Have you ever looked at your aircraft’s rotors and wondered what direction it is spinning? While it’s difficult to tell when the rotors are at full power, you can also tell during startup too. Here’s a guide to the direction of your propellers and why they move this way.
Same way or different?
Almost all commercial aircraft today use at least two engines for safety and to make journeys quicker. But did you know that older aircraft features counter-rotating propellers, meaning that while one turned clockwise, the other went counter-clockwise? While this comes with a handful of benefits, the presence of two different gearboxes meant the feature fell out of favor.
However, the aircraft you’re flying today will most likely follow a simple rotation path. Both engines are seen to spin clockwise (when viewed from the back of the plane) or counter-clockwise if you’re standing in front of the aircraft. This includes popular models like the ATR72 and de Havilland Canada Dash 8 Q400.
Here’s a look at the startup sequence of a DHC Dash 8 Q400 for reference.
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Different types
There are significant benefits to using engines that rotate in the same direction. The first is a lower cost of maintenance. Since both propellers have the same clockwise rotating gearboxes, the price of parts is far lower. Across a larger fleet, these savings are augmented, explaining why nearly all commercial jets use this framework.
However, there are some drawbacks as well. In case of an emergency, these planes have a ‘critical engine’ that causes asymmetric yawing. The left-hand engine is the critical one on most commercial aircraft due to the clockwise rotation. This makes controlling the plane harder if the critical engine fails than if the other one does.
On counter-rotating engines, where both propellers move in opposite directions, there is no single critical one. Since both rotate inwards, there is no asymmetrical yawing if one were to fail. However, given how rare engines failures are and the ability to correct the asymmetry, manufacturers have stuck to singe direction engines today.
There is one modern exception to this rule. The Airbus A400M military transporter uses counter-rotation today to meet specific safety requirements that are required for propeller planes.
What about jet engines?
Now that we know about the direction of propeller-engined-aircraft, what about jets? In this case, the choice is simply up to manufacturers on what direction they choose to install blades. Due to several reasons, mostly to do with weight and inertia, there is no critical engine on large jet engines, making the choice simply one made by conventions or industry norms.
For instance, all Rolls-Royce (except one) and CFM engines rotate clockwise, while all GE (minus the GEnx) and P&W engines rotate counter-clockwise. This makes it easy to spot what direction your plane’s engines will be going.
