# Why does power help us maintain altitude in slow flight?

I was asked this question and didn’t know how to phrase a nice and easy explanation. What I said is- well because drag is now increasing Un- proportionally to lift, we need something to generate more lift, and since pitching up like we would do in normal speeds isn’t an option because it would just increase drag more, our only way to create more lift is increase thrust- and in so increase that vertical component of thrust that acts as lift and help us maintain altitude.

Am I right in explaining it like that? And is there a nicer and simpler way to explain that? Something with the 4 forces and thrust-drag and lift-weight couples?

Thanks!

It is not correct to say that power controls altitude. In slow flight, it is taught that power controls altitude and pitch controls speed. Yes, it works, but you have to always keep in mind that a power change will always require a pitch change and a pitch change will require a power change, if you want to maintain the same trimmed speed, altitude and angle of attack.

As you might very well know, in a slow flight, you reduce your power first and start putting down the flaps to sort of keep the aircraft in an approach configuration. Then you pull back on the stick until you are close to the stall speed. Once the speed you want is reached, you add power as required to maintain speed and altitude. If you lose altitude, a little addition of power will arrest it. But trust me, this will also increase the speed by small margin. This might not be registered in the air speed indicator of a smaller aircraft. Similarly, an increase in pitch in attempt to reduce an increasing speed, will change the altitude. So, it is not fixed. Pitch and power are not two separate entities. They always work together. Try flying a high performance aircraft with powerful engines and with a load of inertia and you will see how true my words are.

The reason why more and more power is necessary to successfully accomplish a slow flight is because you are operating below the L/D speed of the aircraft. Once you are below this speed, the increase in induced drag (due to high angle of attack) increases the total drag on the air frame. To overcome this drag you are required to add power or energy. We call this operating behind the power curve or the region of reverse command. Reverse because normally, you need an increase in power to fly at a higher speed. But behind the curve, to fly at a lower speed, you need more and more power.

Look at the graph below, the GD (Green dot) speed is the L/D speed. As you can see, as you go below the L/D there is a steep increase in drag.

• I think it fair to say that power does control altitude; obviously there are other factors, but an increase in altitude requires an increase in potential energy, which is provided by the engine. In contrast, lifting the nose up will generally not result in an altitude gain, certainly if the aircraft is in slow flight already.
– Frog
Jan 17, 2021 at 9:50

The vertical component of the thrust line is a significant factor. Easiest way to visualize it is to take it to the extreme; an airplane with so much power it's able to hover on its propeller while pointing straight up.

Drop that back down to a deck angle of just under 15 degrees, and a simple vector calculation shows that an engine making 600 lbs of thrust has a vertical component of about 150 lbs, which is effectively adding to wing lift.

Take away the vertical thrust component helping the wings, say by having an engine mount that tilts down to put the thrust axis back to horizontal, and it's as if a 150lb passenger suddenly appeared in the seat beside you. It's not a lot, less than 10% of the total lift say, but it's not nothing either.

Add in extra lift from slipstream effects, and that's probably the major component of it all, in terms oh how power influences sink rate in slow flight.

On large turboprops that have huge propeller discs that may cover 1/3rd of the total wing span, slipstream effects are dominant with flaps down, and in the approach configuration, sink rate is directly and immediately affected by power changes.

Total Aircraft Drag is the sum of Induced Drag and Parasite Drag. Induced Drag reduces with (the square of) speed whereas Parasite Drag increases with (square of) speed. If we plot this we get a somewhat U shaped curve for Total Drag at different speeds.

If we draw a horizontal line representing a particular Drag value - Except for the lowest point on the curve, representing the least Drag point, every other point above lowest Drag on the Drag axis of this curve corresponds to 2 points on the curve, ie corresponds to two speeds. Since Power is proportionate to Drag, every Power setting corresponds to 2 speeds except at the lowest point on the Drag curve.

So, as speed increases in flight, the airplane could accelerate to, and maintain the first/slower speed, or further accelerate using available Power till it reaches the second/higher speed value.

Typically we would not want to fly at the lower of the speeds when the same power can give us a higher speed.

"Why does power help us maintain altitude in slow flight?" - actually, the forces and force couples at play are the same whether high speed or low, but from the the dynamic stability point of view, low speed and high speed differ as follows - lower speed lies in the zone of dynamic instability in speed, ie any drop in speed results higher Drag thus tending to naturally reduce the speed further unless Power is added to the equation. If power cannot be added, the only option is to push the nose down and trade altitude for speed (provided ground contact is not a factor!). Due to the shape of the curve, any hesitancy in taking the counter-action would result in a rapid speed loss requiring more and more power. This is commonly referred to by pilots as "the wrong side of the drag curve". Compare this to the right side/higher speed zone, where a drop in speed would result in lower Drag thus naturally tending to return the airplane to the desired speed without much by way of a Power input.

Similarly, for completeness of understanding, you may like to figure out in your mind, the dynamics of what happens when there's a slight increase in speed.

a nice and easy explanation ...

Aerodynamics ... Cessna 172 ... Slow flight

Things are generally safer when nice and easy so ...

For this type of aircraft, Center of Gravity is checked as part of the pre-flight. If CG is in range, your plane should be "staticly stable", and trimmed for speed. Confirm this with your instructor.

Now, because your CG is in the right spot and you know how your plane works, the nice and easy explanation is:

If my plane is trimmed for a given speed (65 knots for example), adding or subtracting power will make it climb or descend at approximately the same speed. If you have no power, your plane will glide at 65 knots. Adding power lets you control your glide slope at the same speed. Adding more power lets you climb.

Not all planes are set up like this. It is very important to get information specific to your 172, and to talk about it with your instructor.

• Yes thats true, but when we are cruising 90 kts, and want to climb from 2900 to 3000 feet, we don’t add power, even though it will indeed make us climb, we would just pitch up... So not sure “airplane wants to fly trimmed airspeed” is the best explanation for this. Jan 17, 2021 at 14:54
• @leha007 "2900 to 3000 feet" is a minor adjustment, in a landing pattern, slow, it is much safer to control altitude with throttle. Look at the graphs below, what might happen if you pitch up 200 feet AGL and start slowing down more? Not worth the risk. Jan 17, 2021 at 15:58
• It is very important to know that your "energy state" is V$^2$, at 90 knots you have plenty of it, at 65 knots not as much. Adding throttle (thrust) takes time to build speed (pitching down greatly helps), but if you get "caught short" low and slow you have neither the time or the altitude to recover. An unenexpected downdraft once had me skimming across the corn tops 1/4 mile short of the runway (I added power before climbing a bit). Jan 17, 2021 at 16:08