[Crumpp]

Hi Graham,

Let's not confuse our concepts. To avoid confusion and hurt feelings lets define our conditions. I am going to cover some basics again for everyone.

Let’s look at the entire curve and affect of weight. This avoids confusion when we get down in the weeds. When we get in the weeds it does seem like adding weight increases our speed and based on conditions the relationship is direct or inverse depending on what we hold constant. When we put it all together it requires more power to achieve the same performance. Increasing weight means we must travel faster to achieve the same performance and out limits of performance have been reduced.

Credit to Professor David F Rogers United State Naval Academy:




What we notice is that at any point on the L/D curve, the corresponding lighter weight requires less power and our aircraft can achieve a faster velocity. The effect is clear; add weight and our aircraft's performance is reduced. Vmax occurs at a slower speed.

If we get into the weeds on the specific effect:

Where some confusion on this issue seems to lie is in the fact in coefficient form, lift, drag, both have a fixed and finite relationship with Angle of attack. A Coefficient of lift has only one corresponding coefficient of drag and these occur at a specific angle of attack. For the reader, coefficients of lift and drag simply represent the ratio between lifting or drag pressures and the dynamic pressure.

If we keep velocity constant adding weight means the coefficient of lift must increase to meet the new lift required. This means the angle of attack must increase and with it the coefficient of drag.

If we cannot increase our coefficient of lift to meet the new lift required, then we must increase the amount of dynamic pressure to meet the new lift force required. The ratio of lift and drag pressure must remain constant if we are to keep our angle of attack the same. However to meet the higher lift forces required, we must raise the specific values of lift force to dynamic pressure.

The only way to increase the forces and maintain the same ratio at the same angle of attack is to increase the speed.

The confusion lies in the fact we no longer have the power available to overcome the higher drag forces required at the new velocity. Our sustained aircraft performance envelope shrinks and the angle of attack increases as our performance is now thrust limited.

Here is a "big picture" on these relationships:




Does this mean that "heavy" aircraft are "bad"? Aircraft are not a single characteristic but are a system. The effects of weight are easily mitigated through proper design. I certainly would not be so presumptuous as to assume any of the design firms in WWII were incompetent.

Facts are they were all the best and the brightest of their perspective countries. The design contemporary aircraft they produced were the F22 raptors of their day.


All the best,

Crumpp