# Drag and torque of a propeller driven by relative wind alone

#### Jean Claude

##### Junior Member
Title: Negative Thrust and torque characteristics of an adjustable pitch-metal propeller
Author: Edwin P. Hartman
Ref: NACA report n° 464
Comments: This report can be used to predict the drag of a propeller accidentally freewheeling, due to a broken gearbox belt, or transformed into a windmill driving the dead engine, or simply idling.

How can we find the drag of a freewheeling propeller from this report?

Since level 0 of the torque coefficient (Q / ρV2D3) corresponds to zero torque, i.e. perfect freewheel operation, note the abscissa where level 0 intersects the torque curve corresponding to the pitch of your propeller (e.g. 22° at 0.75 R, top graph) (here, nD/V =0.83).
If your propeller has a diameter D of 1.65 m, then at a forward speed of 25 m/s it will reach :
n = 0.83*25/1.65 = 12.6 rpm or 756 rpm
On the lower graph, the drag coefficient curve corresponding to your 22° pitch gives a value of - 0.01.
Since the drag coefficient is Tc/ ρV2 D2 , the propeller thrust will be :
T = -0.01*1.22*252*1.652 = -21 N

The propeller mentioned in this report have D= 9 fts (2.74 m), pitch 17° and V =100 mph (44.7 m/s) would have given
nD/V = 1.03 Hence n = 1.03*44.7/2.74= 16.8 t/s or 1008 t/mn
and T = -0.015*1.22*44.72*2.742 = 274 N or 60 lbs as was well found

It's very clear that rpm and drag increase dramatically as propeller pitch decreases.

Last edited:
Jean Claude,

I can testify personally that decreasing prop pitch will add drag. Pull the power levers back too fast on a C-130, and one can definitely feel the amount of increased drag by one's body being pushed forward in one's seat.

Wayne

Jean Claude,

I can testify personally that decreasing prop pitch will add drag. Pull the power levers back too fast on a C-130, and one can definitely feel the amount of increased drag by one's body being pushed forward in one's seat.

Wayne
I know when I taxi turboprop aircraft, I can control taxi speed with the power lever or prop control, depending on the aircraft. It saves brakes and makes a really neat sound when the prop cycles in and out of flat pitch. Okay, it wasn't really to save the brakes, but the sound is neat!

Jim

I know when I taxi turboprop aircraft, I can control taxi speed with the power lever or prop control, depending on the aircraft. It saves brakes and makes a really neat sound when the prop cycles in and out of flat pitch. Okay, it wasn't really to save the brakes, but the sound is neat!

Jim
It is SOP to use Beta on a C-130 to control taxi speed, and yes it is a cool sound for sure.

BTW, as a child, I lived only a couple of miles from KVNY Van Nuys Airport. At that time, Van Nuys was a joint Civil / Military airfield. The 146th Tactical Airlift Wing, CA ANG was stationed there with 16 C-130s. On cold winter mornings, the atmosphere would carry the sound of operating C-130s very well to my house. I remember lying in bed listening to the C-130s engines operating, the distinct sound of taxing to the runway, and the change of pitch in the sound as the power levers are pushed forward to start the takeoff run. After a C-130 had landed, I would listen to the sound of the power levers cycling between ground idle and Beta as the aircraft would taxi back to the ramp. That was a very happy, soothing sound to me listening to the C-130s.

Wayne

To complete, naca report 464 indicates that the drag of a locked propeller of similar shape to the photo with D = 1.65 m and 20 degrees pitch, at forward speed V = 25 m/s (56 mph) gives a thrust T = Tc. ρ .V^2. D^2 = -0.023* 1.22 25^2* 1.65^2 = - 48 N ie drag = 10 lbs

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