I was just curious, I'm planning on never exceeding the load and do CG etc. on each flight but I was curious about it. Watched a show about outdoorsmen where one guy in Alaska was flying with something resembling a cub, filled to the brim with supplies. Got me wondering.
Also about changing CG as one flies. Slightly overloaded say, but as one uses up fuel, the CG changes under way, and I guess depends on where the tanks are, but was thinking it might make it go to tail heavy.
Generally just interested in experiences with this subject.
Good question - And one that should probably be asked more. I'm reminded of Rick Durden's article "Buzz Jobs: A Frank Discussion" in which he states that the FAA's "just don't do it" stance hasn't worked at all to reduce the number of crashes due to pilots buzzing things, so he talks about what it's really like. It's refreshing to see that sort of discussion because unless people understand the "why", the "what" just ain't gonna happen.
The other thing is, flying at gross vs. solo is different in a lot of the same way as flying over gross would be vs. flying at gross. However, we don't know how the aircraft will perform over gross because the charts stop at MGW.
So... Why is MGW what it is? Depends on the airplane. For some it's a structural limitation, for others it's a required performance limitation (such as the example of limiting flap travel on a 172 to increase MGW). With a few exceptions, we don't really know what caused the gross weight to be set as it is.
We do know that the limitations are such that the aircraft should be able to take 3.8 Gs at MGW without the important bits falling off. That's about it. We see things like the Alaska rules, the Cessna TCDSs, and even things like Max Conrad's legendary Casablanca-LA flight where he took off at nearly double the legal max gross weight of the Comanche 250 he was flying, and we think it must be OK. And, inevitably, we still end up with a fair number of crashes each year due to being too heavy for the conditions being flown in.
First things first: Speed. Many of your V-speeds will be different, related by the equation Vactual = Vbook * sqrt(actual weight / max gross weight). This is just as true when you're flying under gross, so it's a handy thing to know! This equation should work pretty well for your stall speeds, approach speeds, and Vy. It'll work to some extent for Vx, though I would expect more variation there. It does NOT work for Vne - That does not go up with increasing weight. Want to nail that short field landing when you're light? Adjust your approach speed using this equation to keep from floating too far.
Your acceleration will be lower because of the higher weight, yet you'll need to accelerate to a higher speed for takeoff. So, runway performance is a real killer if you're over gross. It's very important to remember, at any weight, that you'll reach 3/4 of your takeoff speed when you've used half the runway you're going to use. So, if you're on a 3000-foot runway and it takes you 1500 feet to get to 3/4 of your takeoff speed, your mains won't leave the ground pretty much until the pavement ends. Don't do that.
Climb rate is a function of excess horsepower, that is, the horsepower that isn't required just to keep the plane flying at Vy. 1 hp is about 33,000 lb ft/min. If you figure out what your climb rate is at a given weight, you can figure out how many excess horsepower you have at that weight and subtract to figure out the non-excess hp. Now, if you subtract that from your available horsepower at a given density altitude and plug in your actual weight, you can figure out roughly - and I do mean roughly - what your climb rate might be. It's likely to be worse than what's calculated, because induced drag will mean that the non-excess horsepower will be higher so you'll have less excess horsepower available for climb.
Excess horsepower can make up for a lot of other flying sins. Now, you're going to give it away? Don't do that.
Now, fuel. Your climb rate is going to be reduced, so it's going to take you longer to climb, and you're going to burn a LOT more fuel getting to cruise altitude. I know the planes I fly that have fuel flow in them burn about twice the fuel during the takeoff roll as they do in cruise. Do you have the instrumentation to know how much fuel you burned? If not, plan on at least double the fuel burn from what the POH says for flying at MGW... And since you already likely loaded minimum fuel on board to get as close to MGW as you could, you probably don't have enough and you're going to run out. Don't do that.
I think that most pilots are thinking about structural failure when they're thinking about flying over gross. I would bet that in reality, it's a couple orders of magnitude more likely that you'll kill yourself with a performance-related issue than a structural one if you fly over gross. Is 5 pounds going to kill you? Probably not. But where's the line, and will you be able to see it before you cross it?
