max. gross?

[rorythedog]

i'm curious as to how the max. gross weights are derived for our planes? my 1978 m20j has a very generous balance envelope, but is somewhat limited by a 2740 gross weight limit. i see that newer versions(mse) have a 2900 pound mgw, and missile conversions are rated at 3200 lbs.

does anyone know what the differences are, or what the limiting factors are? i'm only curious...not looking to break any rules that are the cumulative knowledge of 100+ years of flight.

thanks for any input on this topic! 

[craigsteffen]

[i'm not an aeronautical engineer, just my sense of things from a physics background.]

Max gross is limited by structural strength of landing gear plus body.  The plan has to be able to take a hard landing at full gross without deforming, with proper engineering margins.  (For larger airplanes, this is modified by having a max takeoff weight that's bigger than the max landing weight.)

Max gross is ALSO limited by the power that the engine can lift.  I presume that there's some formula that they use to say that you have to be able to take off with your stated gross weight at so-and-so density altitude, and again with engineering margins.

Forward and rear CG considerations are aerodynamic.  The farther to the rear the CG is, the less pitch stability the airplane has.  Too far to the rear and the plane won't return to its trimmed pitch when the controls are released.  That's bad, so there has to be a margin for it. 

Too far to the front, and the airplane is TOO stable, making it diffiicult for the pilot (or autopilot) to maintain proper climb/decent.  This gets worse at high weights, which is why (for many airplanes) the CG envelope has a notch off of the high-weight, most-forward corner.  Basically with too far-forward a CG, you don't have the elevator authority to lift the nose for takeoff. 

Both ends of the CG envelope are strongly dependent in the fine details of the aerodynamics of the whole airplane, particularly the size and position of the control surfaces.  For instance, making the elevator bigger or farther aft probably makes the envelope bigger, because you have more elevator authority.

[Hank]

We have a saying in the radio control community:

          Nose Heavy flies poorly. Tail Heavy flies once.

If you're too nose-heavy, going up is difficult, and you have a one-way ride to the ground. If you're too tail heavy, you will stall;  proper stall recovery is full down elevator, but you won't have enough to break the stall more than temporarily. You may stall, recover, stall, recover, repeatedly, and if you're lucky enough, a runway will be a very few feet below your lowered gear when you stall the last time. Don't bet on it! Yes, this is also a one-way ride to the ground, and can either be leaf-type fluttering or [much more likely], stall, spin followed by lawn-dart-like impact, pointy end first. And it does not matter if the too nose/tail heavy condition exists at takeoff, happens as fuel burns off, or if the load shifts in cruise from passengers moving around or turbulence tossing things around, or a big dog hopping between seats.

Max gross is related to structural strength of the aircraft and thrust produced by the engine [lift is proportional to speed; more weight requires more speed to lift; more weight requires more thrust to move; more weight reduces speed, which reduces lift. As with airframe icing, it's a vicious circle.]. Flutter of aircraft controls also comes into play, as greater deflections are required to move the aircraft at higher weights, which creates higher forces on the wing tips and tail. Just like with loading, applied load = moment arm x force. Look at the size coonnections of your ailerons to the wing some time, not to mention the little bitty bolt that holds the tail to the airframe.

Some J's have beefier gear than others, and can have their max gross raised. Some cannot. It is serial number specific. IIRC, there were also changes made to the steel cage around the cabin when the gear was beefed up, but take that with a grain of salt. Additional landing weight creates higher loads that must be carried somewhere, so it makes sense to me. But for myself, it doesn't matter as I don't have a J and my max gross is just 2575 lbs.

[Piloto]

Although many of the above items described will limit the gross weight the dominant factor for a 36' wingspan plane is the stall speed in landing configuration. This speed can not be higher than 61kts at gross weight for a single engine plane as per FAR 23. To increase the load carrying capacity without increasing the stall speed the wing would need to be longer (like on the Malibu) or the flaps longer and wider or a fatter wing (Saratoga). Also the tires would need to be changed to a higher weight rating. A plane with a longer wingpspan would require a bigger hangar because it would not fit in the traditional 40' T-hangar. This could limit the marketability since a bigger hangar is harder to get and substantially more expensive.

José

[GeorgePerry]

that's one reason the diamond line of aircraft haven't been more sucsessful.  The DA-40 has a 39' 2" wingspan and the DA-42 on top of the engine debacle has a 44' 6" wingspan.  Niether fit into a standard hanger.