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Propeller Torque

I

Ivan

Messages
62
Country
unitedstates
Hello All,

Does anyone here happen to know what controls the resistance of the propeller to spinning and how to go about adjusting it?

I have read Jerry Beckwith's FS Props document here:
http://www.mudpond.org/fs_props.pdf
but that only appears to cover steady state pitch selection and thrust generation.

Test Case:
Our aircraft has an engine that makes 1150 HP at 2500 RPM.
It has a propeller with a minimum pitch of 29 degrees.

Scenario:
The aircraft takes off.
As it accelerates to maximum speed, the RPM gradually increases but does not reach 2500 RPM until the aircraft has reached 180 mph.
Past 180 mph, the RPMs remain constant as the pitch increases to match advance ratio.

I would like the maximum RPM to be reached at 100 mph. (Sort of like putting a smaller propeller on the aircraft.)

I have tried the following:
Propeller MOI: Seems to have no effect.
Propeller Time Constant: No great effect but the CSU started allow propeller to overspeed.
I have also tested some other values that allowed minor changes, but not to the degree I am looking for.

Thanks in advance for any information.
- Ivan.
 
I understand the discussion you pointed out for me but it isn't really related to my question.

Imagine that I have a 13 foot diameter fixed pitch propeller set up for direct drive on an engine that makes 100 HP at 3000 RPM.

Lets say that this propeller can hit only 200 RPM before the wind resistance to the blades prevents the propeller from turning any faster. I am not concerned about the thrust being generated.

Next, we cut down the blades so that the propeller is only 3 feet in diameter. The engine now hits 3000 RPM because the short blades have much less resistance. What value controls this torque factor that was slowing down the propeller?

- Ivan.
 
Thanks MGH,

I think I need to revise your graph to reflect the Record 512 and engine power from my AIR file and see if I can reproduce the same results I am getting in the simulator.

I assume the following is true:
Pitch changes are instantaneous?
RPM changes are limited by the MOI / Propeller Time Constant?
Intermediate values between stated pitch angles are interpolated linearly?

I am sure once I have digested this, I will have a bunch more questions.

Thanks.
- Ivan.
 
To the best of my understanding:

prop_tc affects rate of change in propeller pitch angle

propeller_MOI affects rate of change of propeller rpm. It should be combined with engine moment of inertia but that doesn't seem to be taken into account

Linear interpolation should be sufficiently accurate
 
After quite a lot of messing around with Excel, I have a spreadsheet that takes the 512 table as input and calculates power required. Problem is that my numbers don't make sense when compared to what I am seeing in the simulator.

I suspect a bad formula somewhere. Still trying.

Also tried lowering the positive values for the two blade angles above and below my propeller's minimum pitch angle. Didn't seem to have any noticeable effect at all.

What formula are you using for power required?

Thanks.
- Ivan.
 
You're using the momentum-blade element theory formula.

Cp is a power coefficient, where are you obtaining that value from?
n is the rotational speed of the propeller in revolutions per second, not minute.
D is the propeller diameter.

It would be helpful to know what values you are using versus what values you are expecting.

There is also this formula that calculates maximum static thrust (ideal):

T = Pio^2/3*(2*p*A)^1/3

Pio = maximum horsepower (in m-N/s, convert horsepower to kilowatts and then 1 watt == 1 m-N/s).
p = rho
A = propeller area.
 
Last edited:
WarpD,

My goal may be a little different than what you are expecting:

I have an aircraft (about 1100 HP at sea level) which is turning a 10 foot propeller. The minimum pitch angle is 29 degrees.

The problem is that it does not hit full RPM until I reach almost 200 mph. (For those of you reading along previously, yes, I changed the test subject to one that was a bit more egregious.)
The engine hits about 95% horsepower by around 150 mph, but I am playing with the idea of simulating a lower activity factor propeller on the same aircraft. I want the new prop to hit full RPM by around 100 mph but with the same diameter and gear ratio. I am trying to simulate an engine that puts out more power than the propeller can absorb.

I do not know how to control this factor.

From earlier discussions, I am taking Record 512 and the Cp values in that table and working them backwards to "Power Required" versus "Power Available" as MGH suggested. The problem is that my numbers don't seem to have much relation to the output I am getting from the engine.

Thanks.
- Ivan.
 
It's an iterative process to equalise the power developed by the engine and the power required to turn the propeller. If they're not equal then the rpm will either increase or decrease.

Calculate Advance Ratio J = U / (D * ωP) based on speed forward speed U (m/s), propeller diameter D (m), and propeller speed ωP (rad/sec)

Get engine power PE (Watts) based on engine speed and throttle setting

Calculate propeller power PP = (ρ * ω3 * D) * Cp(β, J) (Watts) based on air density ρ (kg / m3), power coefficient (Cp) from Table 512,using blade angle β and advance ratio J.

Change variables and iterate until powers are equal.



There's a low_speed_theory_limit in the [propeller] section of the .cfg file. Microsoft says it's the "Speed at which low speed theory becomes blended in (feet/second)" but doesn't explain what it is. I don't think it can be momentum theory because that takes no account of blade angle and propeller rotational speed.

My opinion, for what it's worth, Microsoft got the wrong. The tables should have been thrust coefficient instead of efficiency because propulsive efficiency becomes zero at zero forward speed.
 
Last edited:
Why are you changing all the units to Metric when that is not the typical unit used in the AIR file?

In the spreadsheet I have been working up I currently have several tables:

First Table is Blade Angle on Y Axis, Advance Ratio on X Axis. Cell contents are Cp values from Record 512.
There are three rows:
- First is Angle from Record 512 BELOW the minimum prop pitch.
- Third is Angle from Record 512 ABOVE the minimum prop pitch.
- Second is the minimum prop pitch with Cp interpolated from record above and below.

Second Table is Airspeed on Y Axis, RPM on X Axis. Each cell contains the calculation for advance ratio based on these two numbers.

Third Table is also Airspeed on Y Axis, RPM on X Axis. Each cell contains the interpolated power coefficient based on Tables Above.

Fourth Table is also Airspeed on Y Axis, RPM on X Axis. Each cell contains the Power Required to drive the propeller working backwards from Third Table.

BTW, The low speed theory limit is a requirement considering the thrust is calculated from propeller efficiency and advance ratio. At Zero forward speed, the efficiency is ZERO which means there is no thrust. (Yes I know you stated that also.) Below the low speed theory limit, a static thrust formula is used which makes perfect sense considering that efficiency was chosen to be represented.

Now I just need to figure out why the numbers in Table 4 are not what I am expecting.

- Ivan.
 
Last edited:
Ivan,

I'm not sure I can help with your original question but I'm currently fiddling with the FSXA Grumman Goose. One of the annoying things is when you change the prop lever to get cruise RPM, the plane oscillates significantly. (I have it set to a rotary knob on my joystick.)

Propeller Time Constant: No great effect but the CSU started allow propeller to overspeed.
Click to expand...

If you change the "prop_tc=" value from 0.1 to 0.005 in aircraft.cfg, you will remove most of this oscillation.

As I understand it now (correct me if I'm wrong), this value affects the prop governor (which the Goose POH lists).

When you look at a digital readout of the prop thrust, at 0.1 value, even the slightest movement of the prop lever will cause the thrust to jump either way a LOT (i.e., from 100 to 600 lbs). The prop still works fine at 0.005 but the oscillations are a lot less noticeable.

On your original problem, are you taking the corresponding values of table 511 into view in your spreadsheet?

Best,

Steve
 
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