DaveInLA

The manual recommends pretty much anything from 11x8 to 13x7 prop sizes, with 12x6 used for break-in. I want to use this for a 6.5lb Kyosho Zero. I'm looking for good power for loops and simple aerobatics, but I don't need blazing speed or unlimited vertical. What's a good prop size for my application?

submikester

My Feedback: (6)

I have a similarly sized Great planes spitfire with the magnum 61 4 stroke on it. I used a 10x8 prop and was pretty pleased.

Charley

My Feedback: (2)

I'd try a 12x8 to start with.

CR

DaveInLA

Ok, newbie question. I know what the numbers of a propeller's specs mean in a physical sense, but how do they affect flight? For example, I know that larger diameter typically means fewer revs for any given engine, but if you increase the thrust of the prop (ie, 12x6 vs. 13x5), does that make up for it? For example, how would a 12x6 prop fly with this engine compared to a 13x5? Thanks

Flyboy Dave

My Feedback: (21)

A 12-6 Graupner will give good performance as well as good looks.

scratchonly

Download "Aero Design Prop Selector" on google. My tests show it to be very accurate. Safe download.

NM2K

My Feedback: (14)

Very nice, thanks for the tip and the web page.

My Magnum .61 was not what I would call a super strong performer. It was acceptable with a 12x6 or 13x5, but anything with more load would be straining it a bit. Maybe others samples are different (everyone) from mine. On a small warbird, I too would be tempted to run a 10x8 or 11x7.


Ed Cregger

Harry Lagman

Dave, the first thing that needs to be established is the prop load, to determine the range of props to try out. Your .61 should be propped to turn somewhere in a band from the mid 9000s through to the early 10,000s on the ground at WOT. I would estimate that the following prop sizes (depending on the brand you choose and the prop design) will get you in this band: 11x8, 11x7, 12x6, 12x7. 13x4, 13x5. Within this range, in very simple terms, the bigger the diameter, the more static thrust and less straight line speed you will get.

Theorycrafting the best prop for the job depends on a lot of factors:

1) How aerodynamic (slippery) is the airframe? Does it need more thrust (more diameter/less pitch) to drag it through the air or is it aerodynamically clean enough to benefit from more prop pitch?
2) Is the airframe better when doing higher speed aerobatics (like the pattern guys do) or is it better when doing lower speed, higher thrust work, more like the 3D guys do?
3) Does the engine like to unload once in the air?
4) Just what exactly constitutes acceptable vertical performance and an acceptable speed envelope. Each individual will tend to have his own preferences for each of his models.

I recommend you buy a set of props in the size ranges above and just try them out. It's easy to get mired deeply into theories about which one will work best but at the end of the day, just going to the field with a range of props and trying them out can reveal many surprises and perhaps bust a few myths.

Start with a 12x6 and try other stuff out. Let us know how it goes.

submikester

My Feedback: (6)

Having already done this myself I can say that my best results on a spitfire were 10x8 or an 11x7.

Flyboy Dave

My Feedback: (21)

I submit that you won't be happy with the performance of the Magnum .61 FS....
no matter what prop you put on it. In my original post I thought () we were
talking about the .61 Magnum two cycle....( my bad )....

....put more motor in it.

FBD.

carrellh

Using ThrustHP
12x6 @ 10000, 6.2 thrust, 57 mph
13x5 @ 9500, 7.7 thrust, 45 mph
11x7 @ 10500, 4.8 thrust, 70 mph
My brother has a magnum 61 four stroke. Using 15% nitro he gets a little over 10000 with a 12x6. I'm guessing the +/- 500 rpm going to the other sizes.

Flyboy Dave

My Feedback: (21)

....thank you carrellh ....

....I don't have any fancy charts, programs....or "Theorycrafting"....

....what ever that is.

Your examples:

Using ThrustHP
12x6 @ 10000, 6.2 thrust, 57 mph
13x5 @ 9500, 7.7 thrust, 45 mph
11x7 @ 10500, 4.8 thrust, 70 mph ....

....this averages out to 57.3 MPH, which is....in a word....pathetic. []

....put more motor in it.

FBD.

Harry Lagman

Theorycrafting: The process of tossing around arcane calculations based on real or imagined assumptions, often resulting in conclusions that are erroneous or contrary to those observed in actual tests.


Here's a good example:

Flyboy Dave

My Feedback: (21)

http://www.google.com/search?hl=en&q...=Google+Search

Hey, Harry....I didn't give the silly example of "Theorycrafting"....

....You don't have a Magnum .61, do you ?

Harry Lagman

Hey Dave,

So you think an aircraft with a 12x6, pulling 10k on the ground, will do no more than 57 mph, do you?

I just want to confirm this before my next post...

Flyboy Dave

My Feedback: (21)

[quote]ORIGINAL: Flyboy Dave

http://www.google.com/search?hl=en&q...=Google+Search

Hey, Harry....I didn't give the silly example of "Theorycrafting"....

....You don't have a Magnum .61, do you ?


/quote]


....I didn't hink so.

Flyboy Dave

My Feedback: (21)

....you can fool some of the people, some of the time....

....

submikester

My Feedback: (6)

Yeah, the 61 RFS is not a speed demon motor by any stretch of the imagination. My spitfire is not fast but it certainly is cool.

In the end I think it will end up with an old OS 61 SF I have laying around not doing anything. This is what I used in the first one of these I had.

Flyboy Dave

My Feedback: (21)

....I hear ya' on the power thing. I have a couple Magnum .52's, and they are sweet
runners for sure, but not very powerful....the 61's don't make noticeably more power
than the 52's....in fact the same 12-6 prop is used. Now, my buddy had a .61 in an
40 sized Ultimate bipe, and he pulled the engine out after a few flights because the
darned thing just didn't have enough power. []

The biggest complaint was the plane would lose altitude like mad in a turn from lack
of speed and power. He put a K&B .61 two stroke in it, and it was fine. I was going to
trade my .52's and get .61's....but the slightly larger engine just doesn't make any more
power.

Now, I do run a YS .63 foustroke....and it made a difference by going from a 12-6 to an
11-8 prop, but that is a horse of a different color....and it runs on 20% nitro.

The little Magnum four strokes are really good running engines....but sadly, they just
don't make much power.

I don't have or use a prop calculator....but 10 thousand RPM on a 6" pitch prop would
probably be about 60 MPH or so....I don't know....it certainly would not be very fast.

FBD.

seanychen

My Feedback: (4)

Master Airscrew Classic 12.5x5. It will be the right load for the engine @ 10500 rpm, looks great, and fly scale speed at about 50 mph. For your Zero, you want the prop diameter to be large enough to extend past the cowl and blast enough air.

Richard L has the same airplane with YS 63 w/ Graupner 12x7 @ 12000 rpm, and it flies so fast it's hard to imagine it's a real airplane.

seanychen

My Feedback: (4)

OK Dave, this is not as straight forward as you think. Many factors affect the thrurst, including diameter, rpm, pitch, chord width, airfoil, and profile. From what I've gathered, I've found the following relation ships:

Thrust is proportional to RPM^2
Thrust is proportional to Diameter^3
Thrust is proportional to Pitch^1
Thrust is proportional to Chord^1
Speed is proportional to Pitch^1
Speed is proportional to RPM^1

However, you can't have it all, since for a given engine:
If you go up 1" in diameter, you need to reduce pitch by 2" to maintain the same RPM

The best thing to do is to find the rpm that yields peak power, and prop to allow engine to peak at that RPM.

Magnum 60 4-stroke is a good engine that's also pretty light. I think its peak power is about 10500~11000 rpm.

I don't have this engine, but based on my experience with other engines in this size range, I estimate the 12.5x5 will peak at 10500. It should give about 6.5 lb. of thrust, based on the thrust measurement I've done on similar size props.

The ThrustHP calculator has an error which assumes thrust to be proportional to Diameter^4, but does not account for pitch. chord, or profile. The only thing the ThrustHP does correctly for is speed, which you can easily calculate by hand:

10500 rev/min x 5"/rev x ft/12" x mile/5280inch x 60min/hour = 49.7159 mile/hour, about 50 MPH.

If you want the airspeed to be faster, figure out what MPH you want, and back calculate the RPM and pitch you need. Based on the RPM and pitch required, estimate what diameter you need to drop to to achieve the RPM.

DaveInLA

57 mph might be "pathetic" to sport fliers, but it's certainly more than scale, and that's good enough for me. I'll try 12x6 first and see how that goes.

Anyway, re: the 12.5x5 prop putting out 6.5 lbs thrust, I know that's all just theory, but wouldn't that mean that theoretically, this engine and that prop would be able to hover this plane?

w8ye

My Feedback: (16)

I had two of these engines for several years and I always ran a APC 12-6 and never gave it much thought.

seanychen

My Feedback: (4)

This estimation is more than just theory. This is an adjusted estimation based on my other "real" thrust measurements:

APC 13x4W @ 11000 rpm = 7.5 lb

Anyway, if the thrust is the same as your weight, you can get to a point of hover, but you will need more to pull out. This little Zero won't hover too well even if you have 2:1 thrust to weight ratio. The key to hovering well is to maintain in hover heading, and you will need very strong pitch and yaw authority. Yaw authority is something warbirds lack.

The 12x6 will be a good choice. I recommend the 12.5x5 MA Classic more because of the look, something that might matter to you.

http://www.masterairscrew.com/index....PROD&ProdID=29

NM2K

My Feedback: (14)

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Keep in mind that the projected airspeeds that folks have mentioned usually do not take into account the "slippage" that occurs in the real world.

Grabbing a number out of the air, that has worked fairly well in the past, we usually (we being my flying buddies and myself) used to factor in fifteen percent slippage when using the purely mathematically derived and projected airspeed. So, one would take the airspeed and multiply it by .85 (this deducts the slippage of minus 15%), providing us with a more realistic projected airspeed. Now you would be closer to 40 mph. Keeping in mind that most warbirds/sport models stall at between 25 and 35 mph, you can see that you only have a very small amount of wiggle room at WOT when running 6" of pitch, at its best.

This is why I agree with Submikester that a 10x8 or 11x7 will give you the necessary wiggle room to keep flying under high G turns, etc.

If you are a really slick pilot (I have no way of knowing), you can mess around just above stall with judicious and minimally applied amounts of rudder for turning near the stall point. But if you are the typical "Yank and Bank" R/Cer, you will promptly stall the model and spiral dive into the ground.

It is early and I'm not quite awake, so pardon me if I'm being too blunt.


Ed Cregger