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Propeller recommendation for Suped Up Parkzone P-51 BL

#1
I just completed my build of the old-school Parkzone P-51 BL. This was my very first plane (was not a good choice for my first plane, but learned fast!)....with the help of a lot of glue.

Found the airframe kit on Amazon and decided to make it a speedster. I put in a Power 10 motor, 40 AMP ESC, and run a 4S 2200 battery.

I just flew it last weekend and was a blast (such a good plane). It has a lot more speed, but I think I can find a sweet spot by swapping out the stock propeller which is a 9x6.

From what I've researched, pitch (the second #), in this case "6", translates to top speed and the diameter "9" translates into thrust or power/acceleration, especially at slower speeds.

I need advice on what would be an even potentially faster propeller on this? I'm more interested in top-speed. Think I can hit around 80-90 MPH with the right prop. I went and bought a 8x8 APC prop and was thinking a 9x8 might be great too. Just afraid of over working my motor, etc.

Any ideas, much appreciated!

Chris
Carlsbad, CA
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#2
Propellers are a little more complicated than what you have expressed. The twist in the blade is ever so important as is the shape of the blade.

Slow fly props tend to have broad, large area, blades and a lower pitch whereas Speed props have narrow blades with even tips angled backwards to the direction of rotation in some.

When the prop is spinning at low speed the outer half provides the majority of thrust and the iner half at high revs. I can buy 2 props with the same numbers and get 2 very different results. Running a "standard" prop at very high revs can result in blade failure, (Experience:rolleyes:).

Look for high speed props first (They generally have the thickest central section blades because of the leverage exerted at high speed and thrust), and also have the broadest part of the blades close to the hub.

Remember the pitch rating is an average as manufacturers try to make the prop suit as range of speeds and above a prop is just a compromise on a model because we do not yet have the benefit of variable pitch props.

Perhaps someone else can quote a prop type or manufacturer for you to investigate or evaluate.
 
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Craftydan

Hostage Taker of Quads
Moderator
Mentor
#3
Chris,

Looking at the power 10 on ecalc on 4s (FYI, it's not spec'ed for 4S . . . not that that really matters), On an APC electric 9x6 you're running a little high on current -- just over 34A. It can handle 42A bursts so that's not a big deal if you're careful with your throttle management.

ECalc estimates the pitch speed (The speed the prop would fly the best at) at 75mph -- if you're hitting that, your prop is holding you back. If you're not, it's the drag (not enough thrust) holding you back.

An 8x8 pushes you up to 102mph pitch speed for 30.31A WOT.
A 9x8 pushes you up to 96mph pitch speed for 41.5A WOT. (the lower pitch speed is because the motor is starting to bog down)

. . . which sounds like a no-brianer win, until . . .

Both the 8x8 and 9x9 props will stall at fairly high speeds. How high?

The 8x8 prop needs a 34.1mph airspeed before the prop will come out of stall in WOT.

The 9x8 prop is better, but still needs 24mph airspeed.

Either is a recipe for trouble. Not disaster, but trouble. You'll have to gently coax the plane into the air and she will scream into a torque-roll like mad banshee at low speed if you hit WOT.

With the exception that you may never get up to flying speed on the 8x8, All the numbers look good. Lower current draw, similar thrust at higher airspeeds, higher top speed . . . it's just getting there that's the problem.
 

Konrad

Posting Elsewhere
#4
I assume you are talking about this ship.
http://www.parkzone.com/Products/Default.aspx?ProdID=PKZ1880
Are you aware of this predictive on line program?
https://www.ecalc.ch/motorcalc.php

I might be mis-understanding Hai-Lee, but as a general rule the last 20% of the blade produces 80% of the thrust, this is regardless of the rotational speed of the propeller.

Thrust is just how fast you can move a certain amount of air, mass times acceleration. With propellers you can get the same thrust moving a little air fast or a lot of air slowly.

I assume your motor is around 1100KV so with a 3cell and an 8” pitch your pitch speed will be about 80mph max.. Then you'll have to factor in the airframe drag and I suspect your top speed in level flight will be around 65 mph to 70 mph.

Here is a simple formula to help you see some of the basic relationships of the propeller to power.
Power = K x P x D^2 xRPM^3

“K" is a constant that addresses things like air density, blade shape, number of blades etc. More advanced formulas can extract these variables out, but for our simple 2 bladed props a “K” factor of 1.11 to 1.31 works.

But what I’m trying to show is the relationship of the major propeller parameters. Notice that power is a square function of the diameter and a cubic function of RPM. This means that it takes a lot of power to spin a propeller just a little faster. And as Speed (pitch speed) is just “P” times RPM you need to cut down on the prop’s diameter if you want to increase the pitch speed and still stay within the power rating of the motor and ESC.

On the subject of speed are you aware that the drag on the airframe goes up by the cube of the airframe’s speed? So to go twice as fast requirers eight times the power!

I fear that turning an 8x8 prop with a 1100 KV motor on 3 cells is taxing your stock 18 amp ESC. As it looks like the motor will be drawing close to 20 amps.

A note on speed. It really is boring to just go fast. I know as I use to make engines for, and race, FAI F3D Pylon. The skill and fun of RC flying is putting the aircraft where you want it.

To go around 100mph plus on an 8x8 prop I need to use something like this.
http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFL4205
http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFLM4025B
http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFLB25004S30
http://www.e-fliterc.com/Products/Default.aspx?ProdID=EFLA1060B

I hope this helps.

All the best,
Konrad

P.S.
Sorry I see you are not using a stock set up (4cell). As Craft Dan is saying it can be trouble. But for ROG flights a stalled prop isn't too much of a concern. Even if you lift off before the prop is fully hooked up you will still have enough thrust to over come the airframe drag at these low speed. Just don't expect the best climb rate. On most Pylon racers the prop doesn't hook up until long past the first pylon.
 
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#5
CraftyDan,

Appreciate the detail - makes a lot of sense. I may try then a 9X7 to see what sort of incremental speed I can get without sacrificing too much control at the slower speeds.

Thanks again!

Chris