I recall a story from an RC aircraft newsgroup many years ago in which a hobbyist had built a prototype aircraft, but through some oversight or design error had put the main gear too far to the rear with respect to the center of mass. On the first takeoff roll, when he reached what he thought was a reasonable speed, he pulled back on the stick, and found that he didn’t have enough elevator authority to pitch the nose up. So he waited until the plane picked up more speed, but still could not pitch up. Finally the plane reached the end of the runway, which had a short dropoff - allowing the main gear to drop, and letting the plane achieve a nose-up attitude and begin climbing. The plane flew just fine, so it wasn’t a problem with the center of mass, it was just that the main gear needed to be moved forward. He landed without incident, made the change, and that was that.
Clearly the designers of commercial aircraft have this all sorted out, so I’m curious: when a plane is at rest, how much of its weight is on its main gear, and how much is on its nose gear? If I look at a 747, I see 16 wheels on the main gear, and two on its nose gear, for a ratio of 8:1, which would suggest ~11 percent on its nose gear and ~89% on its main gear, assuming the main/nose tires are of comparable rating (I don’t know). Is this close to correct?
The DC9/MD80 was especially light on the nose gear. At very light aircraft weights it was possible to taxi over bumps and have the nose strut extend almost fully. At which point some microswitches would close indicating the aircraft had become airborne. Which confused a bunch of downstream systems since we were still very much on the ground. That was our cue that either the switches were out of adjustment, the nose strut was over-pressurized, or the CG was far aft.
The 727 was also pretty light on the nose. During un/loading it was normal to extend the tail stairs to act as a support for the back of the airplane. That ensured it didn’t sit back on its tail if the front was unloaded before the back.
Most jets we use steady forward = nose-down control inputs through the takeoff roll to ensure positive nose gear traction. Absent that once the speed picks up the airplane wants to weathervane into the crosswind. Which is not a happy thing.
You could remove fuel from the aft fuel tank or add fuel to the forward fuel tank if it had them. Many commercial aircraft just have fuel tanks in the wings.
We lowered the nose on an F-111 by defueling the aft tank. Some big dummy (not me) sat it on it’s tail bumper by not watching how they were refueling. The nose came down nice and slowly.
I’m speculating here, but IMO **JerrySTL **has it.
Typical airliner fuel tanks are in the wings and the center section where the wings connect to the fuselage. Given swept wings, fuel in the outboard wing tanks tends to make it tail heavy whereas fuel in the center tanks makes it nose heavy. As well, some models have additional fuel tanks fore and aft in the fuselage barrel itself.
On the ground fuel can be transferred between any combination of tanks to drive the weight forward.
Typically you’d expect a tail-sit to have been caused by loading heavy cargo aft without pre-loading any cargo or fuel weight forward. Depending on what was loaded how you could also reverse the process by unloading some cargo. E.g. baggage in the belly of a passenger jet is a different problem to remove versus, say, heavy machinery on pallets on the main deck of a cargo jet.
Though that would require a load crew willing to climb in there while it’s sitting on its tail. Not generally a great idea.