|GM F5J 28mm/6mm Spinner Specification|
|Motor shaft||6 mm|
|Distance across hinge pins||24 mm|
|Prop hinge pin diameter||3 mm|
|Prop blade root width||8 mm|
Review by: Roger Sanders
As a mechanical engineer and machinist, it is clear to me that GM not only makes spinners that are works of art, they are the most aerodynamically clean, precise, best designed, most beautiful, and lowest drag spinners available. GM pays attention to all the little details that make a truly outstanding product.
For example, unlike most other spinners that use a grub screw to mount the spinner yoke to the motor shaft, GM uses a collet. A grub screw forces the spinner to one side of the prop shaft, causing it to run slightly off-center and wobble. A collet assures that the spinner will be perfectly concentric and run absolutely true, which reduces prop vibration and assures that the edges of the spinner will align accurately with the fuselage.
A collet grips better than a grub screw, which reduces the chance of the prop coming loose. There is no ugly hole in the side of the spinner needed to access a grub screw. No flat is needed on the motor or gearbox shaft when using a collet.
Gliders are all about having low drag since this is critically important for best performance. GM is the only spinner manufacturer who rounds off the square edges of their yokes to smooth out the airflow over them. This not only reduces drag, but the yoke looks much better.
GM also recesses the bolt head into the yoke so that it does not extend out into the airflow to cause drag like other spinners that use bolts. The head of the bolt is rounded to further reduce drag and improve appearance.
GM eliminates the typical large nut used to hold the prop bolt in place. Instead, they tap the yoke itself so that the bolt threads into the yoke and is flush with the side of the yoke. This completely eliminates the drag and ugly appearance produced by a nut. Eliminating the two steel nuts also reduces weight.
The GM spinner does not have an air hole in the nose of the spinner. Instead there is a smoothly rounded bolt there that is used to clamp the collet to the motor shaft. This look beautiful and further reduces drag. It also makes is very easy to mount and dismount the spinner since you do not have to search for a tiny grub screw. Also, you will never accidentally strip the GM mounting bolt, like you easily can by over-tightening a tiny a grub screw.
There is some question if elimination of the spinner's nose hole reduces the flow of cooling air to the motor. I note that there are still openings in the GM spinner behind the prop blades' roots where air can enter the spinner and flow into the fuselage, so I doubt that a nose hole is needed. In any case, compared to other spinners I have used (and I have used all the popular brands), have not detected any difference in motor temperature or experienced any motor overheating when using the GM spinner.
GM spinners use a straight yoke. The most popular and common spinners use cranked yokes in an attempt to get the prop blades to lay closer to the sides of the fuselage. Is using a straight yoke a problem?
No. A straight yoke is stronger, lighter, more compact, and lower drag than a cranked yoke.
But it is true that a straight yoke does not hold conventional prop blades as close to the fuselage as a cranked yoke. However, there is an easy solution to this problem -- use GM prop blades!
GM props are specially made to fold into a far more streamlined shape and lay against the fuselage much more closely than conventional prop blades. GM prop blades are designed to be used with straight yokes and therefore are ideal for use with the GM spinner. GM prop blades are also lighter than any other brand of prop because they are made from very thin carbon and are narrower than any other prop blade. Their narrow shape reduces drag compared to conventional props. .
Other straight yoke spinners have their yokes extend out further from the center of the motor shaft than GM spinner yokes. These long yokes cause more drag than the short yokes GM uses. GM spinners use the shortest and most compact yokes possible, which both reduces weight and allows the prop blade to fold as closely as possible to the side of the fuselage. Also, a short yoke looks better.
Another advantage of straight yokes is that they make it very easy to fit a small (size 8) rubber band around root of the prop blades to keep the blades closed up against the fuselage when soaring. This eliminates the problem of a lower prop blade dangling out in the airstream causing drag and looking absurd.
A rubber band goes freely straight through the GM straight yoke spinner from one prop blade to the other. The rubber band does not have to snake around inside the spinner as is the case with a cranked yoke, which interferes with its functioning.
The GM spinner is pure carbon fiber. It is not made of plastic. A GM carbon fiber spinner is much stronger than the more common plastic ones.
The GM carbon spinner's shape is ideal for blending and flowing the air perfectly into the typical, narrow, streamlined nose of the high performance gliders. It is slightly better in this regard than any other spinner I have used.
The GM carbon spinner shell runs absolutely true and is perfectly centered due to its precision manufacturing combined with collet mounting. The design and execution of the GM spinner simply is brilliant.
In summary, the GM spinner is the best there is. When combined with GM prop blades, you will have the lowest drag, lightest, best designed, most beautiful, and highest performance folding prop system available.