# At what angle of deflections do rudders become inefficient?

This article (picture) says rudders for ships can deflect 35° max,so at what aoa does an airplane rudder lose its efficiency,at what " angle of deflection " does it start to stall ?

• Remember - deflection is NOT the same Angle of Attack. Angle Attack is the airfoil vs the relative airflow which may not always be coming from the front. – Dan Jun 8 '18 at 19:21
• Ok....true....will correct that @Dan – David Teahay Jun 8 '18 at 19:22
• Hydrodynamics are my bread and butter... remember, a ships rudder has the prop right in front of it. Any more than 35 degrees and you are probably going to rip the rudder off with the prop wash/rudder forces. There are some other things to think about but that is the major one that you don't have more deflection is just that, keeping the rudder attached. – Ron Beyer Jun 9 '18 at 0:36
• – Peter Kämpf Jun 9 '18 at 9:02
• I think this comment about ships is about really specific types of ships. I've sailed dinghies using more than 35° of rudder to turn. – Dan Hulme Jun 11 '18 at 12:21

The only time you should be using full rudder is when doing a side slip - such as turning the yoke for a left turn but pressing right rudder, so you are descending and looking to your left as the plane goes forward tracking down the runway direction. Normal coordinated turns, you would not use anywhere near that much rudder while turning.

I just found a NASA Test Report on my plane, a Cessna Cardinal, that lists control deflection amounts as a reference for a light general aviation airplane:

Angle of attack, -4° - +20° ± 0.5°

Sideslip, -15° - +15° ± 0.5°

Pitch angle, -15° - +15° ± 0.5°

Roll angle, -75° - +75° ± 0.5°

Rudder deflection, -24° - +24° ± 0.5°

Stabilator deflection, -20° - + 5° ± 0.5°

Right aileron deflection, -20° - +20° ± 0.5°

Left aileron deflection, -20° - +20° ± 0.5°

• Yes!....finally a nail in the head!.... Thanks bro!👍🙅 – David Teahay Jun 9 '18 at 5:12
• This does not answer the question. This table quoted here is grossly missrepresented; the original document list this table as the parameters that the test instrumentation measures and records. It does not present actual control deflection muchless effectiveness at any specific deflection. – J Walters Jun 9 '18 at 11:20
• Most airplanes seem to be around 23-25 degrees. I would imagine water rudders can go to 35 because of the much lower Reynolds number of a rudder in water (much higher fluid viscosity/lower velocity). – John K Jun 9 '18 at 16:42
• OP is not going to be doing aerobatics in a homebuilt ultralight tho. – CrossRoads Jun 9 '18 at 19:49
• What does this editorial comment mean? "Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted." The data is right from the NASA report, which is linked to in the answer. – CrossRoads Jun 11 '18 at 11:44