Hi guys

I am a newbee to 4 strokes any help would be much appreciated, here is my question.

The OS FS 30 rpm is 2500 to 13000 weight 279g
The OS FS 54 rmp is 2200 to 12000 weight 419.6g
The OS FS 70 rmp is 2300 to 12000 weight 467g
The OS FS 91 rmp is 2000 to 12000 weight 742.8g

I am looking to put the 91 in my piper cub but looking at the other engines the rpm's are the same but weight wise obviously there is a difference. For rpm's which would you guys recommend. The weights mentioned are not an issue.

Much appreciated and many thanks (cant decide which one to go for)

Hi!
The rpm figures is with certain props...for instance the OS FS .30 can swing a 10x6 APC prop at around 9000rpm...an OS .91 on the other hand can turn the same rpm but doing so with a 14x6 prop. Some difference in pulling power don't you think!
How big is your CUB?

thanks for your quick response. sorry not to mention, the wingspan is approx 6'6" and the plane weighs 6lb. I am just looking to fly it for fun, no aerobatics or anything.

thanks again

The .52fs will fly a 6lb cub fine. If you later decide to try aerobatics with it, the .70fs would be better.


Mike

Find out how much engine weight will be needed to balance the model. Then, go back to the engine specifications and choose the one that comes closest to the weight. I'm picking that for a 78" span model, even at a light 6lb, the .91 is the right choice. It's always better to have too much power and use the throttle than it is to have not enough.

G'day

I have a couple of Cubs - one if which is about the same size as yours. Initially I put an OS 70 Surpass in it and it was fine but really it was quite a bit overpowered and landing was interesting because the big prop still produced quite a lot of thrust even with the engine at idle. I next tried a Supass 40 which flew it ok but was marginally under powered but I suspect very "scale" so I then put in an OS 52 which was just about right. I later took the 52 out for another project and I put in an ASP (same as Magnum) 61 which is what is in it now. The 61 is the same size as the ASP/Magnum 52 but weighs slightly less and is slightly too much but quite OK. The ASP 61 is a very light engine for its capacity.

When I had the 70 in it, people used to say that the model had two modes of flying - Scale Cub mode (about 1/3 power) and Spitfire mode (any more). It was much easier to fly with the smaller lighter engines. The 61 is about the best I had had in it.

I also have a Hanger 9 quarter scale Cub (a wonderful kit to build). It has a Saito 120S in it. It is quite overpowered with this engine and will take off just above idle. On its first flight it actually tried to take off while I was taxiing.

Cubs have lots of wing area. They are floaters. They do not need a lot of power to fly. Of course, a Clipped Wing Cub is intended to be aerobatic and a bit more power will help in one of these. I am building a 1/4 scale SIG Clipped Wing Cub and it will probably have my OS Gemini 160 in it. Total overkill but it should sound nice.

I don't know whether you have flown a Cub before but if not, there are a couple of things to consider. They float, as I have said, and as a result they can be a bit tricky to land if you have too much power and a big prop and your idle is not low enough. Secondly, they have such long wings that they need rudder to be used in turns or they can suffer from "adverse yaw" effects. I have mixed some rudder into my ailerons for the big Cub and that makes landing easier. Differential aileron helps too - lots of up but not much down aileron.

Enjoy your Cubs. I do.

Michael from Oz

Hi Guys

Many thanks for all the information, I will probably go with the 70.

Once again many thanks

I agree with the .70 size four cycle on a Cub of this size. Your biggest problem is that OS no longer makes the .70 Surpass, but only the .70FL. I've not had first hand experience with the .70FL but it has gotten a bad reputation as a 'cheap' four cycle with some of the guys I fly with. You might want to look at one first hand or talk with someone who owns one before you buy the FL. The rest of the Surpass series is great if you decide to go with a different size.

Edited to add:
I just noticed you are in the UK. The .70 Surpass is no longer sold in US markets, but may still be available where you live.

The FL 70 I had was fine in a large trainer. It was not as powerful as the 70 Surpass but it was quite a bit lighter and light is generally good. Yes it is a simpler cheaper engine but mine ran fine and would be fine in the Cub you are talking about. But then again, so would a Saito 56, 62 or even 72. I am really enjoying my 56 and 62 at the moment. The 56 is about 6 years old and going strong and the 62 a couple of months old and getting better all the time. And if you actually need some weight up the front, the old (and rather heavy) Saito 65 was very popular in the Goldberg Cub which is about the same size as you have.