# How do the blades on a propeller generate thrust?

I am having trouble understanding how this type of propeller generates thrust:

It is so much different than a propeller on a Cessna and thrust would be provided differently. I am also confused of how the turbofan blades in a jet engine provide thrust:

Doesn't the turbofan push the bypass air back really fast providing thrust? How do each of the these propellers (turbo prop and turbofan) produce thrust?

• The propeller on the ATR in your linked image functions in the same basic way as one you would see on a Cessna piston aircraft. Perhaps you are noticing that the ATR prop is in a feathered position, in which the blades are rotated to be in line with the airstream. This is done automatically by the propeller pitch controller when an engine has been shut down in order to reduce drag. On smaller aircraft, like a Cessna with a "constant speed propeller", the pitch can be controlled manually (it's actually indirectly controlled but that detail is best saved for a separate discussion). – Porcupine911 Sep 2 '15 at 1:19
• Propellers work the same way as wings. They produce lift with the vector facing forward and we choose to call it thrust. – casey Sep 2 '15 at 1:40
• Okay so I know how the blades on a Cessna spin and generate lift, but I am really confused with the picture I provided of how that propeller generates thrust. – Ethan Sep 2 '15 at 1:49
• Ethan, Please provide attribution for your photos. – CGCampbell Sep 2 '15 at 15:45

## 2 Answers

The cross section of propeller blade is an airfoil. As the propeller rotates through the air, it generates lift, which becomes thrust as the it is directed forwards rather than vertically upwards as in aircraft wing.

Source: recreationalflying.com

The relative airflow on the blade section (airfoil) is the resultant vector of two things - the forward motion of the propeller (the aircraft) and the rotational velocity of the propeller blade itself. Most of the propellers in use today are fixed speed variable pitch propellers. So, velocity experienced by the propeller blade is maximum at the tip (where the rotational speed is maximum) and minimum at the hub.

Source: americanflyers.net

So, at any RPM, different parts of the propeller are at different angles of attack. To remedy this, the airfoil sections are varied along the propeller. So the propeller is practically twisted along its length, with the root at higher angle of attack. Also, in some cases, the airfoils are changed along the length (thinner ones are near tip).

Source: www.pilotfriend.com

As already noted, the pitch of the propeller blades can be varied in flight. What the photo shows is a feathered propeller i.e. a propeller that is set parallel to the airflow. This is done usually in order to reduce drag in case of engine failure. During normal operation, the blades will not be in feathered position. For example, in the figure below, the propeller of engine 1 feathered while the next one is not.

From wikimedia commons, work by Julian Herzog

In aircraft, propeller pitch is changed with speed, with fine pitch at low speeds and strong acceleration (like takeoff) and a coarse pitch for high speed (cruise).

Source:forum.warthunder.com

In most aircraft, the propeller pitch settings are adjusted automatically by a governor according to requirements. The following figure shows the pitch variation in flight for a similar propeller.

"Propeller pitch on an ATR 72" by Olivier Cleynen - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.

Thrust The thrust in both turboprop and turbofan is provided by accelerating air through fans. In a turboprop, it the propeller, while it is the bypass fan in case of the turbofan.

The thrust produced, T is given by,

$T = \dot {m} * (v_{f} - v_{0})$,

where $\dot{m}$ the mass flow rate of air and $v_{f}$ and $v_{0}$ are the final and initial (unaccelerated) velocities of the air respectively. Usually case of propellers, the mass flow rate $\dot {m}$ is higher, while in case of turbofans, the variation in velocity is comparatively higher.

The picture of the turboprop is probably confusing you because you are looking at a shutdown engine. For a multi-engine propeller airplane a desirable feature is that the propeller with auto-feather when shutdown, so that an in-flight engine failure will cause the prop to be in a minimal drag configuration. This minimal lift, minimal drag configuration is what you posted, and this is not producing thrust. When the engine is started and system pressure (oil, hydraulic, whatever controls the adjustable pitch prop for that particular engine) will drive the propellor pitch angle into lift-producing regimes. Turboprops also typically feature "Beta" propellor pitch, which allows the propellor pitch to be adjustable past zero-lift into negative-lift and is used for landing rollout and (in some airplanes, perhaps not many) slowing down in-flight or steep descents. During normal flight the turboprop propellor is producing lift just like the propeller on the cessna you are familiar with.

The N1 fan in a turbofan (the big one you see when you look at the front of the engine) is a ducted fan. The individual blades of the N1 fan are angled to push air backwards. This works the same way as a big box fan you can buy at walmart. The key difference is the N1 fan rotates an order of magnitude faster than a box fan. The bypass air is accelerated by this fan and exits through the rear of the engine to produce thrust.

• I get it. So the first pic shows the engine in shut down mode to reduce lift then the blade turns while in flight.like a cessna engine – Ethan Sep 2 '15 at 13:08