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Monday, July 11, 2011

Understanding CG


Chris wrote...
The F4U is too small and light for my CG tool from Great Planes. Where should the CG be based on your experience and how do you measure it with something this small? I'm giving the plane one more try before it (or what is left of it) hits eBay. More generally, what flight characteristics are consistent with having a tail-heavy plane?

Hi Chris,
Typically, a warbird will balance between 20% and 30% behind the leading edge of the airfoil chord line, as measured from leading edge to trailing edge at the wing root for a straight wing.  Use the mean/average chord line for a wing with sweep. 25% is a decent starting point, as a rule of thumb, and will typically balance somewhat nose heavy.  

Another geometric indicator is to balance at approximately the fattest point of the airfoil's cross section.

It's more important to understand the concept of setting the CG rather than memorize rules or numbers, so you'll never have to wonder what the hell you are really doing.  I used to teach a thought experiment with a dart, to get the aero concept across.

In order for an object to be inherently stable in the air, the center of gravity (CG) has to be in front of the center of lift (CL).  The CL is the point where the aerodynamic force is felt, or, roughly, the geometric center of the lifting surfaces.  A dart makes this concept easy to understand, but I'll draw it with crayons anyway:
Obviously, if the fins (CL) were in front of the CG, the dart would not be stable in the air.  In fact, it would flip around and fly backwards.  The distance that the fins are set back from the CG determines how stable the dart is.  If you make the moment arm really long, you have an arrow, with arrow-like stability.  If you make the moment arm really short, the slightest nudge sends the airplane darting off in a new direction.

But also, if the CG is near the middle of the wing, it is relatively easy to lift the weight of the plane.  If the CG is way forward of the CL of the wing, you need to create twisting moment to lift the weight of the plane, using  down force at the tail, like a see saw or lever.  That is why the elevator uses negative camber to create a down force, which results in a level-flight balance of forces, or if more is added, an aircraft nose-up attitude or climb.

So...

When the airplane is in perfect balance, you have the desired amount of stability, held in place with a little bit of down force at the tail. 

That mini revelation leads us to the best "CG test."   To test your CG location, set up a comfortable cruise, or, the speed at which you wish to fly your plane effortlessly.   Trim the plane up.  Meaning, find the exact amount of elevator down force required to fly straight and level at that speed, using clicks of elevator trim.

Once the plane is trimmed for level flight at that speed, pull the power to idle, smoothly.  If the plane wants to dive as it slows down (and thus the elevator loses effectiveness) then you know that you needed a lot of aerodynamic down force to balance the plane--the plane is nose heavy.  If the the aircraft wants to climb as it slows in idle power, then you actually had up force at the tail--the plane is tail heavy.

Also realize that the thrust line, if out of whack, can provide another source of aerodynamic balance at speed.  But in idle, that force will evaporate too.  So the best measure of CG is this: do you have a nice gradual "straight-ahead" glide, no dive or climb tendency, as the power comes off.

Finally, it should stand to reason that yaw stability is closely related to pitch stability, for all the same reasons.

Therefore...

A nose heavy plane: 
  • Is usually flyable given enough airspeed
  • Very stable tracker without much "piloting"
  • Takes more effort to maneuver
  • Flies best at higher airspeeds
  • Needs little or no rudder input to stay on course
  • Wants to dive as the power comes out
  • Needs to glide fast and loses altitude quickly
  • Runs out of aerodynamic elevator authority as you slow down for landing
  • If you add too much up-elevator in the glide, it stalls or drops a wing
  • Needs some speed and up-elevator to keep the nose from falling
  • Lands hot, or nose first
A tail heavy plane: 
  • Might be un-flyable depending on severity
  • Darts up or down with small elevator changes
  • Very aerobatic with a lot of "piloting"
  • "Digs in" while maneuvering, exacerbating your inputs
  • Flies best when floating slowly
  • May need constant rudder attention to keep it pointed where you want to go
  • Nose wants to rise as airspeed bleeds
  • Doesn't cover much ground in a glide, even if the nose is up
  • Has too much control authority during landing, may porpoise or dart in gusty winds
  • Needs some speed and down-elevator to keep the tail from falling
  • Wants to stall high and pancake, or land tail first
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