# Why doesn't aircraft weight affect descent angle in a gliding flight

• Leo Liu
In summary, the solution fails to convince me because C_D and C_L can be both written in terms of weight: C_L=\frac{2W}{\rho v^2 S} C_D=C_{D0}+k_1 C_L(W)+K_2 C_L(W)^2
Leo Liu
It's a homework question, but I feel like it fits better in this forum. The solution fails to convince me because C_D and C_L can be both written in terms of weight:
$$C_L=\frac{2W}{\rho v^2 S}$$
$$C_D=C_{D0}+k_1 C_L(W)+K_2 C_L(W)^2$$

Question:

Solution:
Any insight will be appreciated.

The thing is that lift and lift-induced-drag are two faces of the same coin.
Within certain limits of angles and speeds, useful lift can’t exist without the useless drag induced by the wings.

If more lift is needed to keep a heavier load flying at the same speed, proportionally more lift-induced-drag will result (increased angle of attack).
If speed is instead increased to achieve more lift, more parasitic drag will be induced (increased skin friction and shape drag).

A jumbo jet has a gliding ratio as good as a very light airplane.
Both have an optimal speed/AOA for which both drags reach a minimum value, resulting in more economic horizontal flight (less fuel is used for same covered distance).

https://en.m.wikipedia.org/wiki/Lift-induced_drag

https://en.m.wikipedia.org/wiki/Parasitic_drag

Astronuc and Leo Liu
There's another, more simplified answer to the question: a stabilized approach usually has a set angle for a given runway, called glideslope. Pilots will usually have external visual references, like VASI lights or the "sight picture" out the windshield, to help them line up on glideslope, and larger airports have radio beacons to help pilots alight with that slope in poor visibility. ILS systems are more expensive, though, and generally only appear at airports with frequent high-performance and turbine powered aircraft operations.

Flyboy said:
There's another, more simplified answer to the question: a stabilized approach usually has a set angle for a given runway, called glideslope. Pilots will usually have external visual references, like VASI lights or the "sight picture" out the windshield, to help them line up on glideslope, and larger airports have radio beacons to help pilots alight with that slope in poor visibility. ILS systems are more expensive, though, and generally only appear at airports with frequent high-performance and turbine powered aircraft operations.
That answer is not correct. The "glideslope" is perhaps incorrectly named as aircraft are not truly gliding but are using controlled power and flaps to achieve the desired descent angle. The OP was talking about actual gliding.

The glideslope varies by airport, although it is usually close to the clean glide ratio/angle for many planes. However during actual approach and landing with the plane in a dirty configuration with flaps in landing gear down the glide ratio of the plane is much worse or steeper than when clean. So if a plane loses its engine(s) on final approach it will not be able to glide to the runway.

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## 1. Why doesn't the weight of an aircraft affect its descent angle in a gliding flight?

The descent angle of an aircraft in a gliding flight is primarily determined by its lift-to-drag ratio, which is a function of its aerodynamic design and airspeed. The weight of the aircraft does not significantly impact these factors, so it does not play a major role in determining the descent angle.

## 2. How does the lift-to-drag ratio affect the descent angle of an aircraft in a gliding flight?

The lift-to-drag ratio is a measure of an aircraft's efficiency in generating lift compared to the amount of drag it produces. A higher lift-to-drag ratio means the aircraft can maintain lift with less drag, allowing it to glide at a steeper angle without losing altitude. Therefore, a higher lift-to-drag ratio results in a steeper descent angle.

## 3. Does air density affect the descent angle of an aircraft in a gliding flight?

Air density does play a role in determining the descent angle of an aircraft in a gliding flight. In denser air, the aircraft can generate more lift, allowing it to maintain a shallower descent angle. In thinner air, the aircraft may need to maintain a steeper descent angle to maintain lift and avoid losing altitude.

## 4. How does airspeed impact the descent angle of an aircraft in a gliding flight?

The airspeed of an aircraft directly affects its lift and drag forces, which in turn impact the descent angle. A higher airspeed means the aircraft can generate more lift, allowing it to maintain a shallower descent angle. However, a higher airspeed also means more drag, which can cause the aircraft to lose altitude faster. Therefore, the pilot must carefully manage airspeed to maintain the desired descent angle.

## 5. Can the pilot adjust the descent angle of an aircraft in a gliding flight?

Yes, the pilot can adjust the descent angle of an aircraft in a gliding flight by controlling the airspeed and angle of attack. By increasing or decreasing airspeed, the pilot can change the lift and drag forces, which will impact the descent angle. Additionally, adjusting the angle of attack (the angle between the wing and the relative wind) can also affect the descent angle. However, the aircraft's design and weight will still play a significant role in determining the maximum achievable descent angle.

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