how fast will my Katana go? - RC Groups
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Dec 30, 2003, 10:53 PM
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Dancy's Avatar

how fast will my Katana go?

I am trying to coaless everything I have been reading but not having much luck. I bought a Katana from Chief Aircraft and I am trying to figure out the best engine prop combination to make this plane go fast. Our little club has a speed contest each year and I would like to take a stab at it. The current record is 127mph. I have been playing around with the Thrusthp program ( and now I am confused! Although that’s close to my normal state I am hoping for some help from those more well versed it this subject than I. So just how much power (in hp) will it take to get this plane moving? I am hoping to see 130 to 150 mph. Maybe that is too fast for this plane. I don’t know. It seems to be very well constructed and is very light. The specifications are:
· Span – 72”
· Fuselage – 8” wide 66.5 long
· Area 992 sq/in
· Weight 7.8 lbs with all gear except engine, mount, spinner, prop, fuel
· Wing root – 15.5x2 inches
· Wing tip – 8.5x1.125 inches
· Airfoil NACA 0014
It is balsa/plywood conventional construction. The wing has an I beam type balsa spar and is sheeted 100% forward of the spar and 30% aft. Covered in monocoat type covering.
I considered the OS engine 91 VR-DF, a ducted fan engine. I was thinking high rpm (18000), small diameter prop, nearly 5hp in a small, light engine. But according to the ThrustHP program I would end up with an 11x8 prop. That only extends past the fuselage by 1.5 inches on each side. Does not seem like a workable situation to me, plus it looks silly! ThrustHP tells me what the static thrust would be for a given combination would be (13.42lbs) but I don’t understand the connection between static thrust and max speed. The program does list a speed of 136 mph but I don’t trust it because there are just too many things it does not take into account like the width of the fuselage. DRAG in other words.
I have seen some discussion on static thrust to weight ratio and I can make sense out of it as it applies to hovering your plane but how would it be applied to max speed? It does not take drag into account. Everybody knows slippery airplanes go faster. I have heard of this inverse square law as it applies to hp vs. speed now I need specifics.
Back to the original question, what engine/prop combination will produce the results I am looking for, a speed of 130 to 150 mph. I want to know HP, RPM, Diameter and pitch. Perhaps a spread sheet or simulation is available.

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