Veeeerrrrrry interesting indeed. I think the key difference is, not so much the greater HP, but the greater torque (albeit via HP) to turn that larger prop.
One thing I want to avoid is that accident you referred to with too fine pitch. For now I have an electric MT with auto pitch control. That should help me avoid problems like this.
I have a lot of jet time, but little prop time. So when it comes to AOA, I think in terms of the wing producing max lift. The AOA changes with elevator input. A prop is not unlike a wing, but I still don’t have a grasp of what is causing AOA change in a fixed pitch prop. But, usually I have to read things three or four times in order to “get it”.
What gave rise to seeing a need for clipping the larger prop? Was the torque not there? I’m sure you answered this a few days ago, I’ll look over the old posts.
And yeah, it’s possible to overthink this thing.
Fun nonetheless.
BW
The propeller blade is a wing, but used in an impossible situation.
To stall or not to stall that is the question.
A stalled airfoil has some of the smooth flow near the trailing edge separating from the surface and rolling.
This reduces the mass of air that is changing direction (Vector) and the thrust of the blade or the lift of the wing is reduced.
This is a function of Angle of attack to the RELATIVE wind or airflow. When the plane is tied to a tree with a spring scale, or just sitting still, the flow through the propeller disc is nearly at 90 degrees to the zero lift line of the prop. This flow changes for the worst as the propeller RPM goes up. The cylinder of air going through the prop begins to accelerate reducing the (Effective) angle of attack of the blades, however the last bit of the airflow as it enters the disc begins to spin or, follow the blades around a bit. This increases the Angle of attack.
The actual blade angle relative to the crankshaft never changes. The blades path through an ever faster moving column of air produces a lengthening spiral through the column. If you look at the angular difference between the spiral path and the zero lift line of the blade, you see the effective angle of attack.
As the aircraft and, or the air column accelerates the distance between the points on this spiral will be further apart and the effective angle of attack will be further reduced. So the fixed pitch prop changes angle of attack to the relative wind as a function of the aircrafts foward velocity. So your RV whatever may not take full throttle from stopped. But once it is moving 15 or 20 MPH can take all you have.
Because the effective angle of attack of the blades has been reduced, unstalling part of the prop.
In the stationary aircraft:
Also the cylinder of air is moving into the prop because the prop has generated a low pressure area just in front of the disc.
So local air outside of this moving column is at a higher pressure than the air in the column, and begins to compress the column right in front of the disc.
Now the prop tips may not be seeing a column of air moving as fast as the mid blade sees, and the tips may be stalled and nothing else is.
The air leaving the back side of the blades is accelerated violently and Bernoulli using more local unmoving air chokes the fleeing cylinder of fast moving low pressure air tight against the fuselage. The cylinder of air is now spinning (P factor) and its low pressure is causing slow moving local air to mix with it, so it is gaining mass, And raising the speed of the slower air.
So if the prop is small in diameter and turning very fast it may produce a higher velocity column, but of reduced mass. Conversely, a large diameter prop when the same torque is applied will turn more slowly, but move more mass per second, and attract more local air and accelerate that as well. Disc area is the easy way to express all of this.
More is better. More HP is better. More torque is better. More diameter (Disc area) is better. More blades are better. Wider blades are better. Lighter blades are better. But not for everything.
This should get us down to about 1,000 additional factors to think about.
Static thrust. Not just a good idea.
Ask the guy in the black biplane hovering 200 feet above the concrete at Sun&Fun. That's over 1,000 pounds of static thrust.
Lynn E. Hanover
|