Hello everyone
I’ve been under the impression that all MT propeller approved on any Cessna has a hard max RPM limit of 2700. Does anyone know of an exception to this?

I’m aware that the other brands are approved on the 185 to 2850 but I thought the MT was not.

The MT is limited to 2700, yes. I’ve had both. If you’re in the market I would look at the Voyager. It is much much better than both MTs and spins 2850.

The MT Expedition Series 83" prop is designed to hold tip speed at about .86 Mach. On this blade profile, that is the RPM that provided the max thrust. I have tested pull with it up to 2875, and indeed found the thrust (static) was highest at 2700. Once the plane gets moving, reducing RPM down to 2575 to 2600 results in a significant 'seat of the pants' added pull and reduced the takeoff roll.

Facts of fluid dynamics inform us all that the pressure wave that forms just below speed of sound, results in a disturbance of air on the top of the wing. That results in reduced lift and that is what causes the loud blaaat noise.

Turn your voyager or black mac RPM down and you will also see better thrust along with reduced noise....I have tried it myself with a 185 IO-520 and 90" prop.

I have a funny feeling the long HC Voyager at 2850 RPM is non-compliant with Federal Part 36 noise regulations....

But, yes the Voyager 3B prop is a fantastic design and performs very well and in my opinion is the best performing METAL prop out there. If you want less weight, less vibration, zero erosion ever and tough nickel edge protected blades, you will need to go with composite.

Cheers!

john54724 wrote:The MT Expedition Series 83" prop is designed to hold tip speed at about .86 Mach. On this blade profile, that is the RPM that provided the max thrust. I have tested pull with it up to 2875, and indeed found the thrust (static) was highest at 2700. Once the plane gets moving, reducing RPM down to 2575 to 2600 results in a significant 'seat of the pants' added pull and reduced the takeoff roll.

Facts of fluid dynamics inform us all that the pressure wave that forms just below speed of sound, results in a disturbance of air on the top of the wing. That results in reduced lift and that is what causes the loud blaaat noise.

Turn your voyager or black mac RPM down and you will also see better thrust along with reduced noise....I have tried it myself with a 185 IO-520 and 90" prop.

I have a funny feeling the long HC Voyager at 2850 RPM is non-compliant with Federal Part 36 noise regulations....

But, yes the Voyager 3B prop is a fantastic design and performs very well and in my opinion is the best performing METAL prop out there. If you want less weight, less vibration, zero erosion ever and tough nickel edge protected blades, you will need to go with composite.

Cheers!

So what IS the max RPM on the MT? 2700? It is not clear from the website. A rep at OSH once told me it was 2800 but have found no documentation of this. He indicated you do not actually have to adjust the governor you just can just restrict it by the tach. Yes, I can grasp the concept of diminishing returns at high RPM. I currently have a short Mac 403 so I may actually be getting the 300hp out of it at 2850. Efficiency aside I do not like the idea of derating to lower hp.

The following is my standard reply when folks start correlating horsepower to performance.
Horsepower is merely a unit to measure the RATE at which work is done. Thrust is the force which propels an object. A powerplant that is simply developing its full rated horsepower does not linearly equate to thrust. It is how that power is converted to thrust resulting in what we in lay terms call performance that matters. Differing airfoil designs incorporated in the propeller blade result in differing thrust for the same blade velocity just as different airfoil designs in a wing produce differing lift co-efficients for the same velocity. Although horsepower is a long-accepted measure of potential performance, in reality it is thrust that we seek in this context. In subsonic flow rotational airfoil design, faster is not always better.

TR

TR. correct me if I’m wrong. Wouldn’t one want the prop to be designed to create the most thrust at an rpm where the engine is generating the most HP? For instance, an MT could pull hardest at 2600. That’s fine but the engine would only be putting out 270hp more or less. (Just guessing for an io520). Whereas a prop that is designed to have most thrust at 2750,2800,2850, would create the best performance on that engine because the prop is designed with the engine parameters in mind.

My only point is that HP cannot be linearly correlated in terms of propulsion. The driving force (literally) is the force vector of the thrust provided by the physics in play due to the rotation of the blades (airfoils). If propeller X can provide the same thrust as Propeller Y at a lower RPM while producing less power, two identical aircraft will perform identically at this same thrust even though one is operating with less HP and less RPM. Said another way, if propeller X provides more thrust at 2400 RPM than propeller Y at 2600, with all other things equal, the aircraft with propeller X outperforms aircraft with propeller Y even though propeller Y powerplant is producing more power. In a perfect world, the propeller and powerplant would be designed and engineered together as one is useless without the other, exploiting the strengths and mitigating the weaknesses of both. Unfortunately, engineering a perfect solution to every specific scenario is not economically feasible leaving us with choices which should be based on mission requirement. Being that air is compressible makes for some very challenging parameters for airfoil designers and its effects should be understood by operators to obtain maximum efficiency. The propeller that provides max thrust at 2600 RPM on a standard day, will not provide that same thrust at 2600 RPM at -35F. There is X energy available in the pot, spending some of that energy to create noise and shock waves does nothing to get one airborne faster. This is why we go back to my starting point, vice worrying about HP, we should be evaluating thrust curves. You use the term "most thrust", I assume that to imply the maximum thrust available at your maximum allowed RPM. Again, to fairly compare two propellers, I believe you need to look at the thrust provided at specific rotational speeds (RPM). It may be the case that propeller X provides more thrust at 2400 RPM than propeller Y at 2600 for example. One only requires the power (HP) to convert to the maximum thrust available, the use of any more is inefficient use of both propeller and powerplant.
Perhaps if we change the name "propeller" to "thrust vector provider (TVP)" it places their purpose more in context with the standard forces acting upon an aircraft.
My objective is education, I hope I met it.

TR

I have yet to see a printed graph of an aircraft engine on a dyno. I’ve always wondered what the curve looked like.

Why is engine torque not a value ever talked about? If it made great torque at a lower rpm that burned less gas you would think that would be a target power setting.