Hi everyone, today I took my CMEL oral. My examiner and I had to stop the oral and finish it off tomorrow morning due to time constraints. We covered most of the material but we are going to go over factors affecting Vmc along with critical engine factors and other performance questions. I have most everything down solid but there is one thing I can't remember for the life of me. I know when the C.G. moves aft Vmc increases. What I can't remember is why is it that performance increases with the aft C.G. when single engine. It might sound simple to you guys and girls but for some reason I just cannot remember and can't find a good explanation in any of my text. Please help me out! Thanks!

 

It has everything to do with torque. Think of it as a wrench. If you have a longer wrench, it doesn't take as much force on the end of it to turn a bolt. If the wrench is shorter, you have to put greater force on it to turn the bolt. Same thing in the plane. If you have a longer arm (ie. distance from CG to the rudder), than you don't need as much force acting on the rudder to keep the plane from yawing. Since the rudder is fixed, the only way you can make the distance longer is to have a forward CG. Therefore, if the CG is moved back (bad for Vmc), then you have to fly faster to get the same amount of force.

Performance, however, is different. The reason it's better for performance is the same reason as it is with two engines. If the CG is moved aft, you no longer need as much tail down force. This gives less "induced" weight. Less weight = better performance.

Make sense, or do I need to work on my analogies?

 

The reason performance increases with an aft CG is by putting the weight in the back reduces the amount of downforce that the tail needs to actually produce. That part is strictly aerodynamic. So with less down-force needed, there's less drag associated with the tail, and you get maybe 2-4-ish kts improvement. Now the downside is obviously the VMC increase, and also a greater difficulty recovering from stalls/spins.

Elevator feel can also become rather "weak" so over-rotating and flaring could be detrimental if you over-did it and scraped the tail. All this is due to the rearward shift of the CG, and that reduction of the distance between the CG to the tail is now smaller, providing a shorter "arm" for leverage(think teeter-totter with different weights at each end, then shift the balance point closer, too counter-act you'll need more/less weight[down-force] to balance it all out).

I know I over-killed the post, but this is what happens when you're up too late and just looking for something to do. Good luck on finishing out your check-ride.

**Edit: For the leverage, the torque demo above is definately better, but don't forget the aerodynamic advantage of the aft CG**

 

i just finished up the oral and i'm trying to fit the flight in today. it looks like T-Storms up here though so it might have to wait for tomorrow.

thanks a lot guys!