Re: Propeller dynamics

Cox International Wed Sep 14, 2011 2:37 pm

I ran that prop on our throttled "RC Flyer" and the engine idled @ aprx. 5,000 RPM.

Bernie
www.coxinternational.ca

microflitedude Wed Sep 14, 2011 3:01 pm

Cox International wrote:I ran that prop on our throttled "RC Flyer" and the engine idled @ aprx. 5,000 RPM.

Bernie
www.coxinternational.ca

That's pretty impresive!

SuperDave Wed Sep 14, 2011 3:19 pm

Kim wrote:6x3's and 5x3's do it for me in the .049 department...although I DO have a ridiculous 4.5x4 that I got from Bernie

Let's fair with Bernie here. 4.5X is a prop for a Pee Wee .020 not a .049.

Forgive Kim Bernie for he knows not what he is implying. Cool

SD

Cz10 Wed Sep 14, 2011 3:24 pm

Check out the testing I linked above. It seems a 6 x 3 gives the most thrust from a ..049, but there are great differences between different 6 x 3 props.

There is also a very good explanation somewhere over there on another thread on why a 3 blade is less efficient then a 2 blade ( and why a single blade is the most efficient. It has to do with the blades being in the turbulence of the blade proceeding it. 6 x 3 seems to do it for .061 and 7 x 3 for .074. I'm gonna go out on a limb here and guess that. 8 x 3, possibly cut down to 7.5, might be right for .09

SuperDave Wed Sep 14, 2011 3:30 pm

On that note, engine perforamance is best judgeg in flight rather than on a test stand. The varibles are far greater in flight.

SD

Cox International Wed Sep 14, 2011 3:56 pm

Actually, Dave, that is an OEM Cox prop made for 049 engines.

Aside from the 5-40 screw hole the prop has stamped on the hub "Do not use on RR1 - Space Bug - Thermal Hopper". Back in the old days Cox used a white Nylon that was not strong enough for high RPM's.

We had the props moulded on the (very old) OEM Cox mould and made them in the grey competition series as well as in this revolutionary yellow compound. The prop is nearly indestructible and the blades can be bent, not 90 degrees, but a whopping 180 degrees without breaking.

Bernie
www.coxinternational.ca

nitroairplane Wed Sep 14, 2011 4:10 pm

SuperDave wrote:
Kim wrote:6x3's and 5x3's do it for me in the .049 department...although I DO have a ridiculous 4.5x4 that I got from Bernie

Let's fair with Bernie here. 4.5X is a prop for a Pee Wee .020 not a .049.

Forgive Kim Bernie for he knows not what he is implying.

SD

No Dave that is a .049 prop the .020 prop has 2" of pitch but this one has 4" of pitch.
Like I have seen .061s running props this size.

nitroairplane Wed Sep 14, 2011 4:12 pm

Cox International wrote:Actually, Dave, that is an OEM Cox prop made for 049 engines.

Aside from the 5-40 screw hole the prop has stamped on the hub "Do not use on RR1 - Space Bug - Thermal Hopper". Back in the old days Cox used a white Nylon that was not strong enough for high RPM's.

We had the props moulded on the (very old) OEM Cox mould and made them in the grey competition series as well as in this revolutionary yellow compound. The prop is nearly indestructible and the blades can be bent, not 90 degrees, but a whopping 180 degrees without breaking.

Bernie
www.coxinternational.ca

That's cool Bernie but how do you bend a prop 180 degrees?

fit90 Wed Sep 14, 2011 4:22 pm

I haven't seen any mention of the engine's torque curve, HP curve, their relationship, the type of flying the plane is used for or matching the prop to the airframe/engine combo. Personally, I like two channel planes that go faster than I can control. As a result, I like the shorter diameter higher pitched props such as a 4.5x4. While this prop may not deliver a great deal of thrust on an engine operating between 13-17,000 RPM, it can deliver a relatively great amount of thrust when operated at 25-28,00RPM. And yes, a hopped up Tee Dee .049 will turn that propover 25,000 RPM. Plus, to boot, it moves the little plane around at a respectable speed.

If you want thrust and/or H.P. go with a low pitch prop. If you want speed you increase the pitch. Until we develop a practical contrallable pitch prop for our use we typically have to settle for one or the other or just a nice middle ground.

My two cents,

Bob

Carl Wed Sep 14, 2011 4:32 pm

Cox International wrote:Actually, Dave, that is an OEM Cox prop made for 049 engines.

Aside from the 5-40 screw hole the prop has stamped on the hub "Do not use on RR1 - Space Bug - Thermal Hopper". Back in the old days Cox used a white Nylon that was not strong enough for high RPM's.

We had the props moulded on the (very old) OEM Cox mould and made them in the grey competition series as well as in this revolutionary yellow compound. The prop is nearly indestructible and the blades can be bent, not 90 degrees, but a whopping 180 degrees without breaking.

Bernie
www.coxinternational.ca

i have a few of those white nylon props; i even last used them just about 5 years ago! ill have to check out those yellow ones, are they going to be the same as the old yellow ones that used to come with models back in the 70s?

Carl

Cox International Wed Sep 14, 2011 4:34 pm

That's cool Bernie but how do you bend a prop 180 degrees?

Just hold the prop by the hub, grab a blade at its tip and bend it until it faces the other direction lol!

nitroairplane Wed Sep 14, 2011 4:42 pm

Oh snap,
Or rather bend.
I just imagined the hub would get in the way.

Cz10 Wed Sep 14, 2011 4:46 pm

There is something rolling around in the deep recesses of my brain about "square props" for speed. Pitch = diameter. I know I ran 8 x 8 on a .35 for CL combat, and I am thinking it was close to square on a TD fir 1/2A combat... Like maybe 5 x 5 or 4.5 x 5... Too many rotted brain cells.

nitroairplane Wed Sep 14, 2011 4:54 pm

Cz10 wrote:There is something rolling around in the deep recesses of my brain about "square props" for speed. Pitch = diameter. I know I ran 8 x 8 on a .35 for CL combat, and I am thinking it was close to square on a TD fir 1/2A combat... Like maybe 5 x 5 or 4.5 x 5... Too many rotted brain cells.

I run an 8 by 8 on one of my .061 diesels it loves it over 11,000 rpm

Cox International Wed Sep 14, 2011 5:28 pm

[img]

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This is what I meant Smile

Bernie

AT1984 Wed Sep 14, 2011 5:54 pm

Here's my knowledge of props for Cox engines:

I was told the best prop for a TD .09 is a Gray Cox 7X3.5...I found two at a swap meet, and snatched them up. I guess they are hard to come by?
That's how much I know...
lol!

Allen

SuperDave Wed Sep 14, 2011 6:50 pm

fit90 wrote:I haven't seen any mention of the engine's torque curve, HP curve, their relationship, the type of flying the plane is used for or matching the prop to the airframe/engine combo. Personally, I like two channel planes that go faster than I can control. As a result, I like the shorter diameter higher pitched props such as a 4.5x4. While this prop may not deliver a great deal of thrust on an engine operating between 13-17,000 RPM, it can deliver a relatively great amount of thrust when operated at 25-28,00RPM. And yes, a hopped up Tee Dee .049 will turn that propover 25,000 RPM. Plus, to boot, it moves the little plane around at a respectable speed.

If you want thrust and/or H.P. go with a low pitch prop. If you want speed you increase the pitch. Until we develop a practical contrallable pitch prop for our use we typically have to settle for one or the other or just a nice middle ground.

My two cents,

Bob

fit90 If you go back to this thread's beginning, I mentioned those very factors: torque, torque curve etc.

SuperDave

SuperDave Sun Sep 18, 2011 2:54 pm

Yes, Jacob, I'm here. Smile

SD

**Admin** Sun Sep 18, 2011 3:05 pm

SuperDave wrote:Yes, Jacob, I'm here.

SD

Alright, our hobby shop has some wood 6x3 Zinger props, they ordered those because they ran out of the cox 6x3 props.

SuperDave Sun Sep 18, 2011 4:05 pm

I ordered a Zinger 6X3 and found it not to my liking being spoilded by the revered Tornados of old. Of course if Zinger is your only available wooden prop, use them until find something better comes along.

BTW, My Tornados are NFS, thank you.

SD

**Admin** Sun Sep 18, 2011 5:03 pm

Oh I figured you were not going to sell them! A score like that is meant to be kept! I have not actually tried the Zingers yet, I only remember seeing them up there the last time I was there. Actually I have a funny story about trying to buy a Zinger wood prop. I usually only run my engines on test stands and sometimes home made prop rods, snow buggies, etc. I don't fly much at all.

About that story. I was up at the rudest hobby shop around when I was checking out with some things and one of those 6x3 props when the guy behind the counter said "do you have a prop balancer and know how to use it"? I said "no", because I really don't and I have always just used a pin if I thought I really had to. The guy then said "well then, I can't sell you the propeller because at those speeds, the thing will fly apart and hurt someone. You need to get a balancer before I can sell it to you". I just said "oh I don't care, I'm mostly a collector of those small engines anyway, it probably won't get used". And he continued to say, "can sell it to you". I paid for my other stuff and left. The only reason I still go there is because it is less then a mile away from my house.

nitroairplane Sun Sep 18, 2011 5:07 pm

Jacob I remember that story Smile

I can't believe it.
At least he was concerned
About your safety Smile

or didn't want the shop to get sued and
Lose his job.

SuperDave Sun Sep 18, 2011 8:15 pm

In the OLDEN days there was no such thing as a prop balancer. We flew props that came directly from the source unbalanced. I'd only be concerned if there were excessive vibration then I'd use the one that I have been GIVEN me by one of also-aging friends leaving the hobby for the nursing home. Cool

Ditto a laser incidence meter, digital tach, Hobbico DeLuxe Flight Box, 12V starter motor and more. My only guilt is that my friends aren't flying any more. Sad

SD

Mark Boesen Sun Sep 18, 2011 8:45 pm

Hello,

Back in the day, if a profile ukie vibrated more then normal, I'd rotate the prop 180, problem solved.

The problem with a perfectly balanced prop is that a typical 2c engine is not, but only at a certain RPM range.

Remember: Pitch for speed, diameter for RPM

My favorite 1/2a prop was the gray 'Competition' series props.

Mark

gcb Sun Sep 18, 2011 8:55 pm

There is more to an airplane than most realize. If you choose a kit or an RTF, many of these factors are already solved for you.

An airframe needs a cartain amount and type of power. For a small fronal area with low drag, generally used in racing events, you usually need a small diameter and high pitch prop. The pitch may need to be tempered for accelleration out of a turn.

For a large frontal area and higher drag, you will probably need a larger diameter and lower pitch prop.

Now for the engine: Engines have a power curve and torque curve. Most try to run them near the peak of both. With a bit of skill and experimentation, you can get maximum performance. This WILL NOT be obtained from a test bench.

There are things you can alter on an engine to get best performance for your situation. Most know about varying nitro, but along with that you can alter head shims, and venturi size...all to get the correct firing time for a specific prop. Thanks to these forums you can sometimes find info to get you in the ballpark. For those of us not in competition, ballpark is OK.

Be sure to write down your results because things like atmospheric temperature and humidity will alter results.

And for goodness sake, keep an eye on the ball socket. Cool

Ramblings over.

George

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