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Old May 03, 2005, 12:31 PM
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3 blade Prop for spitfire

Hi All
Is there a 3 blade prop you can use on the spit when running stock??? Or even a 4 blade :-) ???
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Old May 03, 2005, 01:44 PM
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There's a GWS 9070 three blade and a GWS 1080 four blade. The three blade looks good on the warbirds but is no better, and probably not as good, as a 9070 2 blade for pulling the bird round the sky on a C gearbox. I'd guess the 1080 is just too big unless you wanted to go to a D gearbox. You can get them from www.allerc.com in the US (also stock the special spinners) and I think Sussex Model Centre have the props over here, not sure about the spinners though.
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Old May 04, 2005, 12:20 AM
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I can order them also www.mygwsshop.com
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Old May 04, 2005, 11:03 AM
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I've tried the 9 x 7 3 blade on my Corsair, Spitfire and 109 and have seen no performance improvement. If you are belly landing as I do you will usually break one of the blades off at least 80% of the time.
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Old May 04, 2005, 12:36 PM
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Quote:
I've tried the 9 x 7 3 blade on my Corsair, Spitfire and 109 and have seen no performance improvement.
You would not see any improvement.

3 blade propellers are less efficient than two, and two less than single blade (yep there is such a beast). The reason has something to do with the blades travelling trough the wash of the previous blade.

So unless you are really into looks (and you cant tell when they are spinning anyway ) just use a two bladed prop.

Mike
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Old May 04, 2005, 01:17 PM
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The Corsair and Mustang all come with a B gearbox (at least mine did). I have installed, but not flown, the 3 blade onto the Corsair with a B gearbox.
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Old May 04, 2005, 03:05 PM
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David,

Although I referred in my post above to a C box you're right, and it's my mistake, the planes have B boxes. I've been trying to decide whether they fly better with a 9070 or an 8060 (GWS two blade) and subjectively I can't make up my mind - can any of the experts tell me what "the book" says should happen? I'm flying on a 2 cell LiPo which can deliver 20 amps if it has to (but it'll fry my esc!)

I keep reading how APC props are the biz but in the UK the only ones I can find are either "slowfly" or "competition". Can someone tell me which I need (don't mind shopping in the US) and do I need adaptors for use on a stock 350 gearbox output shaft?

Sorry to put your thread off track, Jason.
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Old May 04, 2005, 11:14 PM
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Quote:
Originally Posted by JasonParker
Hi All
Is there a 3 blade prop you can use on the spit when running stock??? Or even a 4 blade :-) ???

I run the 3 blade GWS 9070 on my Spit' and Corsair.Both are stock with C gears and 8 cell 750mah AAA packs.At full throttle,the only difference I notice over the 2 blade is the sound.The 3 blade has got a distinctive scale "chop" to it which covers up the whine of the gears.At slow speed fly-bys however,I notice I can creep along with a little less throttle and tip-stall airspeed is dropped due to the extra pull of the 3rd blade.The 4 blade 1080 will work with the old 7.2 volt version in C gearing,but,the 6 volt motor hasn't got the oomph to turn it.My limited experience not withstanding,I'd say give it a shot.

Waldo P.
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Old May 05, 2005, 10:46 AM
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Quote:
Originally Posted by pda4you
3 blade propellers are less efficient than two, and two less than single blade (yep there is such a beast). The reason has something to do with the blades travelling trough the wash of the previous blade.
The only reason to go with more than one blade is to reduce the diameter. If you want more thrust, you either go with more diameter or more blades. Diameter is usually limited by ground clearance or RPM. (Big prop at high RPM can get the tips close to supersonic speed, which is bad).

So, changing from a 9x7-2 to a 9x7-3 would be similar to changing from a 9x7-2 to something like an 11x7-2. DO that, and you need more torque to keep the RPM up.

Alternatively, you could change from a 9x7-2 to an 8x7-3 or a 7x7-3. Unfortunately, doing that gets the pitch and diameter even closer together, which creates other problems.

Your best bet is to change motors to get one that matches the 9x7-3 (or 9x7-4) nicely. After you do that, get the Fying Styro 9x7. It's available with 3 or 4 blade hubs and has replacable blades. It's an *excellent* prop for electric 1-1.5 lb warbirds.
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Old May 05, 2005, 10:53 AM
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Quote:
Originally Posted by Roger Lombard
I've been trying to decide whether they fly better with a 9070 or an 8060 (GWS two blade)
Without knowing the motors well, it's kind of a fuzzy question because those are actually very similar props in a lot of ways. In theory, the lower diameter makes the 8x6 spin faster than the 9x7, which could compensate for its lower pitch. The 9x7 ought to have a bit more thrust, but in practise it also loads the motor a lot more than the 8x6 does so it might not.

I expect it depends on how torquey the motor is-- big enough motor won't lug on the 9x7 and will do better with it. Small enough motor will lug terribly on the 9x7 and do better with the 8x6-- if it's bad enough it would even burn up with the 9x7. It's quite possible that your motor is somewhere in the middle so the peformance difference is hard to see.

Motocalc can give you a more precise answer.
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Old May 05, 2005, 11:12 AM
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A 3 blade prop will give you less thrust, and more speed, at lower amperage.

I use the Master Airscrew 9x7 3-blade prop on my GWS Zero. I have yet to break a prop blade.
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Old May 05, 2005, 11:23 AM
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And if nothing else, they look beter.
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Old May 05, 2005, 12:42 PM
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Quote:
Originally Posted by Red 61
A 3 blade prop will give you less thrust, and more speed, at lower amperage.
Disagree. Thrust vs speed is a diameter vs pitch tradeoff. More blades creates a similar effect to more diameter, which would generally give you more thrust and less speed.

Current is related to the combination of thrust and speed- you can trade thrust for speed without changing it.

Which is not to say that you'll never get more speed or less current out of a 3 blade prop-- it depends on the pitch and diameters you're using.

(Feel free to tell me to shut up and go sand the '38. (-: )
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Old May 05, 2005, 12:52 PM
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Thanks rdeis - perhaps it wasn't clear from my post- its a stock warbird set-up 350 with B box?

The GWS performance sheets don't give a figure for a 9070 on a 350-B on 7.2 volts or for a 8060 on a 350-C on 7.2 volts - all you can see is that on a B box a 9050 pulls an amp more than an 8060 and so a 9070's going to pull even more and if there's no clear advantage in perfpormance all I'm doing is wasting power/losing duration (and maybe brush life?). I can't get my head round the speed thrust thing - if I'm getting more thrust surely I go faster?
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Old May 05, 2005, 02:17 PM
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Quote:
Originally Posted by Roger Lombard
Thanks rdeis - perhaps it wasn't clear from my post- its a stock warbird set-up 350 with B box?
I don't know enough about the specifics of how the motor works, we'd have to run numbers.

Quote:
I'm doing is wasting power/losing duration (and maybe brush life?).
Quite possibly. If you can post the wing area and AUW I'll try to get you an answer from motocalc tonight. If you're up in the hills somewhere post your altitude as well.

Quote:
I can't get my head round the speed thrust thing - if I'm getting more thrust surely I go faster?
Up to a point, yes, but you can get to a place where the motor has all the thrust in the world but the airplane simply doesn't need it, and so it won't go faster. Let's see if I can give a good analogy....

If you're in a car, your tires are about 6ft around. If you spin them at 1 RPM then you're going to move the car forward at 6ft/minute. If you spin them at 2RPM, the car moves at 12ft/minute. That's pitch speed-- the amount of power you're spending to spin them (thrust) doesn't matter, only the RPM and diameter.

Now, if you put that same wheel on a 10 lb tricycle it takes a whole lot less force (rather, torque) so actually rotate the tire at 1RPM than it would if that wheel was on an 18,000 lb truck. That's thrust, and for that determination the speed is irrelevent. All that matters is the friction, which for a rolling chassis is largely driven by the weight of the thing.

You can see how keeping the RPM constant and either increasing the diameter of the wheel (pitch speed) or increasing the torque required to maintain than RPM (thrust) requires more power output from the motor.

In the real world, the prop slips in the air so the airplane won't be travelling at the pitch speed, but it's still the theoretical maximum speed of the airplane, simillar to when a tire spins on ice-- it's pushing the car toward its rotation speed, and the car will never get faster than the rotation speed, but it's not actually going that fast while the tire slips.

How close to the pitch speed you actually fly depends on how much thrust the prop puts out compared to how much drag you get from the airframe.

To exaggerate things for clarity: If you pick a larger motor that can maintain the same RPM, a rediculously big 36x4.7 prop will probably fly your Slow Stick at almost exactly the same speed as the 11x4.7 and a stocker-- but the 36x4.7 will still do it if you hang a big parachute off the back.

OTOH, you could ditch the parachute and switch to a 36x23.5 and (again assuming you can maintain the RPM) fly 5 time faster. Whee!

From here it gets horrendously complicated in a hurry because everything in the whole proplem is actually related to everyting else. It's interesting, but a bit out of scope, I think. There's probably something about it in the archives of the modelling science forum.
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