erich weaver

erich weaver

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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
 
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
 
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.
 
More specifically...

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?.....
 
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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 :D

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...
 
Put simply...

rvbuilder2002 said:
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...
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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.
 
You picked the wrong instructor

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
 
Bob Axsom said:
That shouldn't be anywhere near the list of important things to be verified in a BFR.
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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.
 
sprucemoose said:
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.
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Second this point. Tailor questions to suit the student. IMHO, the FAR's are for the most part pretty wisely written. Being able to suit a BFR to the individual student is one of those wise things.

CFII/MEI
 
Are you telling me?

Are you telling me that you can't tell an experience level in 10 minutes of flight related conversation and a review of the pilot's log book. In this case the instructor gave the pilot an open book pre BFR question and he is asking the the Vansairforce forum for the answer. I'm not at all convinced that that is what was intended when the BFR process was instituted several years ago. The intent should be to reinforce the important things dealing with operations in the national airspace system, weather, flight planning, operations and decision making. Questions that do not have any effect on what the pilot needs to know for safe and reliable flight operations, deflect focus from the essentials that should be reinforced in the BFR and are worse than non-productive.

Bob Axsom
 
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Bob Axsom said:
Questions that do not have any effect on what the pilot needs to know for safe and reliable flight operations, deflect focus from the essentials that should be reinforced in the BFR and are worse than non-productive.
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I don't disagree with that...but an understanding of the effect of CG range on handling is, imho, pretty essential to safe flying!

And I also want to comment that I don't personally think it matters where a pilot gets correct information -- whether it's VAF or a textbook -- as long as the pilot is (a) seeking that information, and (b) getting the correct information.
 
You're all making valid points

I agree with most everything said here. I think the question should not really be "which is fastest?" but should be "what is the effect on handling with CG change?" as Dan alluded to. However, Bob is correct also because of the way the question is given doesn't matter a hill of beans to me as a private pilot because I don't base my weight and balance on the fact I can squeak out another knot of speed. And yes, if I was a CFI I would make the student show me a weight and balance on a Cessna 150 (if that is what he/she was flying.)
 
As I see it, it's the same exact question as, "What affect does CG have on handling and why?" I wonder how many pilots think that moving the CG back just makes the elevator "touchy" and are fairly clueless about the true affect on stability, or even what "stability" in this context means?

Ok...so you ask the question. If the guy gets is right, great. If he gets it wrong, it's a wonderful opportunity for one more pilot to have a deeper understanding of his machine.

Geez....it's not like his CFI asked him to decipher one of these....

http://paragonair.com/public/docs/FAA-Handbooks/8083-25_AC61-23C_PHAK/_61-23C_Fig_05-29.jpg

Now THAT would be going too far...
 
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