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I have been struggling with the following question for a while, would greatly appreciate some help and explanation why.

Scenario:

  • I'm cruising in my Cessna- cruise attitude 2300 RPM trimmed for 95.

  • I pitch up, say 1 inch of back pressure, and hold it.

Initially the plane starts climbing and slows down, but what happens next?

How will the plane look in say 5 minutes with me still holding that 1 inch pitch up, what attitude, speed, and altitude will it have?

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  • $\begingroup$ Are you talking about one inch of back pressure as measured by the back pressure to get that displacement on the artificial horizon or the back pressure to initially get one inch of backward movement of the yoke? $\endgroup$ – Terry Jul 22 at 6:17
  • $\begingroup$ It may interest you to read the well-known aviation book "Stick and Rudder". Chapter 1 deals with the relationship between attitude, speed, weight, and angle-of-attack. $\endgroup$ – Jamiec Jul 22 at 11:00
  • $\begingroup$ You will fly more slowly and climb, but the amount of change depends on the static stability of the plane. Without knowing that, I cannot be more precise. $\endgroup$ – Peter Kämpf Jul 22 at 11:35
  • $\begingroup$ Terry- I think he means just what effect does pitch have on the 4 forces. To answer your question I think he means just any elevator deflection relative to the trimmed cruise position, just to pull the control column 1 inch back relative to cruise meaning the slightest deflection of the elevator upwards. $\endgroup$ – leha007 Jul 22 at 14:49
  • $\begingroup$ My attitude improves with pitch and speed... $\endgroup$ – Ron Beyer Jul 22 at 16:39
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Well, you knew to be specific about the plane, the RPM, the airspeed, and the attitude.

Pulling the yoke back without changing anything else will create excess lift, and a climb begins. This is the only point in the scenario you have excess lift because: increasing AOA increases drag.

The plane then slows down for 2 reasons. First a higher AOA means more drag. Secondly, as the plane pitches up, the gravity vector increasingly contributes to the drag vector. (A descending aircraft, nose "down", allows gravity to contribute to thrust).

Specifically for a 172, there are 2 outcomes. In this case a steady climb is likely at a lower airspeed. If the yoke is pulled harder, the plane continues to pitch up and stall (with more power and/or airspeed it may loop, but I would not bet on it at 90 knots, 75% power from level flight in a 172).

But 5 minutes later? As you climb, the engine produces less and less thrust. Assuming static stability (a hallmark of a properly CG balanced 172, checked pre-flight), the rate of climb will decrease until the plane has insufficient thrust to continue climbing. It will then fly level at its trimmed airspeed. (If you have sufficient oxygen and no head wind, your ground speed will be higher).

But this provides an insight into what it is doing when it climbs in the first place. Since the wing is around 4x more efficient than the prop in producing lift, pitching up and "using the prop to climb" does not work very well. It turns out the mechanism of static stability works as well in a climb as anywhere else. As the plane pitches up and slows, enough lift is lost to cause the plane to sink (even though the nose is skyward). Sinking pitches the nose down, which increases speed, which increases lift. this works at any power setting, from gliding to full power..

We can see, "5 minutes" later, the plane no longer can climb, but still will be staticly stable at its trim speed so... cut the power, and enjoy the glide home at that airspeed (if you can make it), or, at least remember CARB HEAT ON.

And also, consult the POH for that flight envelope chart, and remember, excessive pulling the yoke can exceed G limits as well.

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  • $\begingroup$ Thanks guys. So what you are saying is, from our cruise at 95 IAS say 2000 altitude we will enter a climb, our speed will decrease, and we will slowly climb until say at 10,000 feet finally power required curve meets the power available curve, our plane stops climbing (at say the new speed of 80/70), And speed will increase back to the cruise 95? $\endgroup$ – leha007 Jul 23 at 0:52
  • $\begingroup$ Sorry Robert, not sure Im following you. I understand it will climb and when it reaches power required=power available the climb will stop. But what would the new cruise IAS be? You are saying it will cruise at its Trimmed airspeed. Wasn’t that trimmed airspeed 95? $\endgroup$ – leha007 Jul 23 at 0:55
  • $\begingroup$ No, because you changed trim by pulling the yoke back. $\endgroup$ – Robert DiGiovanni Jul 23 at 1:02
  • $\begingroup$ Ohh, I think Im starting to get it. So our new cruise altitude will be higher, speed will be lower: Thank you. $\endgroup$ – leha007 Jul 23 at 1:07
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    $\begingroup$ @leha007 well, you're getting comments anyways. Trim speed will not change, rate of climb does. If you go to max altitude, then let go of the yoke, your trim speed returns to 95, and your plane will return to its original altitude, because it has insufficient thrust to do 95 indicated at that altitude. $\endgroup$ – Robert DiGiovanni Jul 23 at 1:29
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The general answer goes something like this:

Level flight, at it's most basic involves a balance of 3 factors:

  1. How fast are you going, relative to the wind?
  2. What is the angle of attack (AOA) of your wings, relative to the wind?
  3. How dense is the air that you are flying through, based on altitude, temperature, humidity, etc.?

Speed and AOA are variables that you indirectly control, via throttle (by increasing thrust) and yoke (by using elevator to alter pitch) respectively. The thing is, changing one variable (speed, AOA, or density) requires that one or both of the others change to maintain equilibrium. Want to stay in level cruise at a given density altitude after lowering throttle? AOA must increase. If the AOA stays the same, the aircraft will begin to sink because the current density cannot provide enough lift at that combination of AOA and speed.

That explains some of the "Why". Now let's consider your scenario. You have taken throttle out of the equation. Thrust is fixed and cannot be changed. This means that for any change to AOA, density must provide the equilibrium. Unless you are already at your stall speed, a slight increase in AOA without lowering throttle will begin a climb at slower forward speed because this new combination of AOA and speed are producing excess lift at the current air density. You have traded forward speed for extra lifting force. Because the throttle and new pitch are now fixed, the airplane will simply climb at it's new speed and AOA until altitude and temperature lower the outside air density to the new equilibrium point. At this point, the aircraft will once again be configured for level flight at the new density altitude. Also, because the density will change gradually, so too will the rate of climb decrease gradually. It will probably take a good while to fully level out.

ADDENDUM:

The "lift" topic always sparks arguments, because technically, lift can be generated in any direction, depending on the definition of lift you are considering. An aircraft moving through the air on its side, may still create some small amount of force laterally, which opposes gravity, even though the wings may be producing no force at all. A helicopter in hover is utilizing all of its accelerated airflow for to counteract gravity, but a change in rotor disc tilt divides that total force between lift (opposing gravity) and thrust (opposing drag). An aerobatic airplane hanging on its prop is utilizing accelerated airflow in much the same way. An aircraft flying upside down is creating lift that is simultaneously against the pull of gravity AND toward the bottom of the aircraft.

Additionally, Bernoulli's principles work equally well in any orientation. An asymmetrically cambered airfoil mounted vertically will still create an area of lower pressure on one side than on the other, and is still capable of altering angle of attack to adjust deflection. Is this lift? The simple physics definition would say "No" because it is not opposing gravity. An aerodynamic analysis of the airfoil would say "Yes", the airfoil is creating lift according to its orientation, which in this case, affects only yaw.

It would be nice if the entire world could agree that Lift is the sum of all forces that act in opposition to gravity, and Thrust is the sum of all forces that counteract drag - regardless of whether these forces are provided by airfoil or prop (which is still an array of airfoils!). Unfortunately, for now, clarification is often required when discussing this problem in polite company.

In closing, I would like you to consider this: in your scenario, the reason that your airplane slows down is because, much like the helicopter, your prop disc is being tilted, dividing its power between lifting and thrusting. The drag on the airplane remains, so the added burden of lift is what leeches away the speed. The wings, on the other hand, no longer bear the full burden of opposing gravity, so the lift that they are producing DOES become less, but only because they are now sharing that load with the prop.

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  • $\begingroup$ Thanks guys. So what you are saying is, from our cruise at 95 IAS say 2000 altitude we will enter a climb, our speed will decrease, and we will slowly climb until say at 10,000 feet finally power required curve meets the power available curve, our plane stops climbing (at say the new speed of 80/70), And speed will increase back to the cruise 95? $\endgroup$ – leha007 Jul 23 at 0:52
  • $\begingroup$ Yes, you will climb. Yes, your speed will decrease. 10,000 feet is speculative - it depends on factors that affect the air density and how much trim you have already applied. I doubt that you will climb 8,000 feet from one inch of aft yoke (especially before running out of gas). Yes, when the curves are in equilibrium you will once agin be in level flight. $\endgroup$ – Aaron Holmes Jul 23 at 1:04
  • $\begingroup$ No, you will not increase speed once you get back to level flight. This will be the new density altitude that supports level flight for this new AOA/Speed combination - the air is thinner, so less thrust, so less excess power to make you climb, right up until you are level and can maintain the new, slower speed (80 or 70 kts, whatever it is). $\endgroup$ – Aaron Holmes Jul 23 at 1:04
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    $\begingroup$ @Aaron Holmes thanks, your explanation re-inforces a concept that is in the process of being improved on this site: lift is greater than weight in a climb. A great many people here thought it was less. At any power/trim setting, a staticly stable plane will pitch up (if too fast) or pitch down (if too slow). The throttle determines angle to the horizon and vertical lift determines rise or sink. As a 172 pitches up, the vertical thrust component is insufficient to replace the vertical lift (1:4 ratio), and the plane climb angle equilibrates based on thrust. $\endgroup$ – Robert DiGiovanni Jul 23 at 7:26
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    $\begingroup$ This one will die hard, but it must. First vertical lift is cosine lift, there for the lift must be greater than weight if the plane is pitched up or down. Remember, using the wing is always more efficient than thrust or drag! Second, any vertical thrust contribution is only 1/4 (for a 172) the vertical lift lost from pitch. In a "stabilized steady state" all upwards forces = weight, essentially rendering the aircraft "weightless" and thrust acts against drag. So happy with Aaron, because Rho got in there too. Rho is also part of the Lift equation, as is increased AOA. $\endgroup$ – Robert DiGiovanni Jul 23 at 18:55

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