SKYTRIBE
PROP EXCERPT
 


SOON - A CD that details step by step instructions on how to make your own prop from scratch

An excerpt from Dave's PPG book 'An Insight into Powered Paragliding' now also available in e-book format. Contact Dave to order your copy.

Because paramotors rely to a large extent on this rotating piece of material behind one's head, I thought it sensible to spend a bit of time explaining propellers theory and mechanics thereof. If one examines a propeller one will find an aerofoil shaped device, with a leading edge, a trailing edge and a camber, common to all aerofoils. The ‘front’ of the blade as we know it is called the camber face and back, the thrust face. As with a typical wing we find wing tip vortices where the high pressure area on the flat part of the blade tries to invade the lower area on the cambered part. Our propeller behind our backs ‘pushes’ us through the air by generating a horizontal lift force which we call thrust. The rotating propeller blade causes the static air pressure in front of the blade to be less than behind. This results in a forward thrust on the propeller blade which pushes the paramotor forward. When one considers the efficiency of a propeller blade, it is only that part between 60-90% of the tip radius that plays an effective part in producing thrust. So when a blade angle is listed it is that part corresponding to the 75% tip radius station.

As mentioned the prop size, diameter and pitch is very specific for the engine type and revolution. The ‘effective ‘ pitch or commonly called the pitch or helix angle is the distance in inches that the prop may be thought to advance forward through the air (in a helix motion because of the “twisted “ shape), in the direction of flight in each revolution. One must remember that because we are using fixed pitch props (with a given pitch setting) , there is really only one RPM setting that will deliver maximum efficiency. (Refer back to the engine efficiency graph). As we alter the pitch we either allow the engine to speed up or slow down (because of the drag produced by the prop) until the most efficient engine RPM is attained. It is possible to make your own, and we have done just that.......................

......Increasing the difference between the large and small pulley, we are increasing the torque. However as we increase the torque so must we increase the diameter and or pitch

of the prop to maintain the engine (and prop) in the required RPM zone. (refer to Fig 47 for more about torque) This means the cage diameter will usually increase resulting in an aesthetically unappealing design for many pilots, as the trend seems to be small compact units. Ideally longish props over small coarse pitched are not a bad idea and provide the advantages of span efficiency that have governed the sail plains throughout the world for so long. Lets look at this a bit closer. Suppose we had a reduction ratio of 2.5, a propeller 115 by 30 pitch, giving us 2500RPM on the prop and 6200 on the engine. If we now took the reduction ratio to 2,55 with the same prop, we may obtain 6300 on the engine, however theoretically the prop RPM may stay constant. This is because as the torque is increased the engine and not the prop ‘unloads’ it`s self, with this excess energy being absorbed by the prop. (Remember energy is never destroyed, it is always converted from one form to another).

If we now increase the ratio to for example 2,7, what happens now is ‘unloading’ again from the prop, resulting in a further climb in engine RPM. This is similar to driving a car up a hill in 4th gear. As one changes down (similar to increasing reduction ratios and increasing torque), we are able to retain the same car speed, however the engine RPM increases). When this happens on a paramotor prop, we obtain a concurrent increase in horse power and obviously torque. Because of the now much higher reduction ratio, the prop is not able to absorb all excess energy and thus NOW the props RPM starts to climb as well. Most props have an optimal operating range (Between 2400 -288), anything under this is out of the power band, whilst anything over this is inefficiency due to the fast blade tips. As the prop RPM climbs higher we increase the pitch, diameter, blade chord or a combination of all three. Effectively what this accomplishes is to increase the drag and maintain it within the power band.

Just remember not all props are similar just because they have the same pitch. One 27 pitch prop may be completely dissimilar from another because of the chord and or diameter. When changing props make sure you are aware of all parameters. This may be easily accomplished by using a rev counter and modifying the pitch and or length until the desired engine and prop RPM is attained. (Refer to REV counters in “Instruments & accessories”.)

Contact Dave

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Our 3 in 1 CD contains an
e-version of :
Assembly Hints- Build your own
Component Plans
and your FREE BOOK GIFT
- "An Insight into Powered Paragliding"
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