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Turbo Talk => Gallery => Topic started by: Bob on February 18, 2019, 08:45:15 AM

Title: Flexing Prop
Post by: Bob on February 18, 2019, 08:45:15 AM
For a model plane with a gas engine, the size next larger than an .049, showing the concept such that I’ve not gone through the numbers but intend to build on a 3D printer.

Each blade has a twist feathering from large ellipse at the hub to a more angled slender ellipse having a slight bow toward the rear much like a fishing rod under load so centrifugal force & thrust the blades are run at perpendicular to engine shank. 

Each blade has two defectors stopping air from spilling off just as wings on commercial units,  The hub has a 7 Morris degree taper to transmit torque - same as milling machines but easy to remove.

The clear dome can be pried off against the detent ring gaining access to the screw & prop.

Title: Re: Flexing Prop
Post by: Andy H on February 19, 2019, 01:54:04 AM
Nice work on the propeller Bob.  I'm curious though. I know nothing about propellers, and wondered.  Is the purpose of the deflectors (triangular bits on the ends), simply to channel the air,  or do they add stability.  As I say I'm just curios.

Title: Re: Flexing Prop
Post by: murray dickinson on February 19, 2019, 12:14:29 PM
I remember .049 control line speed and combat from 45 years ago, and the prop tips were cut back to try bring them under trans-sonic speed, because otherwise they were incredibly noisy, producing a sound like a Guinness book of records attempt at the most people cracking whips simultaneously, which didn't make neighbours or municipal council officers happy.   I never worked it out at the time, but with a 7" prop and the speed of sound at 335 m/s, it would take about 36K rpm for that to happen. 
Title: Re: Flexing Prop
Post by: Bob on February 19, 2019, 04:04:51 PM
Ah yes, the humble .049 engine, back in the day (1954) my older brother & I would start ours up on Saturdays at about 6:00 A.M. off our rear porch, our apartment building enclosed a square with some others.  The deafening sound would echo off the brick walls waking up the dead (sleepy heads). 

Then in the winter we’d go down to a lonely deep frozen pit that filled up with rainwater, large square excavations for the new Eisenhower expressway a year before workmen finished the new highway. That area was good for flying our planes having smooth and level areas, no grumpy neighbors either.
Title: Re: Flexing Prop
Post by: Bob on February 19, 2019, 04:38:24 PM

They’d act to channel the generated wind from simply spilling radically off the prop purposing much like the ends on a commercial wing tip.  I don’t know if it that helps efficacy much considering at start-up having to slide your fingers on the trailing edge - a reason to keep obstruction (uprights) inboard the prop’s outline configuration.