how fast will my Katana go?

I am trying to coaless everything I have been reading but not havin
much luck. I bought a Katana from Chief Aircraft and I am trying t
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 tak
a stab at it. The current record is 127mph. I have been playing aroun
with the Thrusthp program
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now I am confused! Although that?s close to my normal state I a
hoping for some help from those more well versed it this subject tha
I. So just how much power (in hp) will it take to get this plan
moving? I am hoping to see 130 to 150 mph. Maybe that is too fast fo
this plane. I don?t know. It seems to be very well constructed and i
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
· 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 bea
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 wa
thinking high rpm (18000), small diameter prop, nearly 5hp in a small
light engine. But according to the ThrustHP program I would end up wit
an 11x8 prop. That only extends past the fuselage by 1.5 inches on eac
side. Does not seem like a workable situation to me, plus it look
silly! ThrustHP tells me what the static thrust would be for a give
combination would be (13.42lbs) but I don?t understand the connectio
between static thrust and max speed. The program does list a speed o
136 mph but I don?t trust it because there are just too many things i
does not take into account like the width of the fuselage. DRAG i
other words.
I have seen some discussion on static thrust to weight ratio and I ca
make sense out of it as it applies to hovering your plane but how woul
it be applied to max speed? It does not take drag into account
Everybody knows slippery airplanes go faster. I have heard of thi
inverse square law as it applies to hp vs. speed now I need specifics
Back to the original question, what engine/prop combination wil
produce the results I am looking for, a speed of 130 to 150 mph. I wan
to know HP, RPM, Diameter and pitch. Perhaps a spread sheet o
simulation is available
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I would have thought that if you are into breaking records, you would start with the engine, choose the prop that gave max thrust, then bolt it on to a least drag fuselage. No expert on this stuff, but it doesn't seem right to take a stock engine and stock fuselage as a given, then try to find a prop that's then a compromise. I just can't imagine Williams bolting in a 1000BHP engine and making do with six inch wheels because thats all the monocoque would allow...
Hope this helps
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The problem here is slightly more complex. You can set the motor up for maximum static thrust but what is important is getting maximum thrust at the top speed.
A fixed pitch prop is always going to be a bit of a compromise. It will be over pitched at take off and underpitched at top speed.
I'd start off by finding what gives you maximum thrust on the bench and then fly it and clock the top speed. I'd then start upping the pitch until you either can't get airborne or the top speed starts to drop.
Don't worry unduly about a lot of the prop being shielded by the body. Not very much thrust is produced by the centre of the prop arc. It's the outside that is turning faster. Later you might want to experiment cutting down a larger prop as this will give you more pitch at the outer edge (the pich reduces towards the tip to even the trust across the disc. If you really are suffering with the body shape this could help.)
Whatever, a bit of experimentation is going to be the only answer.
I can't think that your club record will be too difficult to break. I've got helicopters that are not all that far behind.
It might be worth you picking the brains of the ic boys on RC Groups:
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Have fun
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Classic Sales
PS: I wouldn't go with the DF motor. Use the standard aero version.
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Classic Sales

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