PM 38 Why the difference in prop length

[supercub]

I heard that one of the props, I believe the rear blades are a little shorter in length then the front blades, why is this? Is this typical on counter rotating props? Thanks
Brian

[Coyote Chris]

Re: PM 38 Why the difference in prop length

All I can tell you (from a member of the crew) is that the new props were very carefully designed and yielded a 20 mph increase in cruise speed with the same power settings.

[supercub]

Re: PM 38 Why the difference in prop length

Thanks, I heard that too, they were re -contoured (reshaped). I'm assuming the difference in length has something to do with the way the air comes off the front blade tips, so as to minimize the distrubance of the rear prop tips.......but that's just a guess on my part. Would like to know the real reason. Thanks
Brian

[Bill@Interstell]

Re: PM 38 Why the difference in prop length

I believe it keeps the tip vortex from the front prop from interferring
with the airflow over the back blades.

[wolfee]

Re: PM 38 Why the difference in prop length

Correct me if I am wrong Mike,

I am pretty sure the rear blades are longer than the front blades. At first I thought they were shorter as well, from looking at the first pics posted by Thom and Scotty on their facebook page. Thom corrected me by saying they were longer.

Wolfee

[Tigercat24]

Re: PM 38 Why the difference in prop length

Correct me if I am wrong Mike,

I am pretty sure the rear blades are longer than the front blades. At first I thought they were shorter as well, from looking at the first pics posted by Thom and Scotty on their facebook page. Thom corrected me by saying they were longer.

Wolfee

What I was told by Thom is that the rear blades are indeed longer than they used to be (each set is now 4 inches longer than before), but they are still shorter than the front blades. The fronts are longer than the rears for the reason mentioned above...to keep the front blade prop tip vortices from interfering with airflow to the rear blades...if the rears were longer than the fronts, the vortex would go right through the path the rear blades are making, instead of outside, which, I'm assuming, would cause all kinds of vibration, instability, etc.

We need "Wild Bill" to confirm all of this.

[wolfee]

Re: PM 38 Why the difference in prop length

What I was told by Thom is that the rear blades are indeed longer than they used to be (each set is now 4 inches longer than before), but they are still shorter than the front blades. The fronts are longer than the rears for the reason mentioned above...to keep the front blade prop tip vortices from interfering with airflow to the rear blades...if the rears were longer than the fronts, the vortex would go right through the path the rear blades are making, instead of outside, which, I'm assuming, would cause all kinds of vibration, instability, etc.

We need "Wild Bill" to confirm all of this.

Ahh, I didnt put both props being longer together. That makes sense
now that you said that. Thanks for cleari g that up for me.

[wolfee]

Re: PM 38 Why the difference in prop length

What I was told by Thom is that the rear blades are indeed longer than they used to be (each set is now 4 inches longer than before), but they are still shorter than the front blades. The fronts are longer than the rears for the reason mentioned above...to keep the front blade prop tip vortices from interfering with airflow to the rear blades...if the rears were longer than the fronts, the vortex would go right through the path the rear blades are making, instead of outside, which, I'm assuming, would cause all kinds of vibration, instability, etc.

We need "Wild Bill" to confirm all of this.

Ahh, I didnt put both props being longer together. That makes sense
now that you said that. Thanks for cleari g that up for me.

[Wild Bill Kelso]

Re: PM 38 Why the difference in prop length

Ok here is the short story. Whittington had some notion that he would run the Griffon at 3800 rpm and cut the props to have the proper tip speed at that RPM. There is no way a Griffon would survive at those power settings! A more realistic number is 3000 @ 100 in/hg so the new props were configured to those specs.

Now as far as the difference in length the rear prop is shorter for two reasons. To prevent the vortices from the front prop from smashing into the rear prop. There was also a tech order on the Shackleton that said to make them shorter to prevent cracking of the tips of the rear prop. Let's not find out the hard way that equal prop lengths might have the same result. Even though we are waaaaaaay inboard of where they had issues with the stock blades. Way more meat where Thom cut them at.

The reprofile was a different way of thinking than any other Griffon powered racer yet. Most guys before just cut the tip off and left what Kerch calls the baseball bat part of the airfoil. With fancy 5-axis CNC machines we have these days we swept the tip and cut new airfoils all the way to the tip. What type of airfoils the last 14 inches is the big secret. The results were quite shocking even for me! A 64 mph gain on the course all from the propellers. No other mods were added!

[RAD2LTR]

Re: PM 38 Why the difference in prop length

64 mph out of that change alone, wow. I've also heard the the wing tips could be redone for a little speed as well as a lower profile scoop. I see more speed out of PM over the next couple years. You guys already proved you were fast enough to keep everyone up front honest if not a serious contender. Hopefully with a few years more development we will be able to truly see what PM can really do.

Will

[Wild Bill Kelso]

Re: PM 38 Why the difference in prop length

64 mph out of that change alone, wow. I've also heard the the wing tips could be redone for a little speed as well as a lower profile scoop. I see more speed out of PM over the next couple years. You guys already proved you were fast enough to keep everyone up front honest if not a serious contender. Hopefully with a few years more development we will be able to truly see what PM can really do.

Will

You won't have to wait a few years.

[GRNDP51]

Re: PM 38 Why the difference in prop length

PM doesn't have electronic ignition or something too right? Correct me in that statement if I'm wrong.

[Tigercat24]

Re: PM 38 Why the difference in prop length

64 mph out of that change alone, wow. I've also heard the the wing tips could be redone for a little speed as well as a lower profile scoop. I see more speed out of PM over the next couple years. You guys already proved you were fast enough to keep everyone up front honest if not a serious contender. Hopefully with a few years more development we will be able to truly see what PM can really do.

Will

I've heard the same about the wingtips...those were apparently two areas that the PM team got some advice on for improvement and were taking one step at a time...first the props, then the wings. I can see from pictures that the wingtip flares aren't quite as long...flared...whichever....as they were under Ron B.'s ownership, but I've heard that completely new wingtips are in the future. I know that they tried a different doghouse scoop before Reno, but I was told it failed during testing. Can Kelso shed some light on some of this? (pretty please )

[Wild Bill Kelso]

Re: PM 38 Why the difference in prop length

I've heard the same about the wingtips...those were apparently two areas that the PM team got some advice on for improvement and were taking one step at a time...first the props, then the wings. I can see from pictures that the wingtip flares aren't quite as long...flared...whichever....as they were under Ron B.'s ownership, but I've heard that completely new wingtips are in the future. I know that they tried a different doghouse scoop before Reno, but I was told it failed during testing. Can Kelso shed some light on some of this? (pretty please )

Engine/prop program first. Airframe cleanup next. I'll have a scoop soon, and wingtips are in the works.

[Big_Jim]

Re: PM 38 Why the difference in prop length

Engine/prop program first. Airframe cleanup next. I'll have a scoop soon, and wingtips are in the works.

Isn't it nice to have an 'errand boy' to do all your shopping for you?