Why overstress the airframe? I did say no bumps. Surely the only result is an increased Vr and a reduced rate of climb but otherwise no real additional forces?
I'm just trying to understand. I guess I don't just accept something "because somebody says so". I need to know why.
"Why" questions are best answered by a four year degree in the field you are inquiring about. Then you can wax eloquently on the subject until your listeners ears bleed. Having said that, here's a very, very brief primer.
There are two ratios that are very important in plane physics. The first is wing loading. If you take the area of the wing, and divide the max gross weight of the plane, you get the wing loading. Like this:
2200Lbs/150SqFt = 14.666666... *I don't know the actual wing area of a 172, so bite me
Now, if we start adding weight, without adding wing area we get into the point where the increased angle of attack will be too high in drag for the plane to fly or climb:
2600/150 = 17.333333333... *the wing did not 'grow' any
As the wing loading number increases, the angle of attack of the wing to produce added lift makes the plane hard to handle, and at some point, the wing will need to be going so fast at take off that it will have no reserve lift for climbing out of ground effect. This is notwithstanding the engine power which we'll get to next.
All airplanes, and all wing shapes are not created equal. The 1960 Piper Pace has a rather short, fat wing which is suitable for a range of speeds between about 55-145MPH. No matter how much engine you hang on the 1960 Pacer, at some point, that big fat wing will no long push through the air with any amount of weight and that is the limit of that. The Piper Aerostar has a thin wing more suitable to high speeds, and the takeoff speed is concomitantly higher as well as the wing loading.
Next we cover power loading. Like wing loading, we take the weight of the aircraft and divide it by the HP:
2200Lbs/145HP = 15.17
As with wing loading there are limits involved. To have greater climb, and go a tad faster Cessna has upped the HP of the 172 over the years, and improved the speed a bit, as well as upped the weight as well. Numbers for a 172Q:
2550Lbs/180HP = 14.1666...
Suppose we used the higher weight of the later year, with the lower HP of the early year:
2550Lbs/145HP = 17.59
As with wing loading, power loading has limits. It works together with wing loading to maintain a safe margin of HP to weight so the plane has sufficient reserves to climb. Some planes have higher limits, or one might say lower margins. This is not to say that a plane with 2550Lbs and a 145HP and 150SqFt of wing area, won't work, But the design margins have been compromised, and the operation in all situations would be less than the designer intended. You are - in effect a test pilot because you are operating outside the boundaries of the design parameters.
. So you take three passengers each underweighing themselves by 10-15 lbs then bringing a bag which is really 45 lbs and not "about 30 lbs" as they told you. I know this from experience being a passenger on other flights. Add that all up and the pilot flying might think he is only at gross but could easily be 50-60 lbs over.
