flight review question

erich weaver · #1 10-16-2006, 08:34 PM

I have my flight review scheduled for this Friday and am busy brushing up on all the things I forgot over the last two years. My instructor provided a list of study questions that Im going through and I am having trouble with the following:

"where does an aircraft fly the fastest - most forward or aft limit CG?" Could someone help me out here along with a brief explanation?

thanks

erich weaver

Gramps1 · #2 10-16-2006, 08:42 PM

AFT CG THE FASTEST

Ironflight · #3 10-16-2006, 08:45 PM

You're faster at Aft CG's because the tail has to generate less "downward lift" to keep the tail from going up. Less lift equals less drag, therefore, you can go a little faster.

Paul

airguy · #4 10-16-2006, 08:51 PM

Aft CG will provide higher airspeeds, due to lower drag.

Heres why - the tail forces produce negative lift, pushing the tail down in order to raise the nose and increase the angle of attack to produce positive lift on the main wing. Aft CG will provide an automatic gravity-induced downforce on the tail, requiring less elevator-induced negative lift to be produced by the tail. Less lift required means less induced drag produced by the tail. Given the same amount of horsepower still being produced, a lower induced drag will result in lower overall drag, resulting in slight acceleration, until the parasitic drag of the aircraft increases to the point that the system is again in equilibrium. The end result - aft CG produces higher airspeed. It's also extremely dangerous at low speeds, as it can make the aircraft difficult or (in extreme cases) impossible to recover from a stall.

rvbuilder2002 · #5 10-16-2006, 11:16 PM

With a fwd C.G. position, the C.G. is fwd of the wing center of lift (actually as long as you stay within the recommended C.G. range, the C.G. will always be forward of the Center of lift so that you maintain positive pitch stability for the reason described below). The wing center of lift is actually lifting from behind the C.G. and trying to pitch the airplane nose down. This is why the previously mentioned down force is required from the tail. This down force produced by the tail must now be carried by the wing so it needs to fly at a slightly higher angle of attack than it would to just carry the weight of the airplane...hence, higher drag, along with the drag caused by the horz. tail producing lift.

As the C.G. position moves aft, it gets closer to the wing center of lift which reduces the amount of down force needed on the tail, which reduces the amount of weight the wing has to carry, etc. etc. When the C.G. moves to the same location as the wing center of lift, the need for down force on the tail has been reduced to near zero. This will be the lowest drag configuration but it will also be the point at which pitch stability goes to zero.

There is no longer a Stabilizing interaction between the amount of lift the wing produces, the amount of down force that the tail produces, and a pitching moment of force pitching the airplane about its pitch axis (which is where ever the C.G. location is) at a given airspeed, so pitch stability will be zero or even negative if the C.G. was allowed to get aft of the center of lift.

At least thats what my understanding of it is anyway...
Anyone more aeroengineer minded please jump in and correct anything I may have flubed up. Kevin?.....

David-aviator · #6 10-17-2006, 05:25 PM

Quote:

Originally Posted by Gramps1

AFT CG THE FASTEST

Or pull the power back slightly and burn less fuel.

dd

jcoloccia · #7 10-17-2006, 05:56 PM

I agree with all the above. One thing I've always wondered about, though. There are lots of planes that require down trim at cruising speed. Makes me wonder if a more forward CG would actually be faster in this case since it would allow the cruise with the elevator flat and less parasitic drag?

So really, it seems likes you'd truly get max airspeed at the balance point where the lift=weight and you've minimized both parasitic and induced drag.

I know there's some aerodynamicist (is that a real word?) that can give a bottom line on all of this

For me, personally, it doesn't matter. I'm putting around in old Citabria's and 172's. I could hang an elephant off the tail and still not see 100kts cruise...

FlyerJumper · #8 10-17-2006, 06:01 PM

Quote:

Originally Posted by rvbuilder2002

As the C.G. position moves aft, it gets closer to the wing center of lift which reduces the amount of down force needed on the tail...

In a nutshell, this basically means less deflection of the elevators is needed to keep the nose up. Less deflection means less control surface area out ther creating drag.

Bob Axsom · #9 10-17-2006, 06:14 PM

Aft of course but I'd shop around for a different instructor for my next BFR. That shouldn't be anywhere near the list of important things to be verified in a BFR.

Bob Axsom

sprucemoose · #10 10-17-2006, 06:34 PM

Quote:

Originally Posted by Bob Axsom

That shouldn't be anywhere near the list of important things to be verified in a BFR.

I disagree. While I don't think that knowing the answer to this is a requirement for passing a BFR, I think that questions like this, and the answers given, can give an instructor (particularly one who does not know you) some insight into your knowledge level. With that knowledge, the instructor can tailor the BFR more to your level of knowledge and experience.

For example, if I pose that question to a student and he nails it with a correct analysis, it is unlikely that I'm going to waste his time working out a weight and balance sheet for the C150.