This is theoretically incorrect, Dave.

With the prop sizes that we use, 2" of pitch are the load equivalent of 1" of diameter, so a 14x6 offers the same load as a 13x8, 12x10 and 15x4 props. The RPM will be very close with all these sizes.

A 12x8 will give about the same RPM as a 13x6.

I think that as long as you are inside the engine's prop range you can take 1" diameter and add 1" pitch or viceversa, when you get out of that range, you start taking 2" pitch for each diameter.

Example:

TT91FS -> 11x10 to 13x9 (as the manual says)

11x10
12x9
13x8
14x7 <- we're out of the engines specified range, start decreasing 2" pitch for each diameter
14x6
15x4

I use the prop sweep to indicate volume loading and the results are as follows:

Load
11-10 950
12-9 1017
13-8 1061
14-7 1077
14-6 923
15-4 707

It is also very important to keep the engine in the power band.

Bill

Joe,


I was not referring to a specific engine and its recommended prop size range, but to a specific prop load.


A given engine should spin at a certain target RPM, to show good performance.

Let's take a certain sport .46 engine, with a muffler. It should be set to a ground RPM, between 12,000 and 14,200, where the higher RPM is for best power and the lower range is for best torque (thrust - 3D).

If we target a certain limited range; say 13,200-13,500 RPM, any prop equal in load to a 10x7 will be spun within this smaller RPM range. It could be an 11x5, 10.5x6, 11.5x4, 12x3, 9.5x8, 9x9, 8.5x10... With each of them, the RPM this engine is capable of achieving an RPM between 13,200 and 13,500.

Of course you can increase the load, but then you will get into a lower RPM subset. If you increase the diameter (choose any of the props in the above paragraph) by 1" and decrease the pitch by only 1", RPM will be down to 12K+, which is in a different bracket.
The engine will no longer be able to power this prop into the previous bracket.

And I am talking about props of the same design. If you use narrow blade props, in the new group, you may be able to stay within the higher bracket, but then it is still a prop with the same load as the previous design 10x7...


The general rule remains; 1" diameter is equivalent to 2" pitch.
Increase one and you will have to decrease the other by this ratio, to maintain the same load.

It wasn't me that invented this...

The problem with volume loading only, is that it ignores the greater frontal area & wetted area of the longer blades.

My personal experiernce with engines in the 40 - 90 size, & their associated props, is that 1" of diameter approximates 2" of pitch in engine load. This correlation is best demonstrated with engines that have relatively steep power & torque curves -- they are more sensitive to load, & the RPM changes are more pronounced.

A TT 42 GP is an excellent example. My 42 GP turns 13,000 RPM with an APC 10-6, 11,700 with a 10-7, 13,100 with an 11-4, 11,600 with an 11-5, 10,900 with an 11-6 & 11,100 with a 12-4.

It very nicely fits the 1"dia = 2" pitch correlation.

Hi!
14x6 APC is what I would use or maybe a 13x8 APC.
Master A is no good except the scimitar series that work a little better.

I'll give the manufacturers 2 choices, they can make the MAS as efficient as APC, or make APC look good [sm=devious.gif]

The noise that some of the larger dia Master Airscrews make when on a flyby is awsome. Like a 14-6 on a Super Tiger 90 or a 15-6 on a Y-S FZ91.

Enjoy,

Jim

You are not wrong! A friend and I have the identical same plane. He runs an MAS K-series on his and I run a MAS Scimitar. Although they fly at almost exactly the same speed (mine's a little faster), his just gives the impression of being *much* faster -- all because of the howl that the prop creates.

Spectators (and fellow fliers) love it.

The MAS/APC debate isn't really that important. Most of today's engines have more than enough power for the airframes they're attached to so it becomes an issue of wallet-size and personal preference ahead of raw performance.

That's true and I must admit when I hear it, it tugs on my heartstrings. A guy at the field has a TT .61 Pro with a 13x5 MA in an acro biplane - man does that thing "yowl" on a flyby!

Then I think "the energy consumed to make that sound is not contributing to the all important forward motion", which brings me back to "Mother APC" (or Bolly).

Yes, I love that sound too, and the white ring around the prop looks sweet!

The debate of 2" pitch for 1" diameter VS 1" pitch for 1" diameter...

Another factor which many people don't consider is the chord width. *Usually* when diameter is increased, the chord also gets a little wider, as in the case of APC. So APC 14x8 gets about the same rpm as 15x6.

I would like to point out that APC 13" prop is a thin blade sport prop, but 14 is the pattern profile that you see up to 20". So 13x8 APC will actually get higher rpm than APC 14x6.

As for Master Airscrew nylon prop, the tip flexes under load, so a 14x6 is more like 14x6-4. You will get very good rpm with MA 14x6 on TT 91, but the pull won't be as good as APC 14x6.

Agree. It is only an approximation, & as you pointed out, the approximation works for props by the same manufacturer & of the same geometric configuration. It is still a closer approximation than the 1 - 1 scenario.

I use the 1" Diameter 2" pitch rule too - there aren't too many surprises using this approach. Sean, you make a good point on the profiles of APC props. The "Pattern Profile" actually kicks in on the higher pitch 13", 12" and 11" props; for example, the 13x9 and 13x10 are "Pattern" whereas the 13x8 is "Sport".

I have found that loading through this "Pattern/Sport" divide is still fairly consistent with expectation, at least in the 13" diameter range.

OK on the original question here is the real answer from an old retired aerodynamicist. On the original question (airplane and engine combination) if you chose to use other than a 14/6, 15/6, or 15/4 you will suffer a condition known as "chronic unhappiness". When this occurs take a deap breath, sit down, open a cold beer (other than the USA) a cool beer, relax. Go back to using the 3 props mentioned and the " chronic unhappiness" condition will subside.


Denis

I have a better idea:

1.- Open a Corona and drink it
2.- Open another Corona and drink it
:GoTo#2 until I don't give a damn about "chronic unhappiness"

JOe - how about Tiger/Carlsberg/San Miguel? Hehehe ...

Gee dude, we don't have any of that stuff down here [:@] we only have:

- Sol
- Corona
- Victoria
- Modelo Especial
- Negra Modelo
- Moose Head
- Bavaria
- Heinekken
- Budweiser
- Bud Light
- Coors
- Miller GD
- Miller Lite
- Gallo
- Gallo Light
- Kloster
- Kloster Light
- Superior
- Pacifico
- Estrella
- Tecate
- Tecate Light
- XX (Dos Equis)
- XX Amber

I think I forgot a few

Now I just have to make a choice but I think i'll stick to some wine to make me sleepy

Forget the Carlsberg -- all will be well

Go back and read Sport Pilot post - 14x6 or 13x8 APC - I didn't know anybody still used MAS on anything but trainers -

I generally use the nylon ones on initial flights. Sometimes they actually perform better. The wooden MAS's will often perform better than other brands, but that is mostly slight differances in pitch in differant manufactures.

But after switching over to Bolly wood props, you realize who is the king...

Doubt it, the nylon ones are horrid. Had to cut a half inch of one blade to get one of them balanced. Don't buy that brand again.

Hugh,


I have only been dealing with them for over two years now...

The Clubman series? I always ream them to size and shave off all the sharp 'casting slag'.
After that, I have not needed to balance even one of them.

...And I do check each and every one of them, with a finger-tip balancer, before selling, or mounting them.
No returned samples either.

You must of encountered a 'quirk'


The woods are a real hit here, as are the C/F.
Most competition participants use them specifically, unless they have to endorse a hobby-shop, or something... who sell another brand...

I tried today a MAS 13x8-K but didn't fly (did it on the backyard), Engine gets to around 9.6K with some smoke, I'll fly it tomorrow with that prop and let you know what happens