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The question arise while discussing with some of my student pilot friends that as we have been performing Slow Flight we have to keep increasing the AOA of the aircraft to compensate for every lose in airspeed. What about "FAST Flight", decreasing the AOA of an aircraft for every gain in airspeed. Maybe Negative AOA. I'm aware that lift could be generated with negative AOA from the graph of AOA vs CL. And if you could explain to me why there is a specified cruise setting?

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    $\begingroup$ Straight and level is usually a term meaning "holding altitude and heading", not necessarily the fuselage being level with respect to AoA. So the answer to the title of the question is yes, unless you run out of elevator authority or lift you can fly "straight and level" at any speed $\endgroup$ – Ron Beyer Jan 14 '17 at 13:40
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Yes, with good engines and low drag you can fly straight and level at really high speeds.

What about the AOA? It gets smaller, but only down to a certain limit. At level flight, the weight of the aircraft is equal to the aerodynamic lift:

$$ W = L =C_L\cdot\frac{\rho}{2}V^2\cdot S $$

The lift coefficient depends on the AOA ($\alpha$):

$$C_L=C_{L0}+C_{L\alpha}\cdot \alpha$$

Therefore,

$$\alpha = \frac{1}{C_{L\alpha}}\left(\frac{2W}{\rho V^2 S}-C_{L0}\right)$$

For very high speeds the first term of the parenthesis will get close to zero, so that the desired AOA setting becomes

$$\alpha_{V\rightarrow\infty} = -\frac{C_{L0}}{C_{L\alpha}}$$

This AOA is indeed negative for most aircraft. If your AOA gets smaller you won't be able to sustain level flight.

The plot below shows the AOA needed for straight and level flight on a Cessna 172 and compares the two terms in the parenthesis of the equation for $\alpha$ (multiplied by 10 for better readability). You would need to fly much faster than $V_{NE}$ to get the first term significantly smaller than the second one.

enter image description here
Source: own work with data from Wikipedia and polar.

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In order to fly at top speed with the fuselage level you need to pick the right altitude.

Consider the picture below: It shows a Northrop B-2 (foreground), a Rockwell B-1 (midlevel) and a Boeing B-52 (background) flying in formation at moderate altitude. The fuselage attitude is considerably different between all of them.

Northrop B-2 (foreground), Rockwell B-1B (midlevel) and Boeing B-52 (background) flying in formation

Northrop B-2 (foreground), Rockwell B-1B (midlevel) and Boeing B-52 (background) flying in formation (picture source)

Why the difference in fuselage attitude?

  • The B-2 is designed for midlevel cruise and has a fairly high aspect ratio, which translates into a decent lift curve slope. Here it flies close to its design point and looks about level. A high lift curve slope means that small angle changes suffice to correct lift for off-design situations.
  • The B-1A was designed for supersonic flight first and was modified for subsonic low-level penetration in the B-1B. Here it flies in less dense air and slower than its design point, and you can see that it needs to keep the nose up. This is amplified by the high sweep angle and low aspect ratio with backwards-folded wings, which results in a low lift curve slope, so a relatively large angle change is needed to fly slower and higher than designed.
  • The B-52 in turn was designed for very high subsonic flight, so here it flies in much denser air which requires it to fly with a negative fuselage attitude to limit lift.

Each of the three will fly with the fuselage about level in its design point. This can be generalized for any airplane: All what is needed to fly fast with a level attitude is to adjust the flight level to something close to the design point.

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