schokshi

Hello,
I am about to complete H-9 P-51 ARF. Manual recommends, 2s .40-.46 & 4s .40-.82. Generally I've seen .52-.60 size 4 s recommended in place of .40 size 2S.

If I go by the HP, OS .46AX=1.6 hp
OS 70 4s = 1.1 HP
OS 91 4s = 1.6 HP

Does that mean to match power of .46 2s I need to put OS91?

bubbagates

My Feedback: (32)

Essentially you are correct but HP numbers are very suggestive. The best engine for that plane is either the OS91 Fourstroke or even better is the Saito 100.

kerrydel

My Feedback: (2)

One thing you need to look at besides horse power is torque. Most engines don't list it, but you can get an idea of how they compare by looking at what props are recommended for each engine. The .46 can use an 11x8 or a 12x7, the .70 can spin an 11x9 or a 12x8, and the 90 can take a 14x7.

Personally, I think the 70 4s would be just fine on this plane. There's no need to overpower a plane that's meant to be a trainer.

Kerry

britbrat

Many manufacturers wildly inflate their HP numbers -- OS being particulary guilty.

The 70 4-S will be close to the 46 AX 2-S, but maybe a bit weaker. A 91 4-s is certainly going to outperform a 46 2-S, but at a penalty in bulk & probably weight. An 80-size 4-S is probably a more sensible choice for a 4-S.


However, this simplification ignores the power delivery characteristics of the 2 types. You have to decide for yourself what you want from the model.

A hot 46 2-S will rip around very nicely, but with the associated wail from it's high-rpm operation. An 80-size 4-S will also blast around, but with the typical put-put sound of a single cyl 4-S -- your choice.

Mr67Stang

The 4 stroke will give your plane a more scale feel. The diference when you go from 2s to 4s is torque and torque is what makes your prop turn not horse power. The 4s turns a substantialy lower RPM but with its greater torque will spin a larger prop. Your .46 2s would likely have an 11x6 to 11x7 prop on this plane but if you go with the .70-.80 4s would likely have a 12-13 inch prop with similar pitch. Even better go with one of those cool 4 blade scale props.

schokshi

I thought HP is proportional to RPM & Torque. I am not sure if you can only look at the torque to determine power of the engine. 4S will have high torque but lower RPM.

Mr67Stang

Horse Power plays it's part. HP is importaint for sustaining momentum/work under a load but the torque is what gets it there and the more you have the more prop you can swing. Then 1+ HP can easily keep a 13 even a 14 inch prop spinning near peek RPM for a 4s, while the equivilent 2s might turn that same prop but it will never reach peek RPM due to insuficient torque. Thus we reduce the prop size for the 2s and rely on the higher RPM to produce the same thrust with a smaller prop. Esentialy when you chose your engine, 4s or 2s, you are trading HP for TQ or vise versa.

B.L.E.

The difference between that .91's 1.6 horsepower and the .46's 1.6 horsepower is that the horsepower of the .91 four-stroke was measured at an rpm that model airplane engines actully turn in sport aerobatic airplanes. The .46 makes it's horsepower at something like 17500 rpm. To get that engine to rev up that high, you would probably have to put a 9 inch prop on it like pylon racing planes use. Unfortunately, these small props are a total mismatch for most trainers and sport planes so the engines power mostly gets used to accelerate air backwards instead of pulling the plane forwards. The engine makes 1.6 horsepower but the prop wastes most of it. That's why .46 two stroke usually uses a 11 to 12 inch prop. Its the best tradeoff between letting the engine rev enough to make horsepower and prop efficiency. In real world use, that .46 is probably making no more than 1 to 1.2 horsepower.

chashint

If it was me and money was not a factor I would assemble the plane and add weight where the engine mounts until it is balanced, then I would buy the 4 stroke engine that comes closest to balancing the plane.
Prop clearance would also be considered, I personally like a 13 inch prop on a 70 and a 14 inch on a 91 4 stroke engine.
Based on the published weight of the plane a 70 4 stroke will give you spirited performance that should allow giant round loops but not unlimited vertical.
If money is a factor go with the 2 stroke engine.

da Rock

Loosely speaking..........

Horsepower is torque X rpm

Since 2strokes operate at higher rpm, they could produce the same torque as a slower engine and they'd have a higher horsepower rating, right? loosely speaking.....

The real measure to compare would be thrust. Nobody publishes that for good reason. But there is a thrust program somewhere around here on the internet. If you were to plug in the prop size and rpm for the appropriate size 2stroke and get the thrust from that prop, and then compare that to the trust from the appropriate size 4stroke and it's prop, you'd have a better comparison. And it would show thrusts that were more equal than were the horsepower ratings of the two engines.

fredscz

My Feedback: (47)

First question -do you have the PTS version [sold as a trainer] or do you have the larger H-9 P-51? The PTS would be great with the Saito 82 or 91. Both are much lighter and have more power than the os versions [OS and Saito are both fine engines but OS's are heavier]. Weight would be the biggest influence on my decision. OS46 and Saito 82/91 about the same weight.
If you have the larger P-51 then either of the 91's, or the 100 would work fine. The Saito 82 would have the power but you would probably be adding nose weight. I fly the H-9 AT-6 with an 82 because that is what I had to use at the time and it does fine. [APC 14x6 9850rpm on 15%]. I was going to put a 100 on it but it flys so well I may just leave it alone.
Fred

checkmate91

Here's my perspective: my big heavy trainer (which is quite acrobatic incidentally) was fitted with an OS40 LA and variously 11x4 or 11x5 prop. fine in air, rubbish on ground when grass anything other than shaved. I fitted an ASP 52FS (magnum or SC in the US) with a 11x6 prop and the thing just leaps off the ground. it's great. Now, the os40 produces 1.0 hp the 52FS produces 0.9 but the torque is completely different. Don't get me wrong, I love the simplicity of my little OS 40 but love the sound and the grunt of my 52. I think some of the other correspondents have suggested a 70fs (or 80) in your p51, that would sound great and if your kit asks for a 46 it would do the job...

schokshi

Thanks all for the replies. I am leaning towards Saito 82.

skiman762

Nobody has mentioned that a P-51 is not a beginners plane

B.L.E.

It's not nessesarily the "torque" that's responsible for this. That .52 likely just makes more horsepower than that LA 40, period! The real question that needs to be asked is "how much horsepower does a LA 40 make at 11,000 rpm, with a muffler?" My best guess is no more than 0.7. When O.S. says that the LA 40 makes 1 horsepower, I suspect that they mean that a test engine managed to make this much horsepower on a really cold and dry day with an open exhaust and a tiny prop that let it scream to 17,000 rpm and it managed to do this exactly once in a row. Or, someone could have just pulled that number out of his rectal cavity.

The fact that 4-stroke engines make their peak horsepower at actual usable propeller rpms means that when they say ".9 horsepower", it's .9 horsepower that a sport plane can actually use, not a hypothetical horsepower that is only available if you had some kind of reduction gear on the engine to match the engine's ultra high rpm to a efficient prop turning at 9000 rpm.

schokshi

Skinman762,
Actually I solo'd on LT-40. Mustang PTS in my second plane

skiman762

Oh ok good deal warbirds can be a handfull sometimes but are alot of fun.

donkey doctor

Hello; Chiming in on the torque vs. Horsepower debate; What these engines do is provide thrust to propel our planes. Regardless of the HP numbers, it should be available thrust that we are comparing. Other things like easy starting and a nice sound and a usable mid-range and a reliable idle, don't figure in a naked thrust measurement. I prefer four strokes, and would likely go with the Saito 82.

MinnFlyer

My Feedback: (4)

Ok, when I first saw this I noticed Bubba saying an OS 91 or Saito 100 is ideal, but he thought you had the 60 size H-9 Mustang.

For the PTS, go with an OS 70 Surpass (That would be my choice) or a Saito 72

The Saito 82 is WAY more than it needs, and this plane is a little nose heavy even with a 2-stroke, so I would really avoid going too big with the engine.

(Note: H-9 recommends a 4-stroke up to an 82 simply because they MAKE an 82. I've got a PTS, and a 70 is as big as I would ever go with a 4-stroke)