Spiral slipstream: 3-blade v 2-blade prop?

[RV7A Flyer]

Discussion over beer tonight: Is there a difference in the amount or strength of the spiral slipstream between a 2-bladed and a 3-bladed prop? (Basically, given an identical airframe, does one or the other require more or less trim, if one were to need a trim tab, on the rudder, and if so, how much is attributable to slipstream vs. other factors, like P-factor)?

Just idle musings while enjoying malt and hops...

[vic syracuse]

This has actually been studied in great detail, and it turns out there are many, many factors that come into play, and I will try to list them here. However, know that many great minds have been working extensively over the years to finds answers to your question. I am sure they will continue to search for answers and share them with us here.

First, and you may have already thought about this but it really begins with understanding the type and thickness of the Primer that was used underneath the paint. Do not underestimate this.

Second, another less understood variable is the configuration of the airframe---nosewheel or taildragger.

Third, there is a real, but less understood impact on whether the trim is manually activated or uses a servo. The configuration of the stick buttons is also a factor. The impact of the autotrim function, if connected to a yaw damper is even less understood.

Fourth, it seems that airflow is disturbed in various ways depending on whether it is a tip up or a slider canopy.

And since the 3 bladed props are well known to be much slower, the forces directly affecting the trim tab are much less, requiring a trim tab proportionately larger, in some cases approaching the need for an extra rudder. I think this was solved many years ago on the P-38 Lightining. It, too had 3-bladed props so they needed to add the extra rudder, requring a second fuselage to support it.

There are certainly some less-understood impacts, such as the tire pressures, type of oil being used, size of the oil cooler and the location of various air-oil separators. These also are being studied, but perhaps will be solved sooner over malts.

Use the Search Function to find data on the various topics listed above.

Good Monday morning!

Vic

[DaleB]

Vic, all I can say is... you're my new hero.

[Mel]

And don't forget; a painted aircraft is affected much less than a wrapped one.

An aircraft with manual flaps is also affected less than if they are electrically operated.

[F1Boss]

I think you could toss in a discussion the turbo-encabulator in your note - it would tie in nicely!

BTW I always wondered about that second fuselage on the P38...

Carry on!
Mark

[vic syracuse]

I clearly forgot about the turbo encapsulator, as well as a few other things. It's early Monday.

Don't forget it really matter how the prop is controlled, such as using a vernier cable or a throttle quadrant, as well as the type of governor.

Then there is the very complicated impact of having a smoke system. It seems there is some real debate as to whether adding oil to the exhaust makes the slipstream MORE or LESS dense. The smoke particles are larger than the air molecules that they displace, so the impact on the trim tab should be more, thereby requiring less rudder. However, the oil sticks to the aircraft, thereby changing the W&B and CG, all at the same time. I think it will require the placement of many video cameras to actually solve this one. Some will need to be outside, and at least one will need to be inside focused on the ball and switch to see the actual timing and impact. However, the required increase in throttle due to the sticky airframe may negate the measurements. It's really getting complicated now!



Vic

[krw5927]

On a serious note, when I changed engines and props from an O-320 spinning a Sterba wood prop to a Titan O-340 stroker with a 3-blade Catto, I found I needed to remove the trim wedge I had previously installed on the rudder.

I assumed it was due to differences in the spiraling slipstream, but never looked into it any further. I was just happy to get rid of that trim wedge. Not scientific, of course, but a practical example.

[ppilotmike]

...about the differences in mass/rotational momentum, depending on how the interior is finished out!

[jdeas]

Originally Posted by krw5927

On a serious note, when I changed engines and props from an O-320 spinning a Sterba wood prop to a Titan O-340 stroker with a 3-blade Catto, I found I needed to remove the trim wedge I had previously installed on the rudder.

Outside of torque remaining the same:
Gyroscopic (different weight blades)
P-factor (2 vs 3 blade)
Spiral slipstream
Parasitic drag
*different cg

It would be interesting to know to what extent these changes would make. While I appreciate the levity of the group, it is an interesting thought that came up as I was trimming my rudder for a centered ball.

and yes Vic, I do believe the 3-blade is slower but it sure can climb

[n82rb]

you forgot the biggest factor, the shape of the canopy release handle.

bob burns