This is a discussion about that mysterious subject of the Center of Gravity on the Hobie Hawk sailplane (or any airplane for that matter). If you are new to aviation or are just curious about how the location of the balance point on your Hobie Hawk can affect its stability, hopefully I can shed some light on this sometimes confusing subject so you can set the balance point on your airplane more effectively.
I do not intend to throw out a lot of differential equations at you to explain this subject as it would only confuse the issue to the point where I could possibly confuse both of us!! To set the balance point you really only need to know two things:
1. What is the skill level you are flying at?
2. What type of flying will you be doing?
The question regarding skill is important. If you move the center of gravity it will affect the pitch stability of the airplane. Generally, moving the Center of Gravity (CG) forward produces the most stable flight characteristics and moving it aft can cause complete instability and loss of control. Most airplanes have a "spread" where the center of gravity can be located and the airplane will fly just fine. Stray outside this area and a lot of undesirable pitch problems and just bad performance (L/D) will result.
The type of flying you are doing is also important. Most thermal duration contests will require an extended period aloft and good piloting skills in detecting thermals are paramount. Your airplane can help with this task when the CG is set in proper position, especially with light thermal activity. With the CG set as far aft as you and the airplane can comfortably stand, it will change pitch attitude at the slightest whisp of lift and que you to the presence of lift as the pitch will be very sensitive to external forces acting on the airplane. Of course if you are heading to the slope in at 30 knot gale this is hardly an issue!!
To understand the relationship between CG and stability we need to bring in another factor that affects stability : Center of Lift (ie. Center of upward force) or as it is also known Center of pressure. The CG and CP (Center of Pressure) interact to produce a net force that affects pitch stability. There is ALWAYS some force generated by this relationship and it is counteracted by the horizontal stabilizer. This force will try to cause the nose of the airplane to pitch down or "Tuck". In flying wings this is counteracted by the reflex or turned up portion at the trailing edge in the airfoil.
There are three types of stability : Positive, Neutral, and Negative. When the CG is at the forward limit, the stability is Positive and as it progressively moves aft it becomes Neutral and then Negative. Positive stability is when you can dive the airplane and let go of the stick and the pitch will immediately rise to recover from the dive. Neutral stability is when you do that same dive and let go of the stick and the plane keeps right on truckin' into the ground in a straight line. Negative stability is when the plane will actually decrease in pitch or "tuck" as the dive progresses. That's BAD!!!
I have made some diagrams to better illustrate the CG/CP relationship. In order for the airplane to be stable in the pitch axis it must have the center of pressure located behind the CG. If the CG moves aft past the Center of pressure disasterous pitch excursions will result as the stabilizer can no longer counteract the "tuck" tendancy and will actually work against recovery efforts. The likely flightpath of an aft CG loading would be one that would look like a series of dives and stalls resulting in the eventual crash of the airplane.
OK. So that's what happens with an aft CG. So why not keep that CG way far forward and be safe about it?? Well, we could do that but some undesirable things will be happening that you might not even notice until you see your contest standings. I've heard this a lot on the flying field, " …Dang!! Why cant I get my nine minute flight with the same plane he has??!! This plane is a piece of crap!!!". Piloting skills is one thing but that aside, if the CG is too far forward a lot of drag is being created by the stabilizer to counteract all of that weight in the nose. Think of it this way : You add an extra ounce of lead in the nose to give better stability. The horizontal stabilizer must now counteract the extra weight in the nose with a negative force to balance it out in flight. So now there's more weight in the nose, the stab is pushing down also and the greater negative incidence of the stab now creates even more drag and consequently a very steep L/D glide angle. No wonder it flys like crap……Another thing that can happen with a forward CG is that at slow speed such as on landing, the stabilizer can actually stall and the nose can suddenly drop without warning!!
Most kit manufacturers will supply beginner and intermediate modelers with a CG location the is very much on the positive side of the stability spectrum. In other words, they have you stick a little extra weight in the nose to have the airplane fly a bit more stable so that if the novice gets into a dive or stall the airplane will be very quick to recover. If you are comfortable with your airplane and your flying skills you can move the CG to custom fit the task you will be performing with your airplane. The next section deals with the issue of "Dive testing".
Go to any F3B contest and almost all the contestants will have "dive tested " their Sailplanes to find the "sweet spot" for their CG location. What is the "sweet spot" you ask? Ask ten guys and you might get ten answers. Most pilots will get that CG as far aft as they can for a couple of reasons. The first one is that with an aft CG the pitch of the airplane will be more sensitive to thermals as this is closer to a "Neutral " stability setting. The second reason is that they want to remove as much weight as possible from the nose of the airplane for the same reasons mentioned earlier. Depending on the piloting skills involved and factors with the airplane such as the type of airfoil, the Sweet Spot could be anywhere forward of the center of pressure.
To dive test the airplane all you need is a calm wind morning with no thermal activity yet. This is the ideal condition as few outside factors as possible will be influencing the "test". Send the airplane up on the high start or winch and be sure that the plane is trimmed to fly nice and level (hands off) at cruising speed. Start the "test" from left to right or vice-versa. This way you can better see what the pitch is doing during the recovery phase. Put the airplane into a 30 to 40 degree dive for about 3-5 seconds (be sure the wings are level) and let the controls neutralize and watch what the pitch does. If it pulls nose up real hard in a recovery, this means you've got a forward CG and most likely too much ballast in the nose. If the plane keeps going in the same pitch towards the ground than that's Neutral stability and some folks fly this way in contests. If it "tucks" and dives even steeper for the ground, this is negative stability and can be corrected by adding more weight to the nose. Remove or add weight in small increments as necessary to get the stability for the task at hand. Some airplanes that have their CG set close to neutral (such as the Hobie Hawk) might have a tendency to tuck at higher airspeeds (The Center of Pressure actually migrates aft at higher airspeeds) so be sure to set the CG where you will feel comfortable with it in the speed ranges you will be flying in. That's it!!
Here are some diagrams to help explain all of this: