One-engine-inoperative aerodynamics is one of the major areas students struggle with when first learning how to fly multiengine airplanes. In this article I will address the principles of flying a multi engine airplane when one of its engines fails.
Side-slip Vs Zero side-slip
With a multiengine airplane with all its engines operating, sideslip is eliminated by keeping the ball in the inclinometer centered, just the same you are used to do with a single-engine airplane. This is called zero sideslip and is the condition where the airplane is presenting the smallest profile to the relative wind, creating minimum drag.
Because of the asymetrical thrust, A centered ball is no longer an indication for zero sideslip if one engine fails.
Even with the ball centered, on a multiengine airplane producing asymmetrical thrust, we are still flying in a side slip due to the lateral force created by the rudder.
The only equipment that can indicate slip in an asymmetrical thrust condition is a yaw string attached to the windshield.
To achieve zero side slip with one engine inoperative we should use both rudder and ailerons. Place 1/2-1/3 of the ball out of its cage towards the operative engine and use about 2° of bank towards the operative engine ("raise the dead" engine). The opposing forces of horizontal component of lift and rudder side force will eliminate the sideslip.
Zero side slip will give us best performance and directional control.
Remember that loss of directional controlled is caused by asymmetrical thrust. The reduction of power will restore directional control, but will also decrease performance.
When a light twin loses an engine, it loses 80% of its climb performance, due to the increased drag and decrease in excess
power required for a climb.
Not bad on Vmc in general, but also a few dangerous misinterpretations. Please download my newest paper from my web site.
I am a graduate USAF Test Pilot School and know how Vmca is tested and what the conditions for validity of the red-lined Vmca are.
Amir Fleminger
- Thank Harry
|2008-11-20 01:11:52
Could you specify what "dangerous misinterpretations" please?
Thank you for your comment.
Harry Horlings
- VMC
|2008-11-20 20:41:04
Thank you for the opportunity to clarify Vmca; there is a huge misinterpretation of the real value of Vmca and the conditions under which it is valid.
The clarification below is taken from college books (Dr Roskam), my Test Pilot School books and from FAA and EASA flight test guides.
The given definition of Vmc is copied out of FAR 23.149. This FAR however is for airplane designers and for the certification of the airplane. The engineer designing the vertical tail selects a small bank angle of max. 5 degrees away from the critical engine to design the fin with rudder, because then the fin can be made smaller (and lighter) and the sideslip can be reduced to zero. Once the number of degrees is selected (and the tail built), the pilot has no choice but to use the same bank angle for making the listed and red-lined Vmca valid and for the drag to be minimal. If this bank angle is not used, a small twin might not have positive climb performance anymore.
The red-lined Vmca is the worst case Vmca, i.e. all of the factors that have influence on Vmca are at their worst case value, the thrust is max. and the bank angle is the same as used for designing the vertical tail. The red-lined Vmca is therefore always a safe Vmca at max. thrust setting, provided the pilot maintains the small bank angle away from the inop engine, and hence does not make turns, if the airspeed is close to Vmca!
The line "Vmc must not exceed 1.2 Vs" is for the tail designer only, not for pilots; the tail should not be made too small. The pilot does not have to know which engine is critical, because there are no separate procedures for critical and non-critical engines. Any engine failure has to be handled the same way; Vmca is determined while the critical engine is shut down, because then Vmca is higher, but this is already included in the worst case Vmca.
If the props have an automatic autofeather system, this is used to determine Vmca. If it does't work, the Vmca will indeed be (much) higher than the red-lined Vmca. If no auto-feather, windmilling is used for determining Vmca during flight-testing, and this Vmca is published.
Vsse is the speed below which an engine should not intentionally be shut down. But it is OK to decelerate below Vsse in order to 'experience' Vmca.
The most unfavorable CG for Vmca is aft CG, a more forward CG decreases Vmca, because it increases the distance to the rudder making it more effective. The most unfavorable weight is low weight; a higher weight decreases Vmca provided the pilot maintains the small bank angle away from the inop engine. If the wings are level, the weight effect is zero (side force = W sin (phi) = 0), but Vmca is at least 8 kt higher than the red-lined Vmca and the drag is not minimum. If banked into the dead engine, Vmca increases considerable. This happened very often during turns while an engine was inoperative, espacially during the final turn if a high power setting is required for a while. Vmca increases so much that control is lost; the tail is too small for maintaining control.
Please refer to the downloads page on my website www.avioconsult.com for much more additional material on Vmca, including accident reviews as seen through the eyes of a flight-tester.
Fly and come home safely.
Harry Horlings
- Vmca
|2008-11-23 15:56:11
In addition to post 5 below: Please check the website of the University of North Dakota (UND) via link:
http://media.avit.und.edu/f4_Inop%20Engine% 20Trainer/f1_Inop%20Engine/060302/mainmenu.php
There you can change all kinds of factors and see what the effect is on Vmca, etc.
