Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

91fx speed prop for GP F4 phantom?

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Old 01-04-2003, 01:49 PM
  #1  
Ragz
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Default 91fx speed prop for GP F4 phantom?

I have a 91fx on a GP f4 phantom. The plane weighs about 12lbs dry. Its heavy. I have flown this with an APC 13x8, but I want some more speed. What do you guys suggest that I put on this engine for extreme speed. I am using a regular stock muffler. This plane fly really well, although a bit of speed would add more realsim to this beauty. I have an APC 13x11 pattern prop...but I reckon that would be tooo much....How about a 11x11 or a 12x11 etc.

Ground clearance is also an issue...two 13x8's have chipped on slightly less than stellar landings.

Thanks in advance.
Anurag
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Old 01-05-2003, 04:06 AM
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Razor-RCU
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Default Hmmm

I would think the 12 x 11 would be the ticket of the props mentioned there... Maybe someone will post who runs this motor and give a tried and true combo.!

Take it easy,
james
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Old 01-05-2003, 05:00 AM
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Default 91fx speed prop for GP F4 phantom?

A try to put a good pipe on it...you won't regret it.
The OS is not a high revving engine, but quite powerful if piped.
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Old 01-05-2003, 07:31 AM
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Ragz
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Default 91fx speed prop for GP F4 phantom?

any ideas where I can find the right pipe for this beast?

Anurag
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Old 01-05-2003, 08:13 AM
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Default 91fx speed prop for GP F4 phantom?

Take a look Weston UK , the Genesis pipes are very nice.

Also, take a look at Just Engines , they probably are one of the most complete engines and accessories suppliers.

Of course this might not mean much to you seeing you're from India...I really would't know where to get stuff in India.
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Old 01-05-2003, 08:36 AM
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Default 91fx speed prop for GP F4 phantom?

What fuel and glow plug are using. What rpm is the 91 FX turning the 13/8 at?

Keep in mind the a 13" prop provides higher static thrust numbers than an 11" or 12" prop ever will. It's simple math. This heavy bird at 12lbs. most likely needs a lot a ground to get airborne? Much like the real pig...

If you want to go faster, first try increasing the pitch., Tune the engine with new prop and go for a ride. A tuned exhaust system would be very helpful in accomplishing your goals. Weston makes good stuff, check with a dealer for the right application. Mac's Products here in the USA has everything you need. Engine savvy is a absolute requirement when attempting to extract every last bit of power. If you don't own a tachometer, I'd invest in one.

http://www.macspro.com/
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Old 01-05-2003, 10:58 AM
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Ragz
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Default 91fx speed prop for GP F4 phantom?

Im using a home brew with 10% nitro and 20% castor (no synth). The plug is an O.S. no.8.

i got a tacho...i havent checked the rpms yet...will do that and revert. Is a 13x11 too much for this engine?

thanks for the info.
anurag
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Old 01-05-2003, 01:53 PM
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Ragz
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Default 91fx speed prop for GP F4 phantom?

I just cheked the above mentioned sites for pipes..and there are so many options. Can someone suggest the one for my application?

Thanks,
Anurag
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Old 01-06-2003, 04:35 AM
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Default 91fx speed prop for GP F4 phantom?

I got a .91 fx on an intruder 90 by world models. Got it for christmas just broke it in with a 12x9 prop, my plane is under 8lbs and it has unlimited vertical, I tried to hover a little today but it keep pulling out of it and is super fast with this prop, I have an 11x11 that I have been waiting to fly it with, I should be able to test out this combo Tuesday, I'll let you know how it goes.
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Old 01-06-2003, 10:55 AM
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Default 91fx speed prop for GP F4 phantom?

great...will await your results...i have a 13x11 and a 13.3x13.5 both apc...any idea how these might fare?

anurag
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Old 01-06-2003, 05:53 PM
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mugenkidd
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Default 91fx speed prop for GP F4 phantom?

The 13x11 may work just make sure that the engine is clocking over 10k max RPM on the ground if not then the engine is overloaded. I think the 13x13 would be too much for this engine. I'll let you know how the 11x11 does tomorrow.
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Old 01-07-2003, 01:48 AM
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Default 91 fx prop

The rule of thumb, or old wives tale, as I have been told, is that 1 inch in diameter equals about 2 inches in pitch, for load purposes. Thus, a 14x6 and 13x8 have about the same load, and are supposedly the ideal props for the 91 fx. Extrapolating from there, 12x10 and 11x12 would be a good place to start, if you wanted to add pitch to the mix, without adding load. The 12x9 and 11x11 would allow more rpms, which is also good. I suspect that a 13x11 would be too much prop for the 91.

Your results may vary.

Paul
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Old 01-07-2003, 06:28 AM
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Default 91fx speed prop for GP F4 phantom?

From what i have been told is that you multiply the dia with pitch...eg. 14x6=84. So load factor is 84... 13x8=104. Is there any truth to this formula?

Anurag
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Old 01-07-2003, 04:35 PM
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Default 91fx speed prop for GP F4 phantom?

Anurag,

I remember seeing a formula years ago that had you doubling or squaring the pitch and then multiplying it by the diameter. I don't remember it exactly.

I don't know how accurate any of these formulas or rules of thumb are, but a straight multiplication of pitch x diameter to give a load would seem to be less accurate, because it has 1 inch of pitch equalling 1 inch of diameter in load. I don't think that is the case.

Paul
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Old 01-07-2003, 10:59 PM
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Default 91fx speed prop for GP F4 phantom?

There's also a scale factor at work here...

Example: on a .40 engine you go from a 10" to a 11" prop, that is an increase in prop diameter by 10 %.
However, if you do the same on a bigger engine, say you go from a 20" to a 21" prop, that's only an increase by 5%.

If you look at the prop disc area this difference is even bigger (squared).

So just multiplying diameter and pitch is not really the way to go...

Ideas? Anybody?
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Old 01-08-2003, 03:45 AM
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Default 91fx speed prop for GP F4 phantom?

hey rags I tested the 2 props, I was getting about 11,100 on the 12x9 and 11,200 on the 11x11. Although the 11x11 had higher top speed it was hard to tell, the acceleration of the 11x11 was poor, the 12x9 provide a lot more pull. I was even able to "hover" my pattern plane upright for about 15seconds then pull out of it without loosing any altitude with the 12x9 and found it a lot more fun to fly with. If you were trying to haul around a heavy plane I would imagine the 12x9 would be a good way to go.
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Old 01-08-2003, 06:16 AM
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Default 91fx speed prop for GP F4 phantom?

thanks mugenkidd...i will order some 12x9's immediately...

anurag
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Old 01-08-2003, 01:44 PM
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Default 91fx speed prop for GP F4 phantom?

My solution to get more speed and power out of my OS .91fx was to replace it with a Jett .90!

I tried MANY different props with this motor, and none of them produced the kind of power I was looking for. I had this on a .60 Cermark F-16.

I put the Jett on a .60 F-20, and the flights are head turning!

Good luck.

Gary
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Old 01-08-2003, 02:06 PM
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Default 91fx speed prop for GP F4 phantom?

sounds like a cool idea..but expensive I will first try out a 12x9...then think abt the Jett engine idea.

Anurag
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Old 01-10-2003, 09:39 PM
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Default 91fx speed prop for GP F4 phantom?

Just flew my midget mustang a week ago with a 15 cc pipe. 13" length (glow plug to max dia. of pipe) 13x10. It was turning 11k on the ground. Nice smoke trail with 10% blend of castor/synthetic. 5 1/2 turns out on the high speed needle.

Plane flew awsome!!!!!! Took a while to calm down. But that magic comb cited above should get you close...
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Old 01-21-2003, 06:56 AM
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Default 91fx speed prop for GP F4 phantom?

Gary, How much does your cermark f20 weigh? is it any where close to the 12lbs my f4 weighs?

I am ready to order some props and would like some more advice on the assortments to order. These will be shipped to India, so ordering 1-2 doesnt make sense. I was thinking of 12x9, 12x10 and 11x11, 2 each to start with.

Can someone give me the exact model number of the tuned pipe that I should buy for this engine?

Thanks,
Anurag
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Old 01-21-2003, 01:54 PM
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Default OS .91 pipe

You need to call up Performance Specialties and talk to Dave Shadel, the owner and pylon racer. He has 2 different size Ultra Thrust mufflers for the OS .91FX. He claims it likes to rev and turns it 14,000-15000 rpm. Talk to him about the best props for speed.

I bought the longer muffler for the lower rpm range with a 14-6 prop and it definitely works.
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Old 01-22-2003, 11:24 AM
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Default 91fx speed prop for GP F4 phantom?

Where can i find performance specialties contact details? any website?

thanks
anurag
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Old 01-22-2003, 01:56 PM
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Default 91fx speed prop for GP F4 phantom?

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Old 01-22-2003, 11:46 PM
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Default 91fx speed prop for GP F4 phantom?

Anurag,

My F-20 is around 8lbs, how come your is so heavy?
Is there anything you can do to lighten it?

I find it hard to believe the Ultra Thrust pipe is going to get the 91FX to 15K. If it does, my guess is, it wouldn't last too long.

Why risk the cost of the pipe, AND the chances that it may blow up on you, PLUS you'll also void any warranty by the MFG.


If your ultimate goal is to increase your speed, then lighten the plane, and get yourself a high performance motor.

I've been there, done that....many times....trying to get these production motors to turn some rpm's without blowing the head off.. All it did was cause me aggravation.

I think I paid $360 for My Jet 90L. No tax, free shipping.

Now add up the OS 91FX, tune pipe, and risk factor...it don't add up to me.

Gary
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