0
$\begingroup$

In straight and level flight, lift is determined by speed and angle of attack. If we are heavier, we must go faster to generate more lift, at the same angle of attack. At the same speed, angle of attack must be increased for more lift, hence the saying "stall speed increases with weight".

We know lift is less than weight in a descent, because the drag vector has a vertical component opposite weight.

We know lift = weight in level flight.

We know the wing is much more efficient than the prop at producing lift. By simply raising the nose in a condition of 2400 lbs weight and 600 lbs thrust, sin angle × 600 obviously does not equal 2400 - cos angle × 2400.

We know in a climb there is a vertical component opposite lift.

Does this not in effect make the plane "heavier"?

Does this not allow us to keep optimal AOA, and steady state (not curving up or down) flight, at a slightly higher airspeed when climbing?

Should we not pitch to a slightly higher airspeed in a climb for maximum aerodynamic efficiency?

$\endgroup$
3
  • 1
    $\begingroup$ For some reason you closed your own question, which has the answer: aviation.stackexchange.com/a/77045 $\endgroup$ – ymb1 Apr 20 '20 at 15:54
  • $\begingroup$ @ymb1 fair enough, this question is a more refined version, dealing with an increased vertical force requirement in a climb. Do you think that statement is true or false. This discussion continues because many points of view are correct. $\endgroup$ – Robert DiGiovanni Apr 20 '20 at 16:15
  • $\begingroup$ Now, based on the $Lift$ equation, will a plane flying at its best AOA require more or less speed to create more vertical force and even if I double my thrust from 300 to 600 lbs, does that make up for 4x less efficiency than the wing? Have a look at Quiet Flyers Cessna 152 chart. Tell me how else it would be that way? $\endgroup$ – Robert DiGiovanni Apr 20 '20 at 16:18
1
$\begingroup$

We know in a climb there is a vertical component opposite lift.

Does this not in effect make the plane "heavier"?

Does this not allow us to keep optimal AOA, and steady state (not curving up or down) flight, at a slightly higher airspeed when climbing?

No, a plane is in effect "lighter" in a climb than in level flight.

Climbing "unloads" the wing-- the lift vector is smaller in a steady climb than in level flight

For any given angle-of-attack, the airspeed must be lower in a steady climb than in level flight.

And for any given airspeed, the angle-of-attack must be lower in a steady climb than in level flight.

In a sustained vertical climb, lift must be zero, with thrust supporting the entire aircraft weight (and also overcoming drag).

Covered in detail (including vector diagrams) here-- Does lift equal weight in a climb?



Note: the question stated that the goal was to maximize "aerodynamic efficiency". This would seem to imply that the goal is minimize fuel burn per altitude gain, or something similar. Assuming that the engine is equally efficient at all airspeeds, it would seem that this would be achieved by maximizing the L/D ratio, in which case the concepts stated above are relevant. Since angle-of-attack is tied to L/D ratio, any shift in the correlation between airspeed and angle-of-attack will change the airspeed that we should fly at, although the shift may be too small to notice in actual practice. For shallow climb angles the Lift vector is really only reduced by a very small amount, and since Lift for a given angle-of-attack varies according to airspeed squared, the resulting shift in the airspeed associated with a given angle-of-attack will be quite small.

While it's good to understand the basic physics at play, we should be clear that in actual practice, it is by no means always optimum to climb at an airspeed that is lower than the airspeed corresponding to the attack that yields the maximum L/D ratio in level flight. It depends on what parameter we are trying to optimize and on what other variables are at play. In most aircraft, these other variables will produce changes in the optimum airspeed for climbing that completely dwarf the slight shift in the relationship between angle-of-attack and airspeed that is caused by the "unloading" of the wing during the climb.

We are more often interested in climbing at the maximum possible rate, or the maximum possible angle, than we are in climbing in the most aerodynamically efficient manner. Particularly in aircraft with piston engines, the difference can be enormous.

In theory, for a fixed amount of Thrust, the steepest climb will be obtained at the angle-of-attack that gives the highest possible L/D ratio, but this will not be the fastest climb. Climb rate depends on the difference between power-available and power-required. Even though the speed where minimum power is required is actually slightly below the best L/D speed, in an aircraft with a fixed-pitch prop the speed for best climb rate is usually above the best L/D speed, because of the greater power available when the engine can turn at a higher RPM. On the other hand, since Thrust equals power divided by speed, in a piston-engine aircraft more Thrust will typically be available at low airspeeds than at high airspeeds, despite the loss of RPM. Since climb angle depends on the difference between Thrust and Drag, in a piston-engine aircraft the steepest climb angle typically occurs at an airspeed a little below the airspeed that yields the maximum L/D ratio, i.e. at an airspeed a little below the airspeed that yields the minimum amount of Drag. For general aviation light airplanes with piston engines, these kind of effects related to available Thrust or Power will usually have an effect on the airspeed that yields the maximum climb rate, as well as the airspeed that yields the maximum climb angle, that completely dwarfs the effect due to the slight "unloading" of the wing by the climb angle, and the resulting slight shift in the airspeed that is associated with any given angle-of-attack.

Example-- from this Cessna 152 data sheet--

Best climb angle 55 KIAS

Best glide speed 60 KIAS

Best L/D speed-- not stated

(Theoretical best L/D speed is slightly faster than best glide speed because the drag from the windmilling or stopped propeller biases the best glide speed to be slower than the best L/D speed.)

Best climb rate 67 KIAS



Related ASE questions on the relationship between Vx, V best-glide, and Vy:

How can best glide speed be lower than best rate of climb speed??

Is Vy closer to Vbg with a variable pitch prop

$\endgroup$
6
  • 3
    $\begingroup$ Best way to illustrate these things is to take them to an extreme by imagining the climb attitude getting steeper and steeper, and eventually the wing is completely unloaded and the propeller thrust is providing all the lift (assuming you have a thrust to wt >1:1). $\endgroup$ – John K Apr 20 '20 at 13:57
  • $\begingroup$ @John K that's right, and you only reduce AOA when drag from wing exceeds vertical lift contribution, then it's all engine to climb. The NF-104 story is a great one to read. A great pilot wanted to use to wing, and may have wound up saving so much fuel it was too much weight for the rocket at the end. $\endgroup$ – Robert DiGiovanni Apr 20 '20 at 15:57
  • $\begingroup$ Best rate of climb 67 KIAS, more thrust available at lower airspeed, points to less drag and/or more efficient use thrust, the wing. Viola! But you are right in that the advantages are very slight, and quickly drowned out if one tries to go still faster. I am interested in preserving optimal AOA for safety, if it can be done. $\endgroup$ – Robert DiGiovanni Apr 20 '20 at 16:05
  • $\begingroup$ @RobertDiGiovanni -- "Best rate of climb 67 KIAS, more thrust available at lower airspeed, points to less drag and/or more efficient use thrust, the wing." -- actually I think it simply points to more POWER available at higher airspeed, due to more RPM available. $\endgroup$ – quiet flyer Apr 20 '20 at 16:07
  • $\begingroup$ @RobertDiGiovanni -- I would like to see someone give a good answer to your older question aviation.stackexchange.com/questions/56084/… . As I understand things the answer ought to be "yes" but I can't confirm from practical experience. $\endgroup$ – quiet flyer Apr 20 '20 at 16:09

Not the answer you're looking for? Browse other questions tagged or ask your own question.