Effect of Propeller Rake...

[John Leidle]

    Recently I was informed by someone that in a prop driven boat rake will determine  the angle at which the boat travels forward... meaning more or less rake will raise the nose or raise the transom.  Is there any of the same effect in an airplane & does it affect toy plane like what we fly?
  john

[Howard Rush]

No.

[Steve Helmick]

While Howard's answer is interesting    I wonder if the data presented    has been thoroughly tested?

Some basic tests could be done with one of John's planes that uses APC props, to see what happens. Use hot water to sweep the blades aft about 10 degrees. Pay close attention to tracking, and pitch, of course.

From what I know about boat props (bass boats, in the 60-75 mph range), I think it would definitely do something, and my guess it would be BAD! Maybe even VERY BAD! Report on your tests, John...  Steve

[phil c]

Raking the prop blades can make a major impact on the noise.  Look way back to the first APC props.  They were designed to be quieter and had the blades raked 5-6 deg.  The problem was that with more than a couple degrees(what they currently use) the blades were very subject to bending and fatigue failure.  Even on sport planes after a dozen or so flights the blades would be showing chordwise cracks in the plastic.  I tried some on combat planes.  After 3 flights you could just snap the blade tip off with a light twist.  Never had one break in flight though.  APC uses good plastic and reinforcement.

[John Leidle]

    Good idea Steve,, the boats I'm refering to are the piston powered Unlimeted hydroplanes speeds upwards of 135MPH  I know they are capable of going faster but these guys dont want to bust them up going 150 mph.  I just might try a test or 2.
  John

[Randy Powell]

I have several props (mostly carbon fiber) in which the blades of the prop are curved back. The tip of the prop terminating aft of the nominal hub somewhat. My assumption (possible wrong) was that when running, the prop would maintain something close to no backward curve and instead be perpendicular to the hub. Seeing several in flight pictures in which the prop appeared to be bowed forward under load, I just thought that the blades starting off with a rearward bend would help to combat this.

Hmmm....

[Chris Wilson]

I have several props (mostly carbon fiber) in which the blades of the prop are curved back. The tip of the prop terminating aft of the nominal hub somewhat. My assumption (possible wrong) was that when running, the prop would maintain something close to no backward curve and instead be perpendicular to the hub. Seeing several in flight pictures in which the prop appeared to be bowed forward under load, I just thought that the blades starting off with a rearward bend would help to combat this.

Hmmm....

Randy, if the purpose of carbon fiber is used to create stiff flex free props then how would they 'be bowed forward under load?'

It doesn't make any sense to me that they would be purposefully flexing at any point.

I believe that aft rake places the center of pressure further back onto the hub, or more importantly further back towards the CG.

[Howard Rush]

What do you mean by rake, John?  The standard definition is sweep toward the tip like this (Steve Fitton picture):

[John Leidle]

   Howard,
  Looking at the propeller from the side , if the prop looks like a 12 inch ruler it has no rake. If looking at the propeller from the side and the propeller tips are arced back towards the rear it has rake, the guys at the hydro museum use this rake as an adjustment to raise or lower  the transom or bow which ever you prefer . I wondered if it has the same effect out of watter as in water..
  John

[Howard Rush]

I think the analogous effect for airplanes would be an increase or decrease in the amount of lift you'd get from the prop at a given angle of attack (sideforce at a given sideslip angle).  I don't know what effect bending the prop tips back has on this.  I would expect not much.  I did a little searching, but didn't find anything on the subject.  The guy who would know is Stuart Sherlock, http://www.supercoolprops.com/index.php .  When you ask, though, don't count on his definition of "rake" being the same as the hydro guys'. 

The way an increase or decrease in the amount of lift a prop has at a given angle of attack would affect a stunt plane is a difference in stability and a difference in line tension for engine offset.

[Dean Pappas]

Perhaps the prop rake in a race boat has its effect on boat trim because only part of the prop is in water at full tilt.
There is no analogous situation in air.
Dean P.

[John Leidle]

   Good point Dean.
  John

[Howard Rush]

Perhaps the prop rake in a race boat has its effect on boat trim because only part of the prop is in water at full tilt.
There is no analogous situation in air.
Dean P.

If that's the case, sure.

[John Leidle]

   Thanks Dean,
   You got my mind started. I wonder if the greater degree of rake is kinda like moving the prop aft a couple inches?  What really  surprised me is these props are tiny .. you have 1500 HP Allison driving a 14 inch prop... shaftrun anybody?
            John

[Chris Wilson]

Prop rake in marine applications is often used to fight the effects of cavitation - it progressively presents the blade to the flow of water unlike a straight blade would.

I have always considered 'rake' in our applications is done more for noise suppression at the tips and the amount we can use is very limited by the high aspect ratio of the blades designed for use in air.

Cheers.

[RandySmith]

   Thanks Dean,
   You got my mind started. I wonder if the greater degree of rake is kinda like moving the prop aft a couple inches?  What really  surprised me is these props are tiny .. you have 1500 HP Allison driving a 14 inch prop... shaftrun anybody?
            John

In Water..BIGGG differance

Randy