rear CG means lower AoA?

cointyro

New Member
I was reading through my new copy of Gleim's PP knowledge test prep book, and came to a section on W&B. Namely, one of the question explanations stated that a plane with an aft CG flies at a lower angle of attack, and also stalls at a lower speed.

I was trying to reason this out in my head and couldn't figure it out.

In straight and level flight, lift must equal weight; and lift is proportional to the coefficient of lift times the amount of air deflected vertically by the wing.

A lower AoA means a lower coefficient of lift; and slower flight means less air vertically deflected by the wing. This would suggest, in my interpretation, that if you're in straight and level flight, if you decrease your AoA AND your airspeed (a double whammy), your lift drops especially and you'll descend.

So how does an aft CG cause lower AoA and lower stall airspeeds (in level flight) when compared to normal CG? In either case lift must be the same...

Does this have to do with the fact that induced drag (the horizontal component of lift, usually directed rearward) is typically less at lower AoAs?

Thanks for any clarification on the matter! In my studying, I first have to understand the why behind aircraft behavior ... this makes it easier for me to reason out the answer when I've forgotten the "memorized by-rote" answer.

Now if I could just figure the reasoning behind compass deviations (ANDS, etc.) ... I think I'll stick to the memorization tools for that
 

stuckingfk

Well-Known Member
This is how I look at it. When you have have rear CG the planes wants to pitch up in normal flight, so you counteract with pitching down and trimming it off. Thus, you have a lower AOA and a lower stalling speed, takes longer to get to the Critical AOA. When your CG is foward, the plane wants to pitch down, so you counteract by pitching back and trimming that off, now you are flying at a higher AOA when compared to a rear CG to keep straight and level. Also, a higher stalling speed because you are closer to that COA.

This same principle explains why you can fly faster (not much) with a rear CG than with a foward CG.
 

Baronman

Well-Known Member
Good Question!

Ok...Let me just tell you a little story I heard first...I heard that a pilot at a certain airline got a huge bonus because he had suggested flying with an aft CG could save the airline on fuel costs because it would be more efficient....He was right! It was a little more efficient and multiplied by the millions of miles flown, there was big savings...$$$

So....An aircraft flying at a lower AOA makes less lift. (With the production of lift we have drag). Less lift, less DRAG! Less drag means we go faster or farther!

So, why don't we need the same amount of lift?

We know the 4 fundamental forces (Lift, weight, thrust, drag), and that lift must equal weight in unaccelerated flight otherwise we go up or down.

Well, most people forget that the horizontal stabilizer does not create an upward force but a DOWNWARD FORCE!!! Let's consider 2 scenarios.
1. We have a forward CG, so the tail must make a great downward force to keep the nose up. The wing must make enough lift to equal the weight of the plane PLUS this downward force. ALot of lift needed...More angle of attack please!! Greater AOA = DRAG....(Baaaaad)

2. We have an aft CG, the nose doesn't want to go down on it's own. We don't need the tail to make such a great downward force. The wing makes enough lift for the weight PLUS the smaller downward force of the tail. Less angle of attack, less induced drag, FAAAAAAAST PLANE!

So, the airlines by moving more of our Hamburger eating big buts to the rear of the aircraft were doing themselves a favor. Their planes didn't have to make as much lift (and the drag that goes with it) because WE were helping the horizontal stabilizer hold the tail down and the nose up!

I want my money back! Hope that helps.
 

cointyro

New Member
Wow, great quick answers.

You stated "We have a forward CG, so the tail must make a great downward force..."

This presumes that the "point of lift" for the main wings is aft of the CG. If the CG was aft of the "point of lift" (is there a more technical term for this point?), then the tail would need to make less downward force... and this would reverse your conclusions.

In taildragger (conventional? lol) configurations, the CG would be aft of the "point of lift"... but the Gleim manual doesn't make this distiction (i.e. it doesn't say "rear CG means lower AoA ONLY when CG is forward of "point of lift"...)...

Am I missing something still?
 

Baronman

Well-Known Member
Ops...I forgot to answer the part on stall speed.

Remeber the lift equation?

Lift = Angle of Attack x Velocity (simplified version!)

We need to make as much Lift as we have weight (and downward tail force right!?!)

Well, a heavier aircraft must make more "Lift."

Our AOA is limited to the critical angle so we have to have more speed to make the required lift. Thus a higher stall speed.

What are some ways we effectively increase weight? A forward CG means more downward tail force, so that's one way!
Increasing load factor is another.....
 

cointyro

New Member
OK Baronman. I'm starting to understand somewhat better.

The drop in in AoA AND drop in airspeed is indeed a double whammy on total lift. HOWEVER, the decrease in total (vertical) lift required to maintain straight and level flight due to a better "balanced" airplane [horizontal stabilizers need to "push downwards" less] more than compensates for the drop in lift due to lower AoA and lower airspeed.

Ummm, right?

I'm still curious, however, if this conclusion would hold for taildraggers or other craft where CG is aft of the "point of lift"...
 

Baronman

Well-Known Member
The center of lift you're talking about has a name, "Center of Pressure."
The CG is normally located in front of the COP, if not the aircraft would become much more unstable. That's were you get into Static Instability etc. (I think*)


mmm...I don't know where you're getting the part about taildraggers having an aft center of pressure. Their gear is just glued on in a different place!! Once they're in the air, the same aerodynamic rules apply.
Hey, some Cessna 150s have been converted from tricycle to "conventional" hehe. But, I don't think the plane knew the difference once it was in the air....At least no 150 has complained to me about it. ;-)
 

E_Dawg

Moderator
[ QUOTE ]
I'm still curious, however, if this conclusion would hold for taildraggers or other craft where CG is aft of the "point of lift"...

[/ QUOTE ]

I think your confusion stems from the inherient instability of taildraggers on the ground, not in the air.

Remember that once airborne, no one cares where the wheels are; the problem with taildraggers on the ground is that the CG is indeed aft of the mains, whereas in trikes the CG is forward of them. But the wheels have NO bearing on the 'point of lift'; only the design of the aircraft does. Stable airplanes will have a CG foreward of the center of lift, and taildraggers definately qualify.

Anyways... as long as the CG is fore of the center of lift you'll have a stable airplane because the nose will return to it's original position after an upset of a trimmed condition. In other words, if it stalls, the nose drops; if the CG is AT the center of lift the nose will pretty much just sit there after a stall, and if the CG is AFT of the center of lift... I hope you brought a parachute.

Reason being that once you lose the lift during the stall, the position of the CG 'takes over' and the nose will follow.
 

cointyro

New Member
Ahh, thanks SkyGuyEd. I guess that makes sense... the center of pressure (center of lift) MUST be aft of the CG in the air regardless of where it is on the ground.

Great posts Baron, I really appreciate it!
 

aviator

New Member
If you look at the majority of Burt Rutan's airplane designs (like the long EZ) he does away with the horizontal stabilator all together. He moves it to the front (where a lot of expieremental pilots feel it belongs) in the form of leading edge cannards. Now all surfaces provide positive lift and enhanced stalling characteristics are an added bonus.
 

cointyro

New Member
Hey Aviator...

front canards only enhance stalling characteristics if they have a smaller angle of incidence than the rear wings right?

That is, their COA must be lower than the rear wings?
 

E_Dawg

Moderator
No, it must be higher than the main wings.

That way the cannard stalls first, before the main can.
 

Baronman

Well-Known Member
I think the arguement Canard vs. Rear tail is a tricky one..

You're right, with a Canard both wings are making lift upwards instead of one up and one down. So that's good right?

Well, a disadvantage of the Canard is that the forward wing will reach its Critical Angle first, thus never allowing the rear wing to reach it's maximum angle of attack and not producing it's maximum lift.

I have yet to see heavy lifter with a canard design. But lots of fighters nowadays have canards!! The Eurofighter (Typhoon) and some of the Saab fighters for example. Also I was reading, many cruise missles are canards....
 

cointyro

New Member
[ QUOTE ]
No, it must be higher than the main wings.

That way the cannard stalls first, before the main can.

[/ QUOTE ]

SkyGuy Ed... higher COA means it wouldn't stall until after the rear wings... which is the reverse of what happens. The canards must have a lower COA than the rear for the canards to stall before the rear wings... i.e. the COA is reached on the canards before the wings, right?
 

E_Dawg

Moderator
Oh I get what you mean... COA = CRITICAL angle of attack.

I thought you were asking whether the cannard would need a higher angle of incidence.

Anyways... I guess the answer is whatever it takes to ensure the cannard stalls first; either higher angle of incidence or a lower COA.
 

Bob_Beziat

New Member
Coin

Baronman had a great explanation!!

In the Airbus A-310-300, the horizontal tail holds some 10,000 lbs. of fuel. There is a "center-of-gravity control computer" that transfers fuel in and out of the tail tank to keep the CG back around 37% aft of MAC. The usual CG on the 737 is somewhere around 20% aft of MAC (plus or minus).

In the A-310, keeping the CG aft unloads the tail so the wing doesn't have to create more lift to compensate for the downforce the tail creates, and results in better fuel efficiency.

There may some other aircraft that do this as well - I think the MD-11 has some fuel in the tail.

Bob
 
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