Engine decision
 

Okay, a few of you may have read my previous posts on deciding on an engine for my CMPro Extra 300s 90.. and i have come up with the following engine's to chose from, can people please comment on the different engines from either experience or know someone thats used one, and tell me which is the best choice. I am not looking at a 91 as i want to swing a 15*6/8 to 16*6/8 prop.

These are the engines i am looking at to power it: Thunder Tiger Pro 120 and the O.S. 1.08FSR. In order to compare them properly i will put specs of the OS91fx in there too, so you can see why i am struggling to decide.

Thunder Tiger Pro 120
3.5hp @ 15000
35.5oz

O.S. 1.08 FSR
3hp @ 16000
28oz

O.S.91FX
2.8hp @ 15000
19.3oz

The model is spec'd at 3.8 - 3.9kg's or 8.3 - 8.6lbs, and has a wing loading of 76-78g/dm2, and a wing area of 50/dm2. The thing i am worried about is to keep weight at a minimum with the best aerobatic power.. i guess the OS is okay, but the TT is quite a bit heavier than the OS but has a lot more power.. so is the extra 0.5hp worth the extra 9oz of weight in the Thunder Tiger? And furthermore is the OS 1.08 worth the 0.2hp in the gain of 8.7oz? It seems that the power increase doesn't do the weight any justice.. If you take the OS 1.60 for instance it only weighs in at 32.6oz and puts out 3.7hp, which is lighter than the 120 TT but is only 0.2hp up from the TT120 and its a 160?

But there is a big variance in weight between the models.. and given that they suggest an engine from .90 2c up to a 120 4c, then they are basically suggesting an engine weight of between 19-30 oz roughly that is if you take into account the weight of different brands of engines, but in stepping slightly over this mark, regardless of the extra power is it going to make it a pig to fly?? I don't want no million mile an hour landings cos our field isn't really that forgiving for high speed landings. However i understand that usually Extra's and Cap's usually require a little extra weight to balance, is it usually in the tail or nose that they need the weight?

Please help! James.

RE: Engine decision
 

James:

The horsepower figures given by engine manufacturers are pure hype. They bear no relation to reality. I have the TT 120, the OS 160 and the OS 91, and I can tell you that in the real world, the OS 91 and TT120 are very close in usable power. The OS 108 (which I have flown in the past) is a torquey old chunk and will pull a big prop better than either of them. The OS 160 is in a completely different category. It probably has 50% more usable power than the TT120.

BTW, the 91FX prefers a 14x6, the TT120 likes a 16x6 and the 160 turns a 17x6 so fast the prop tips go supersonic. Here in the US, the 160 is clearly the two stroke of choice for 120 size aerobats. (All props are APC)

You might also want to consider the Super Tiger 140 (2300). It offers a higher power to weight ratio than even the 160 and turns a 17x8 close to 10,000.

Don't be overly concerned with the weight. In reading the threads it sounds like your Extra has a tendency to come out tail heavy. You will hate it if you sacrifice power for weight and then have to add lead to the nose to balance the airplane.

Jim

RE: Engine decision
 

James:

I just did the math and now I see that your airplane is smaller than the usual 120 size.

Perhaps the 160 and the ST 2300 would be too much for that airframe. The 108 would be a great choice. It is an reliable as a stone, which is why it has been in the OS line-up for so long. If you want to look for a little more power, the GMS 120 is a big step up powerwise from the TT120 and weighs about the same.

The more I think about it, the more I think the 108 would be the engine for you. Use a flat pitch prop like a 17x6 or a three blade 15x8 Graupner and you won't overstress the airframe.

Jim

RE: Engine decision
 

Thanks buddy!! Thats the info i was exactly after!! I've had excellent info from everyone so far, that just had the little bits i wanted.. i wasn't going to go passed the 120 size two stroke as i think thats just way to much power, cos as you mentioned, realistically i think it should be a big 90 size plane rather than a small 120. I have narrowed my choices down to the TT120, Magnum 120 4 stroke and the OS 108. What do you think my best option from those three is? The prices are about $370 for the magnum, $400 for the TT120 and about $500 for the OS here in aus. I don't want the plane to be too quick on low speed flying, as hot landings are tricky at our airfield because of the lack of approach space to get it right. Thanks james.