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Author Topic: Propellers  (Read 1407 times)
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Hepcat
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« on: February 22, 2018, 08:52:28 AM »

In IndoorFF>Ministick>#126.  'Flyguy' was asking for comments on the shape of propeller blades. I have responded here because my comments are not limiited to Ministicks and indeed avoid any reference to the sort of blade shapes required to flex and change pitch.

 FlyGuy  I have listed below my thoughts at the moment but with no equations or test results to prove anything is best.
I take notice of Larrabee’s work on MIL propellers because he was dealing with propellers on light aeroplanes at low speeds and proved their worth by making the first successful man powered aeroplanes.  This amounts to putting most area in the centre of the blades and not at the hub or tip.
Square tips were utilized in the 1939-45 war to absorb the ever increasing power of the engines without increasing the diameter (which would have required new undercarriages) so that does not concern me. Some argue that a square tip has a better Reynold’s number but I feel that does not outweigh the reduction in drag of a rounded tip.
The root part of a propeller blade doesn’t do much other than to fix the blade to the propeller shaft. It is probably best hidden under a spinner, which works well on an outdoor model but is not often seen on indoor and I usually cut off 10-%-15% of the blade and just use a spar to join the blade the hub.
 Over the years I have wasted hours doodling propeller shapes but any preference is only based on ‘that looks about right’ and my mantra ‘if the chord and blade angle at three quarter radius is right the propeller will work’. Recently, to save dithering, I try to restrict myself to using a shape as in the picture below.  I have drawn this assuming you have CAD because the elliptical arcs are drawn with three ‘clicks’ of the mouse
There are just seven straight lines to draw. A horizontal one, CT which is the radius of the propeller from centre to tip. Draw vertical lines at 75% CT and 45%CT which locate the design chord and the widest chord respectively. The circle on the 75% line is the same diameter as the design chord and the circle on the 45% line must have a diameter of 1.2 times the design chord. Draw a vertical line where you want the root of the blade to be. I usually choose about 10% to 15%CT. Now all there is to draw is two semi-ellipses as I have done in green.  With my CAD program that is only three clicks for each quadrant (in my case:ABC,ACD,ADE,AEB) and I guess others will be similar.
John
 
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John Barker UK - Will be missed by all that knew him.
piecost
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« Reply #1 on: February 22, 2018, 01:14:26 PM »

John, I am always interested to read your musings about propellers. I wonder what your thoughts are about minimum induced loss pitch/planform that was pioneered by Larrabee? This gave the performance improvement that enabled a human powered aeroplane to cross the channel. It proved revolutionary for effeciancy improvement for a single flight condition. But it does not account for all flight conditions or all aero aspects of a real propeller. For model use; it sometimes gives highly tapered planforms where the tip chord may be so as to cause problems due to Reynold's effects.

Do you consider that rubber model propellers have a wider operating range than other types?
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OZPAF
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« Reply #2 on: February 23, 2018, 02:36:44 AM »

That is an easy and quick approach for many typical propellers John.
Thanks
John
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USch
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« Reply #3 on: February 23, 2018, 09:31:04 AM »

Thank you John for your treating of the ever present PP (Propeller Problem).

Now on this behalf I was wondering if your famous spread sheet PROP PICKER could be modified to be used for electric propelled models. Instead having a power input in terms of rubber weight and section using Watts or Amperes and Volts as input. I am sure it could be done but am unable to do the task.

Urs
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Flyguy
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« Reply #4 on: February 23, 2018, 11:11:21 AM »

Thanks John, always interesting to read these things. With respect to indoor props, I think there are some additional complications because they are flaring props, so that must have an effect on the 'optimal' blade shape, I wonder if the effects of shape on flaring have ever been examined? Probably not for human powered planes but maybe, I don't know. The good part is that I can always simply try these things to get some answers, but that takes a bit of flying time, but sooner or later.
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DHnut
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« Reply #5 on: February 23, 2018, 03:15:35 PM »

John,
        were you ever involved in the work on Puffin while you were at DH?  I liked your straight forward explanation of the factors involved because there is still art involved in prop design for models. I had this demonstrated to me recently when I changed from a home made prop to a Tern prop and there was a dramatic jump in performance on the same sized rubber. The same happened when a I changed a Hanger Rat prop and the model required a retrim. Both props were made to the plan but clearly there were variations in pitch. It is an area that we often do not pay attention to.
Ricky
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tross
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« Reply #6 on: February 28, 2018, 09:05:53 PM »

Thanks for that John.
Always good to see how you approach these things.
Very elegant. Smiley

I have a planform generator set up to give similar shapes,
but sadly my planes don't fly well enough to use them. Roll Eyes

TR
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Instructions: Step One...Assemble the pile of sticks shown in pic "A" to look like the model airplane shown in pic "B"........
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