Propeller slip vs propeller efficiency

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MrManH

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MrManH
Hi everyone,

I'm currently studying for my CFI and making sure that I have a good grasp on various topics of aerodynamics. I was fed pat answers as a student that made sense, seemed logical but turned out to be completely false. This impeded my comprehension and ability to learn other topics until I spent days rebuilding my foundational knowledge through research. I want to save my future students from that and teach them correct information from the start.

The latest studying I've been doing has been around the propeller. The PHAK is one of the only sources that mentions "propeller slip" and yet it doesn't truly define it. All it states is that it's the difference between the geometric pitch and effective pitch of the propeller. In other words, how much less than expected does the propeller advance in one revolution.

It doesn't actually explain what causes the propeller to "slip" and then loosely ties that concept to propeller efficiency by saying:
"Propeller efficiency varies from 50 to 87 percent depending on how much the propeller slips"

I understand the different factors that cause the propeller efficiency to be less than 100%, but I don't understand what propeller slip actually relates to. The FAA talks about it like it's some kind of independent, in and of itself factor.

I called Catto props to talk about this and apparently "propeller slip" is not even a term that they use.

The conclusion I've reached is that the FAA is using propeller slip to illustrate the efficiency being less than 100%, but it's not a factor in and of itself.

What do you think?

Thanks!

Slip.JPG
 
Not sure if this will help any, and is maybe even a bit off topic, but this touches on one of my pet peeves. Some of the FAA exam questions on aerodynamics and engine operation are based theories and concepts that are questionable at best. Problem is, in order to pass the tests, one must regurgitate the questionable information. Net result is that instructors have to teach it too, even if they know it's questionable or wrong.

Case in point: how does a wing generate lift? Even though students are taught a bunch of nonsense on this topic, it doesn't really matter. That's because they are taught correctly about airfoil behavior -- how lift varies with AOA, stall conditions and so on. You don't need to know how a wing generates lift in order to learn that. Nobody I know of thinks about how lift is generated when maneuvering at low speed -- you're thinking about the conditions under which the wing will stall.

Okay, I got that off my chest. I apologize in advance to those I may have offended.
 
Air being a gas and not a solid is what causes the propeller to slip. The concept is not very useful other than for answering FAA questions. I've never felt a need to teach a student about it.
 
No offense taken. What you're saying is a valid point about theory vs practical knowledge. Coming from a scientific background however, I have a desire for truth and accuracy and anything less than that hinders my comprehension and feeling of authenticity when discussing the topic. It doesn't mean that I'm looking for mathematical formulas or complex equations, but I certainly have an aversion to oversimplification to the point of total inaccuracy.

For example, a lot of instructors will teach their students that a windmilling propeller generates a lot of drag because it's like holding a disc of the diameter of the propeller against the wind. That is so incorrect and misleading.

Anyway, back on my topic of slip vs efficiency :)
 
This concept is flawed and using an illustration to display something that is theoretical and not physical is misleading and confusing.

They seem to want to describe how an airscrew (propeller) does not displace the air as if it were a solid. Thinking about a spinning prop on a stationary airplane kinda messes up their whole scheme.

Somebody got paid to put figure 5-45 in a government publication. Ignore it.
 
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dmspilot said:
Air being a gas and not a solid is what causes the propeller to slip. The concept is not very useful other than for answering FAA questions. I've never felt a need to teach a student about it.
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Indeed, but technically the medium isn't the reason for slippage. There has to be another factor that is "holding back" that prop. To my understanding, that factor is the efficiency being less than 100% therefore the propeller doesn't advance its full pitch like a screw would in a hard medium.
 
IK04 said:
This concept is flawed and using an illustration to display something that is theoretical and not physical is misleading and confusing.

They seem to want to describe how an airscrew (propeller) does not displace the air as if it were a solid. Thinking about a spinning prop on a stationary airplane kind messes up their whole scheme.

Somebody got paid to put figure 5-45 in a government publication. Ignore it.
Click to expand...

Thanks, I accept that answer :)
 
This is relevant to my interests...
 
"Slip" is a misleading term, it's not really what's happening. It's used to describe the difference between the forward motion of the prop's pitch line and the aircraft's forward motion, which is less. But an aircraft in level flight has a wing with a positive angle of attack (pointing above the flight path) and we don't say the wing is slipping.

Prop efficiency is defined as the thrust power produced by the prop divided by the engine's output power. It comes out to be proportional to the so-called "slip" but it's a different thing.
 
When you are flying straight and level, your wing is slipping 100 percent because you are not climbing at the wing's angle of attack. Prop is no different.

Then you have the problem of the propeller pitch. Typically, the pitch determined by measuring the angle of the relatively flat back side of the blade because that is the easy way. But, this is different than the pitch you would get using the angle of the chord of the airfoil (leading to trailing edge) and different from the pitch you would get measuring from the zero lift angle of attack. Plus, pitch is a function of the angle and the radius at which you measure the angle and for many propellers the pitch will actually vary from the hub to the tip.

Then...
From the picture, it implies that the slip is the difference in aircraft displacement vs. the distance per revolution derived from one of the pitch measurements. So, when the aircraft is stopped, by this definition, slip is 100%. But from the perspective of the airfoil on the propeller, you get a whole different result since the flow through the propeller disk is accelerated and is flowing backwards with respect to the airframe. This leaves one to wonder what is the value of tossing out numbers for "slip".
 
I believe the concept originates from ships. The classic way to describe prop slip is to compare it to a screw. If a screw has 10 threads per inch, then turning the screw ten full rotations will drive the screw in a full inch. In theory, without slipping a propeller will do the same thing. The analogue to threads per inch is prop pitch (although their relationship is technically inverse). If the prop is pitched more, then you expect one full rotation of the prop to result in more linear motion of the airplane forward. However, the air, unlike wood or other solid materials, gives way and the prop slips just like if the screw lost its grip on the wood. The more drag pulling on the plane, the more the prop will slip for a given rpm.
 
I believe part of the issue is slip varies with RPM, density altitude and airspeed for a given propeller and pitch angle. Above a DA of 25,000 they are not efficient. That is why you cannot get the definitive answer you seek.
 
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I am very far from an expert on this however ...

I feel that you may be looking through the wrong end of the telescope. I think you should start by figuring out how a propeller produces thrust. The slippage bit will likely fall out of that.

Trouble is, to do this effectively I suspect that you might be hitting the Math quite hard.

Also -
To repeat Capt. Geoffrey Thorpe from above
"When you are flying straight and level, your wing is slipping 100 percent because you are not climbing at the wing's angle of attack. Prop is no different."
 
The typical prop will be operating at about two to four degrees of angle of attack when in level cruise. The AoA will be higher than that in the climb, particularly with a fixed-pitch prop. The variable-pitch, or constant-speed prop changes its pitch to keep the AoA at the most efficient for whatever the airplane is doing at the time. It allows the AoA to be low enough to let the engine reach redline RPM for takeoff and climb, and allows the pitch to increase in cruise to keep the AoA positive enough to get as much speed as possible for the power setting.

Still, there are some unusual situations. On my Jodel I put a fairly long propeller when I restored the airplane. It had a 44" pitch and 76" diameter. That propeller gave me a cruise speed that indicated, according to the math, that the prop had zero AoA in cruise. I couldn't figure that out until I read that a prop that does that is often too long. One has to remember that the flat-bottomed airfoils used in many props (and wings) can generate lift at AoAs as low as minus 4 degrees, and that prop was likely doing something close to that. It also had the startling characteristic of an uncommanded RPM reduction as the forward speed came up in the takeoff roll. The RPM would drop about 30-50 RPM, quite audible, speed increased well, and I can only presume that the AoA was falling into an efficient range as the speed increased.
 
MrManH said:
For example, a lot of instructors will teach their students that a windmilling propeller generates a lot of drag because it's like holding a disc of the diameter of the propeller against the wind. That is so incorrect and misleading.
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If you can explain that, you ought not have trouble with something as simple as prop slippage. Tell us how windmilling drag should be explained, please. I.e., how does it vary — is there a limit to how much it can be or is it infinitely high, etc. I've never seen a really good treatise on this.
 
Dan Thomas said:
The variable-pitch, or constant-speed prop changes its pitch to keep the AoA at the most efficient for whatever the airplane is doing at the time.
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I’m not sure about this. The control on a CS prop governor is referenced to RPM. The prop pitch/angle of attack, is adjusted by the governor to increase or decrease the rotational drag such that RPM is maintained as set by the pilot. This may or may not equal the most efficient for what the airplane is doing.
 
MrManH said:
For example, a lot of instructors will teach their students that a windmilling propeller generates a lot of drag because it's like holding a disc of the diameter of the propeller against the wind. That is so incorrect and misleading.
Click to expand...

I found this interesting (PDF dl):
Stationary and Windmilling Propeller Drag study

Maybe the prop slip thing is a carryover from the nautical stuff.
https://www.mercuryracing.com/prop-slip-calculator/

Page 148 in the Aerodynamics for Naval Aviators is a good place to look for prop efficiency and windmilling barn doors.
 
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Shepherd said:
I always equated propeller slip with cavitation.
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Cavitation is something that only happens with boat propellers, when the low pressure in front of the blade causes the water to vaporize (boil). It doesn't apply to air propellers.
 
MrManH said:
what causes the propeller to "slip"
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Airframe drag.

Assume, for the moment, that we defined the "pitch" as a function of the angle at which the propeller airfoil generated zero lift (slightly lower than either of the geometric pitch measurements). If the aircraft were moving exactly at that "pitch per prop rev." then the propeller would be generating exactly zero thrust which would mean that the airframe would have to have zero drag to keep moving. As airframe drag increased, the prop would have to accelerate more air through the disk to generate the force to overcome the drag - this means more "slip" because the air flow through the prop disk is moving rearward faster (w.r.t. the airframe) or, the airframe is moving slower w.r.t. the larger atmosphere.

Change from the pitch based on the angle for zero lift to a geometric value, and you get an offset in your slip calculation, but the effect is the same.

When a propeller is windmilling, it is doing work on the engine to cause it to rotate and pump air through the intake/compression/expansion/exhaust strokes plus all of the mechanical drag - that takes engergy - that energy comes from the air flow spinning the prop - and that be a drag. You can reduce the windmilling drag by opening the throttle all the way and reducing the work lost in the engines internal pumping cycle.

You got to fight, for your right, to PARRRRRRRRR_TAY!
 
Take an airplane and try a few different fixed pitch props and you’ll get an idea of how some are more efficient at acceleration and climb while others are better in level cruise. The airplane’s power and weight play a role. You can experiment some using a constant speed at different settings for takeoff and cruise and get similar results.

Notice that seaplane props are always long and cruise props are short. One thing I recognize with my Whirl Wind is how blade twist and chord varies from hub to tip, which makes sense. My big Mac? Not so much. I don’t have a clue about what makes some props work better than others but I notice when they do.
 
if you’re going to teach pilots, it’s no more complex than “A prop with a 70-inch pitch means that if it were perfectly efficient, it would pull the plane forward 70 inches for each revolution. Due to ‘slip’, which encompasses a multitude of factors, that prop may only pull the airplane 40-56 inches, or about 57-80% of the theoretical perfect efficiency.”
 
My prop slips a lot. I can never climb as fast as I want to and cruse speeds are slow...:rolleyes:
 
x
WillFly4Food said:
I’m not sure about this. The control on a CS prop governor is referenced to RPM. The prop pitch/angle of attack, is adjusted by the governor to increase or decrease the rotational drag such that RPM is maintained as set by the pilot. This may or may not equal the most efficient for what the airplane is doing.
Click to expand...

And increasing or decreasing the pitch allows the engine to generate power most effectively as well as keeping the AoA within efficient limits. As the AOA gets too high, such as in a climb, the RPM falls and triggers the governor into reducing the blade pitch to get the AoA back into range. Diving does the opposite: the AoA decreases, reducing prop load, increasing RPM and causing the governor to reduce the RPM.

RPM figures into AoA as much as forward speed and blade pitch do.

A good read: http://www.pilotfriend.com/training/flight_training/fxd_wing/props.htm
 
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Prior to the beginning of the FAA in 1958, the CAA rules stated that in depth study of aeronautical minutia must not be undertaken. They were convinced that over thinking of the flight characteristics of airfoils, including propellers and wing forms... Would cause the same problem that were discovered with bumble bees.

We know they cannot fly... They are too stupid to know this, so they continue to fly even though they cannot....

The overarching concern is that if pilots discover "facts" and physical laws.... They will realize "heavier than air" flight is impossible.....

Unless the world is flat.


Please be careful gentlemen. I intend to take to the air again...
 
LaserRingGyrosandwich and the other folks who mentioned nautical stuff are on target. Never heard of slip in regard to airplanes only ships.
 
The so-called "slip" is nothing more that an angle of attack more than zero. It's just a term, and some people read too much into it. Like a wing, AoA is a variable depending on the lift (thrust) required and the speed of the airfoil. In slow flight your wing has a high AoA to generate lift equal to the weight of the airplane, and in high cruise the AoA is low because the higher speed generates the required lift at lower AoA. In slow flight there'd be lots of "slip."

It can't be used as a measure of efficiency, like some texts say. Efficiency would be measured by comparing HP into the propeller and thrust generated. It would take some mathematics and physics to calculate the acceleration of the mass of air affected. That's not one of my strong points.
 
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