The braking effect of a windmilling propeller is the same thing as the lift on an auto-gyro. From the figure above, it’s the Ly component that causes it to spin and the Lx component that causes it to brake (or lift in case of auto gyro). It’s the principal difference between an auto gyro and a helicopter. On an auto gyro the air through the rotor goes upward. In a helicopter it goes downward.
Cobalt wrote:
The reverse statement “When the propeller is being turned by the engine at a given RPM, every blade contributes the same amount of thrust so a three-blade propeller would have 150% the thrust of a two-blade prop.” is obviously not correct, since when an aircraft is converted to a three-blade prop, the operating RPMs are similar.
Exactly, because – as I wrote – the engine power is limiting. There is no such limitation when the prop is windmilling.
Cobalt wrote:
when an aircraft is converted to a three-blade prop, the operating RPMs are similar.
The propeller RPM is kept the same in part for the sake of operational efficiency, though tip speed will still be a factor. As the added blade will generate its own thrust per revolution, more blades would equal more thrust, if the engine had to power to turn it for more thrust. But, the engine is already operating at its maximum power, so the blade fine pitch settings are reduced to the engine will still turn it at maximum RPM. The result is a flatter pitch prop, which produces more drag when idling or stopped in flight. When I changed from two blade Hartzell to three balde MT on my flying boat, the maximum RPM remained the same, the takeoff thrust was similar, but the power off drag was greatly increased, to the point of making a power off approach challenging. Selecting coarse pitch reduced drag to being acceptable for a power off landing. This effect would be desirable for short approaches, other than to say I’m not really trying to accomplish short approaches in that plane. It’s manageable with technique, but is noticeably different. This is the fourth two blade metal to three blade MT conversion I have done (though the first I have owned), and the characteristics have been similar with all four planes in terms of a drag increase.
When I cruise with the MT, I now reduce the RPM much more than I would have with the Harzell, and then I Can feel the pull, with less noise, and slightly better fuel economy. Where I would have cruised 24 square with the Hartzell, I now cruise 24", 2100 RPM.
Airborne_Again wrote:
There is no such limitation when the prop is windmilling
No windmilling prop should turn the engine faster than its max RPM. And it definitely does not do that during a power off approach.
Pilot_DAR wrote:
The result is a flatter pitch prop, which produces more drag when idling or stopped in flight
AHA! So do I understand correctly
Now that makes sense.
Cobalt wrote:
No windmilling prop should turn the engine faster than its max RPM. And it definitely does not do that during a power off approach.
I never said it did! (How could you read that into my post!?)
I said that with the engine running, the amount of power and thus thrust produced depends on the engine. Whether the prop has two or three blades doesn’t matter. (Except that there may be a difference in prop efficiency.)
With the engine stopped, the amount of drag the prop produces doesn’t depend on what power the engine produced when it was running (obviously). It will depend on the propeller blades. So everything else (particularly blade area) being equal, a three-blade prop will have 150% the drag of a two-blade prop.
Cobalt wrote:
but it achieves this at different blade pitch settings, typically a finer pitch than the equivalent two-blade prop
I did say that, too.
Airborne_Again wrote:
At a given rpm and power output a three-blade prop will be in finer pitch (by design for a fixed-pitch prop or by the propeller governor for a variable-pitch prop).
Pitch is a relative parameter, an assumption at best, it’s not an absolute thing. Different propeller manufacturers even “measure” the pitch differently. Lift is Cl*A/2*rho*V² also for a propeller, and changing Cl of the foil has the same effect as changing the pitch. Only the V part is different for different sections. From 2 to 3 blades is simply a reduction of A, the blade area of each blade. The pitch stay the same, or the power would be different at the same RPM.
If a 2 blade propeller produces the same thrust as a 3 blade propeller, and the rpm, D and HP are the same, then the effective pitch is also the same. The blades of a wooden prop must have a thicker foil than an aluminium prop due to structural reasons. This means the geometric pitch of a metal propeller is larger than a wooden propeller for the same effective pitch (same thrust at the same rpm).
Also, well designed wooden fixed pitch props will vary the (geometric) pitch with thrust. Large thrust creates a finer pitch than less thrust. This is to enable higher rpm at low speed, thus more HP and more thrust. The change in effective pitch is typically 4 inches, depending on manufacturer.
LeSving wrote:
This means the geometric pitch of a metal propeller is larger than a wooden propeller for the same effective pitch (same thrust at the same rpm).
Yes, the airfoil of the wooden props is different to that of metal props. That said, I have seen the same differences in idling drag between two and three blade equipped Cessna 185’s I have flown. There are many variables. The important part is to learn to expect a difference with a different prop, it is very rare that a flight manual supplement is provided to describe an idle power drag difference, so, you’re a test pilot when you pull the power off…
I had dinner today with my friend who designs, builds and sells constant speed, two or three blades aircraft propellers for a living. He confirmed the following regarding two blade versus three blade:
1. From aerodynamic first principles, total combined blade area is constant for the same service regardless of the number of blades. However structural (strength and/or stiffness) issues typically make it necessary to increase individual blade area slightly on three blade prop, meaning the combined area of a three blade prop is usually a little greater than a two blade prop for the same service. That’s interrelated with the typical slight drop in propeller efficiency with three blades versus two.
2. All constant speed props regardless of number of blades are designed for the same blade angle of attack under power, within a fairly narrow range, to maximize L/D. No different than a wing.
3. Braking regardless of number of blades comes from aerodynamic and engine braking effects combined, although he had not looked into it deeply he thought engine braking would be a greater effect.
4. Some props (including some 3 blade MTs) are set up with the fine pitch stop providing redline rpm when stationary at full throttle, but most others are set up maybe 100 rpm below redline. The latter means a coarser fine pitch stop, which provides less braking effect.
Re. aerodynamic braking, I recall from a book I have on helicopters that there is a maximum rate of descent that cannot be exceeded in autorotation (you would need power to descend faster). I don’t have the book to hand but will see if I can find it to check the theory.
