- This topic has 6 replies, 6 voices, and was last updated 14 years, 6 months ago by
.
-
Posts
-
Question:
Does anyone know the safe RPM speed limits for the Graupner CAM Folding Prop series? I’m most interested in the 7.5, 8, and 9 inch blade sizes. These are fiber reinforced blades with a 6 mm yoke drilled for a 2 mm attachment screw.
I know that Hobby Lobby sells the blades … has anyone inquired about speed limitations?
DickThe purchased folding prop ass’y from Graupner is spec’d at 16,000 RPM but that’s due to the h’ware it’s self. Jim J. and I have both spun the Graupner fiber re-infored blades using Jim’s hub ass’y up to 22,000 RPM without any failures yet. For ref. the APC Props have a formula for save prop RPM. It is 190,000 divided by the prop dia. As an example if you are running a 7″ prop divide 190,000 by 7 and you’re save RPM is 27,143. This same formula holds true for there gas props. It is not to be used on the slow park flyer props!
Scott
All,
I know that Dave Edmonson has a supply of folding props used in F1J and F1C that are suitable for the very high speeds we are likely to see in the future. They are carbon fabric reinforced blades as opposed to the injection molded type offered by Graupner. I am not sure about the exact size range available but it doesn’t cost anything to check.
I used the 7.5 x 4 Graupner blades with a Jennings hub on the currently missing Astrostar project with great effect. The calculated RPM was in the range of 20,800. There was no apparent problem with the blades even to the point of re-use after a lawn dart episode that severely bent the hub, which was replaced. It’s the plastic hub that comes with the stock prop assembly that is the limiting factor I believe: the stock hub is prtty wimpy and is limited to 16,000 RPM. I only report personal experience and opinion, not verified by any professional certification. In the words of one aircraft engineer “one picture is worth 10,000 words and one test is worth 10,000 calculations”. YMMV.
Jim has done us all a real service by making the hub available. Thanks Jim.
Cheers!, PeteMy pleasure Pete. I have spun the CAM 6 X 3 on the bench with a 2800 kv motor at 12v. I do not have a method for measuring RPM other than calculated. But if the motor rating is accurate, that is over 33,000 RPM. I hope to test with 4 cells soon. I am not sure this will be the best approach with a 5 sec motor run, but the discovery is what makes this so much fun. Time and effort will tell.
Jim, I agree … 4 cells is the way to go for the future. Voltage is our friend. That’s what prompted my question about RPM limits.
BTW, I think you will find that measured RPM’s will be a bit less than calculated RPM’s. That’s because a portion of the supply voltage is dropped via the i x R of the motor. Nevertheless, your 6 X 3 CAM folder was turning mighty fast. That’s impressive and reassuring.
DickGuys,
I’ve just been trying out an Axi 2208/20 with a pair of Graupner Cam 7.5×4 blades and one of Jim Jennings’ aluminium hubs (thanks Jim – they arrived this morning). I wasn’t sure what to expect in the way of RPM so connected it all up with a Hyperion Litestorm VZ, 700mAh, 3S 30C pack (it says on the back that they’re made by Enerland) and I reckon that this was just about what Pete R had in his Astrostar. ESC is a Jeti 18 Advance Plus. The drive signal comes from a Z-Tron timer. All good so far!
I ran it up a couple of times a mid-range speed to make sure that all was safe then gave it a run at full bore and ran the optical tacho past it. The result was about 13,300, pretty good but much lower than the 20k which has been spoken about.
The Axi website for this motor gives a performance table and 7 nicads on the Graupner 7.5×4 folder is quoted at 10k8 so I don’t think I’m too far out with my reading.
Undoubtedly Pete’s Astrostar goes up fast on this arrangement, but for all that, the prop probably isn’t going anywhere near as fast as the 1820KV figure would suggest – after all its only the no-load speed.
Anyone got any views?
Cheers – Tony Shepherd
- You must be logged in to reply to this topic.