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Old 07-16-2017, 10:17 AM
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Originally Posted by rhklenke View Post
Chris Gunner's son is flying an Extreme Jets Cougar (as stated above, basically the same airplane) on a Kingtech K45 engine - 9.9 lbs of thrust. It has a bit of a longer takeoff roll, but it flys *great* on that engine.

You do *NOT* need a 1-to-1 thrust to weight ratio in an aircraft to fly well - after all, the real Cougar had thrust equal to around 1/3 of its weight...

That being said, if you look at the specs of the newer 80N engines, namely the Bee 80 and Merlin VT 80, they are very close to the size and weight of the newer 60N engines like the K60. However, their fuel consumption is quite a bit higher. There are tradeoffs, but you don't *have* to have an 18 lbs thrust engine to fly this plane...

Bob
I appreciate your point of view.
I do have a couple points to make though. I try not to avoid speaking in absolutes, because it backfires. So, I never said you need 1:1 thrust ratio. And I never said you "have" to have 18lbs thrust to fly this plane. Even with an 80N engine, my ratio will be about 80% (not 1:1), which is a nice place to be especially launching on soft mat or grass. The surface condition can make a big difference. Perhaps I should have emphasized my biggest concern is the takeoff. It's all grass fields here. One in the air safely it will fly. But ya gotta get there, right?

Most of my comments were focused on the factory stating the plane's flying weight is 15lbs,which really can't happen. So if one is to make an informed decision on engine size, you need to start with an accurate flying weight, especially if the factory is willing to print one. If you feel comfortable flying a 21 or 22 lb jet off grass with 12lbs of thrust, go ahead. I'm betting most people wouldn't. When you buy any RC plane, the box usually has an engine choice range, like .60-.90 size, etc. Most people wisely shoot for the middle, or higher part of that range. So if you want to invest now $1000's in a plane and go the other direction, that's your choice. I'm betting most people wouldn't.

Though interesting that the original F9F had a very low power/weight ratio, that says nothing about how this RC F9F will fly on a small engine. Most RC planes have much higher power/weight ratios for a reason. Airfoil reynolds numbers go awry as wings get smaller and smaller and finally approach RC plane sizes. I think it's because air molecules don't scale the same as wing area. Also, drag (surface area) and weight are not linear, but are some weird cube function. So OF COURSE the the original F9F's power/weight ratio was low. So are most full scale planes compared to their scale RC counterparts. Also, they had lousy engines back then. But unlike RC jets today, back then they didn't have any choice in the matter like we do. Oh, and they also had 5000 foot concrete runways.

The saying goes, "the longer your takeoff roll, the narrower the runway".

I still am impressed with TopRC's and Caiman's willingness to fix this. And I still personally would recommend an 80N engine for this plane. Sure, get the lightest one you can find. Another good choice would be the new K70 from Kingtech, which has 70n power in the same weight package as the K45/K60. But the final specs are not in, and it's not shipping yet.
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