I did a search on Power to weight ratio, but by definition, that's not exactly what I'm looking for. I really don't care to factor in airplane weight, and wing loading into the equation. I was just wondering how you would figure out a standard equation to tell you if an engine's horsepower relative to it's weight, was a good figure or not. Meaning one engine could put out lots of HP, but if it's really heavy, then maybe a lighter engine, with a little less horsepower is better. And is there an equation to rank all the engines? Am I making sense?

Yes it makes sense but manufacturers HP claims are generally considered useless-

Depends on the size of engine (46 sized are fairly close in weight-)

The airplane being used- (Some need weight up front or need light-weight up front to balance without extra weight-)

Etc

Why are horsepower claims useless? Are they lying? Do you actually have to mount a certain engine onto a certain airplane to decide for yourself whether it is a good engine or not? I'm not being mad or angry or anything, I'm just shopping for an engine and want to know the best way to get the most for my money. I see some engines that have high horsepower ratings and they are really heavy. So, maybe the weight doesn't matter?

I have a Seagull Edge 540. It's a .61 to .91 2 stroke sized, .91 to 1.20 sized 4 stroke. I have an OS .91 surpass on it, and it is underpowered and flies like a brick. If I put a 1.20 4 stroke on it I don't know if the nose would rise. It's just really heavy. So I guess for this situation, I need to use a 2 stroke for power, because they weigh less? And I don't know what 2 stroke to use because I don't have one of my own that is this large. But I'm back to the same question about 2 strokes. I could go round and round all day about this. If HP claims can't be trusted, then I should just pick one.

HP ratings should list a RPM at which the listed (or peak) HP was obtained. Often this RPM is above the ideal RPM range for the engine. In addition, it isn't always clear as to how the engine was setup, i.e. which fuel, what nitro, did it have a muffler or was it open exhaust, etc. You also have no idea on power curve with a peak HP rating. For these reasons (and probably others), the HP rating listed for a engine doesn't mean a whole lot.

If you want the most bang for your buck, 2-strokes offer cheap power. However, there are some advantages to 4-stroke engines, mainly they develop their peak HP at a lower RPM which means they can turn a larger prop compared to a similar sized 2-stroke.

Typically, the weight of the engine doesn't matter a whole lot unless we are talking about an abnormally heavy/light engine. Obviously CG can be effected, but overall plane weight (i.e. wing loading) won't vary much between diff engines in the same displacement range. However, the power diff between engines of similar displacement can be large, which is significant.

I'm not familiar with your plane, but the engine you are using is at the low side of their suggested range. A bigger engine will help the underpowered problem. As for the brick problem, does the model weigh what it should? Is is balanced properly? A nose heavy plane can feel !QUOT!brick like!QUOT! in the air.

If it was me, I'd probably use a YS110 (that's a four stroke) for your plane.

HP ratigs are useless because there is no standard for testing. One company may simply take an rpm reading an do te math to calculate HP, while another might use a measured thrust to calculate HP and yet another company may have some sort of dyno rigged up.... So, if you can't compare apples to apples, then the numbers are useless.

The engine manufacturers aren't lying about horsepower, but they are misleading you. Given torque in foot-pounds, horsepower is torque times rpm over 5252. The torque curve of a typical model engine is a fairly shallow rise and decline so the horsepower tends to be aligned with rpm. This is how boat and ducted fan engines put out so much horsepower, they turn extremely high rpms.

If you read carefully the .61 engine comparison in Model Airplane News by Dave Gierke, you would have noticed that all of the .61s tested were putting out about 1 horsepower at the rpm of a sport prop, 10,000-12,000 rpm. This is a far cry from the nearly 2 horsepower claimed by some .46 sized engines.

The fact is, just about all sport engines put out very close to the same power on the same prop and same fuel. In my opinion, there would be as much difference in individual engines of the same make as there would be between different brands.

Given the above is reasonable, the best power-to-weight ratio would be with the lightest engine. If you are talking .46s, they all weigh the same so you are talking very small differences. I own, or have owned, several different brands of .46 size engines. Personal preference, ease of tuning and mid-range performance is really the only difference. Right now I have OS, Magnum, GMS, & Thunder Tiger. I had Fox, Rossi, K&B and MVVS.

I see the same thing for .61s and .91s.

When it comes to 4-strokes, YS makes the most power. As for the others, you can cover them with a small towel. The only difference I see is the Saitos, in particular, the .72 and .91s, are lighter. I would think this is the reason Saitos are seen a lot in fun fly planes. The Saito .91, for example, is only 3 ounces heavier than a typical .46. Many profile fun fly planes need nose weight with a .46 so a much more powerful Saito .91 makes an excellent choice.

Thanks everyone for all the replies. That helps a lot. One more question, though. The engines like Webra and Rossi. There are some others, but those are the ones that come to mind. These engines claim much higher horsepower and they are much more expensive than their competitors. Do the Webra and Rossi deserve their high price?

Ed, you said you have had a Rossi. You also said YS puts out more power than the other 4-strokes. Is the same true, in your opinion, for a Rossi that is priced much the same as a YS(higher than others). Rossi's .46 and .53 2-strokes in particular are pretty expensive.

Several years back I had an oiginal design sport/fun fly type airplane. I had picked up a Rossi .40 from another flier and wanted to compare it to my other engines. I made interchangeable motor mounts with it and also a small case Fox .40 I had. On 10-6 props, the plane was slightly faster with the Rossi, but had more climb with the Fox. I flew it most of the time with the Fox. The plane was more nimble and climbed better. Rossis weigh a ton. Personally, I don't think the extra weight is worth the money, but I like light.

Looking at bang for the buck, it's probably going to be hard to beat a Thunder Tiger .46Pro. I would put a Tower .46 muffler on any of them. It'll get you 700-800 rpm just for bolting it on. Costs $15. The new OS .46AX, I have one, is a sweetheart to run and powerful, but $20 more expensive.

For larger sizes, I wouldn't buy a .61. You can get a .75 or .91 built in the same case, same weight so why go small.

Downside: You are still stuck on the HP thing... just let it go. Been there done that. When I first started RC I made spreadsheets with engine HP, weights, costs etc.... waste of time. It meant nothing. Some engine makers, like YS, don't even publish HP ratings, but we all know YS engines make monster power.

As for Webra and Rossi... I really don't know much about them. I do know many Rossi engines are used in pylon. Set up properly with a small diameter high pitch prop I know they scream. But put a sport prop on and who knows. Probably lask luster performance like what Ed just posted.

I just looked up your plane on the web... seagull edge 540, 60 sized. ~9lbs finished. The OS91FX you have in there now should pull a 9lb plane just fine. It won't have unlimited, but it should be close to 1:1 if proped properly. Are you sure your engine is working properly? Plane weight? What prop are you using?

9 lbs is a little much for a 91 4s(unless you are flying scale!) For the hell of it try a 15x4w on the motor. The YS 110 will put out as much power as everyone else's 120 so keep that in mind if you go looking for a new motor.

HP claims by RC engine and car manufacturers often result more from the marketing department than from engineering, since you can measure HP in several ways, none of which are consistend between manufacturers...

Regardless, HP is more relevent to climb performance than speed (sorta like a VW bug going uphill with a stock engine vs one from a Porsche).... and power loading (LB/HP or HP/LB) is a sensitive gage of climb performance, so an engine with more HP will first be noticed as having better climb performance with the same prop.

Practically speaking, I agree with JohnW, since there is no way to get a reliable HP number, all engines of the same displacement and construction (e.g.; standard .46 ABC engines) will put out pretty much the same HP, plus or minus a small percentage... If you need more HP, use a .60.

Also, RC airpcraft are blessed with HUGE HP/Weight ratios compared to our FS counterparts... hence the ability to hover a model prop driven airplane, which is extremely rare in the FS world...

My $.02

Jim

Regarding power to weight. I have found if you group airplane sizes, weights, and wing loadings, then look at the engines and thrust numbers, you can arrive at a power to weight figure. Average the findings and come up with a low and a high, and know how to power a plane.

Hi!
As Ed and Jimmbbo has already said "all engines of the same size have more or less the same power" and that the power or hp ratings the modelengine manufacturers state is worth nothing ....I totally agree with this but there are still certain things to consider when choosing an engine and that is that there are small power and weight differences. For instance a Rossi 6,5cc (.40) or 7,5cc ( .45) is a rather good engine, sturdy built and rather powerful as sport engines go, but it is 50 gram heavier than most other 6,5-7,5cc engines. In a small fast airplane this weight could be too much. So picking a light engine is what I think is important
Another example that hp ratings is noth wort a thing is comparing a .40 pylonracing engine like the Nelson FAI that produces 3,5hp at 27000-30000 rpm or better still the Dutch MB pylonracing 6,5cc engine which produces more like 4 hp at 30000rpm.
Would these engines be better to use in a ..lets say an aerobatic type of aircraft like the SIG CAP 231 EX (span 185cm) than a 25cc (1.50) engine producing around 3hp ?! Hell no! These small pylonracing engines develop their power at such high rpm that we need 7 1/2" props and super sleek airplanes to make them perform...but perfom they will reaching speeds around 375-400km/h (over 200mph).

So picking the right size of engine for your airplane is very important.
I don't know the size of the airplane you speak of but expect it to be around 170-175cm in span as you had .90 engine...... so going to a 120 fourstroke (or a 110 YS) might be the right way to go.
Choosing the right prop is also a very important thing to consider..You can take two fliers using exactly the same engines and fuel...but using different props ....there will be a large difference in performance between these to engines/airplanes.
Among competition oriented flyers there is a general knowledge that APC makes very good propellers. For the larger engines 20cc and up the German Menz (wood) and Tjeck Mejzlic(carbonfiber) are good.

Regards!
Jan K
Sweden

You can often get accurate numbers for weight, but never any good ones for power. So doing a power to weight study would be futile.

Yes (well kinda), and if not reading info from this and other sources, then yes to the second.