Which propeller

[Michael Dearden]

I am building a JP Tiger Moth. The propeller recommended by them is a plastic (APC) 12 X 6. I think a wooden prop would look better on this classic plane so am intending to use one. However, I have "heard" that wooden props are less efficient that the APC ones so do I need to go up a siz, to a 13 X 6 for example? Are wooden props less efficient?

[Pete Rieden]

It probably is less efficient, but I'd try a 12-6 first and measure the current. It really depends on the characteristics of the motor and battery.

Be advised that wooden props are extremely fragile and can easily become a "1 per flight" consumable on a small taildragger flown from a grass field.

PDR

[Tim Mackey]

Depends on the make really...in my experience a decent beech / cheery wood prop is fine. I would not change sizes, and fly it to establish its performance. Alternatively, a little work with a paintbrush and some acrylics can make reasonable impression of a wooden prop.

Underneath the brown and cream paint is a GWS orange plastic slow fly prop.

[Robert Bromwich]

HI

If your after the affect then a wooden prop is fine i use them on some of my electric planes, i have one on the flair attila and that one i had to cut down to get the right amp draw. It works great ive had about 50 flights at the moment no problems

Sorry about the mess behind i was moving my stuff into the new room i had just finished

Rob

[Simon Chaddock]

Michael

A wooden prop can be just as efficient as a plastic one but they are a bit less tolerant to abuse.

Most Tigers had wooden props but for scale effect it is not the fact they were wood that matters (most were painted anyway, usually black) so much as their shape.(see here)

Air flow does not scale directly so a true scale prop will not be as efficient as the same size one properly designed for model use. It all depends on how scale you want your Tiger to be.

[Terry Whiting]

Michael.

If you are changing prop types and sizes, I would only do so with the aid of a Watts meter.  A standard  APC  is not as efficient as an  APC  E . Go for efficiency and get greater duration flying time. 

It looks as though Rob has reprofiled his wooden prop......  

[Michael Dearden]

Gentlemen. Many thanks for all the good info. I have just bought a Noedym watt meter and will be using it to check out the electrics. Not quite sure yet what it is going to tell me apart from the current drawn by the motor.

I think what would be a really useful tool is some sort method of measuring actual thrust produced so that one could compare the actual pull one was getting from a particular prop and at what current. Then you could really tweak things for optimum efficiency and therefore maximum flight times.

[Tim Mackey]

static thrust is pretty easy to test - just use a spring balance or scales  - but it does not necessarily tell you how well /poorly the model will actually fly. I often do static tests, and always user a whattmeter to test different props etc, but often find that the actual flight is the only real way to assess the suitability.

[Michael Dearden]

Quite agree there Timbo but surely if one can tweak props etc to give more thrust at less amps then one is going to get better flight times. I very rarely fly fast, I like to cruise around so after take-off usually throttle back as far as I can while maintaining full control (sometimes!!).

[Pete Rieden]

Static thrust tells you nothing other than what the performance will be like during prop-hanging. Any attempt to optimise props for static thrust simply favours lower pitch props (in the same way that your car will always have more traction in the lower gears). If your prop pitch is too low then you'll have to run higher revs for a given airspeed, and higher revs means more frictional losses in the motor and more aerodynamic losses in the prop, so as a measure the static thrust figure is best ignored.

If you want to optimise efficiency then you need to do it under dynamic conditions, and the only real way of doing this is to fit a datalogger which monitors airspeed, current, RPM (or throttle setting) and altitude. You can buy model-type systems that do this, but be advised that getting meaningful data from airspeed and altitude sensors is extremely difficult - googling "pitot position error" should give an inkling of just how difficult this is even in the full size world where a lot of analytical and wind-tunnel data is available. The errors I'm talking about here are not small ones - they can easily be of the order of +200/-80%, that being the difference in calibration factor required for a probe depending on yaw and pitch angles alone...

PDR

[Nick Rigg]

Timbo - Interested to see the paintwork on the plastic prop but isn't it difficult to keep it balanced?

[Tim Mackey]

nah.... tis on very thin and to balance I just add a further coat as required at the tip.

[Michael Dearden]

Blimey! Is balance THAT critical that a dap of paint will affect it?

[Tim Mackey]

Definately. In fact, many people suggest the best way to balance a prop ( even on a larger IC model ) is to use a little nail varnish or similar on the lighter blade. If you use good quality magnetic balancer ( which I do on every model - no matter wat size  / power ) very small differences can be easily seen.

[Michael Dearden]

Wow I didn't realise that! Now I'll have to buy one of those! A "magnetic balancer"?? Where pray can I obtain such a thing?

And to think that I had to enlarge the mounting hole in my prop and just ran a bigger drill through it! Didn't seem to vibrate I must say.

[Tim Mackey]

Fifth one down the page.

This is the one I use, but there are several different types from several different suppliers 

[Former Member]

[This posting has been removed]

[Tim Mackey]

Yes, and to add that - many props will actually be held perfectly well with just 1 x end of the shaft attached to a single magnet which reduce friction even further. This particular model has no threaded cone - they are both just friction fit on the shaft.

[Ultymate]

As Timbo knows I fly large scale aerobatics and I always use clear varnish on the "light" blade to balance the blades then if the hub needs balancing I stick a strip of vecro with epoxy and coat that with more epoxy as needed to balance the hub

[Terry Whiting]

Ultumate, 

 hub balancing, thats a new one on me. What is your means of balancing a hub?, you have me really intrigued. 

[Former Member]

[This posting has been removed]