Worse is that the wing fuel tanks will tend to change height in relation to the engine when the plane banks, or flies maneuvers. Usually you normally use a 4 to 6 ounce fuel tank with a .25 engine. With many .25 engines you may be able to fly 20 minutes with a 6 ounce tank. The smaller engines usually wind up having fuel draw issues if the fuel tank (actually the fuel line length) get too large. Trying to adjust the needle valve to work with extra large fuel tanks may turn out to be a exercise in futility. But in any case let us know how it goes, you may actually come up with something that works.

No just wondering about wing tanks. And yes. I'd have to beef the electric plane up and not be sure if it will be strong enough.. Oh well, back to ARF shopping if I decide on a small engine glow plane.