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Old 09-11-2011, 04:47 PM   #11
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Re: Which prop size to use for windy days

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Merv, your analogy explains my observations with some of these props.

Now, since I fly an aerobatic plane, I prefer the 11 x 7 prop; others transform the same plane to a 3D one and like the 12 x 6. Would this make sense?

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Old 09-11-2011, 07:51 PM   #12
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Re: Which prop size to use for windy days

You got it!

There has been some work done with in flight variable pitch props (full scale use a version called constant speed props) but they are limited in usefulness due to the fact that you still can't change the diameter.

So we are stuck with picking one gear that kind of does the job but not efficiently in the full speed range. You have to sacrifice speed to get short take off/climb & vice versa.

In my opinion, the most neglected trim change in setting up a new airplane is trying different prop sizes. Most guys just slap on the prop that someone said was "right" & never try anything else. Some of the most dramatic performance improvements are gained here! It can turn a "dog" plane into "my favorite".
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Old 09-12-2011, 09:02 AM   #13
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Re: Which prop size to use for windy days

Lots of talk about wind but few responses about props for Bobby:

Originally Posted by bobmic View Post
I was wandering which prop is better for windy days for a small plane (43in wing span, ~800gr). Currently I am using an APC 12x6 with an Eflite Park 480 BL outrunner. Should I consider an 11x7 to have more speed in a windy day or it will not make a difference.

As mentioned I am using an APC 12x6 and also have a TGS Sport 12x6E from HK which I have not tried.
Making a suggestion for prop blindly on the forums won't do you any good, Bobby.

Wind or no wind, you need to know HOW your motor/prop combo will perform first. There is no point putting on a different prop that will overload your ESC/Battery. I do however agree with what others have been saying on the forums. You dont have to change your props on account of wind. Find the best prop combo for your power system and it will perform equally well on a calm day as well as a windy day. All you have to do is look at full size fixed pitch aircraft and see they fly just as well on windy days and calm days.

While it is true that a coarser prop will pull you faster, and a finer pitch prop will make tak-off's easier and provide braking effect for landing, it is imperative that you match the prop diameter and pitch to the power system.

With electrics, Data is everything. You need to be able to asses a motor/prop combo to get the one that works best with your motor/ESC/Battery. To do this, you need something that can measure power while you rev up the motor with different props.

The best way to do this is with a datalogger of some sort. The most popular one is the Watts-Up and similar devices that you plug in between your battery and ESC.


Then you run the motor on the ground and read-off what the current draw it. Having this kind of data is a real eye-opener:

I personally use the Eagletree Datalogger.


You can use it on the bench like the watts-up, but it can also be left in the plane and flown, allowing you to record and later download and analyze performance while in flight. For example, here is one I did on a friends jet:


You can see the data-logger allowed us to correlate throttle positions with in-flight performance allowing us to accurately predict flight time and battery strain.

Bench/flight testing different prop combos allow you to find the best prop that is a compromise between load applied to the battery and pulling power. That is really what Bobby was asking, isn't it?

Anyway, that's how I would do it.
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Old 09-12-2011, 01:21 PM   #14
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Re: Which prop size to use for windy days

Thank you Max,

I have a wattmeter and this was the first test I did to make sure I am within the Motor and ESC capability. The data logger is a completely different ball game though but I think it is a bit of an over kill for my plane and flying capability.
I got the idea of just trying a couple of props and see which one gives the best performance for me.
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