Ex. 11 - Slow Flight

A. Flaps add drag, but also lift. If you lift the flaps all at once, you may lose that little bit of lift that kept you just on the edge of a stall, and the plane may stall.

B. So long as your airspeed is in the white arc, flaps can be extended or retracted freely, as far as their structural integrity is concerned.

C. The electrical system is capable of handling flap retraction and extension with no problems. Indeed, after landing, when loss of lift is not a concern, you can raise the flaps all at once.

A and B are wrong because a lower airspeed always corresponds to a higher angle of attack (on both the "front" and the "back" side of the power curve. C is incorrect because parasite drag is essentially air resistance, which decreases with airspeed. D is the right answer because a higher AoA is indeed required for lower airspeed, which means that the aerodynamic force, which is perpendicular to the wing chord, is pointing not just up, but also considerably backwards, creating induced drag. Overcoming this additional induced drag requires more power.

A. You need to be below the speed for best endurance to be in slow flight. Many students confuse that with "slow cruise" -- which is not slow flight, but rather straight-and-level flight at reduced airspeed (but still above best endurance).

B. Airspeed for best range is higher than airspeed for best endurance, so flight in this range of airspeeds is not slow flight.

C. Correct answer.

D. A stalled plane can perhaps still be considered flying, since wings are generating some lift, but it's really more like falling...

Ailerons are affected the most because they are positioned closer to the wingtips, away from the fuselage and thus from the propeller slipstream. Elevators and rudder are still in the slipstream, and thus are more effective, since more air is going over them.

Options A and C referring to sea level are obvioulsy incorrect. What iif your airport's elevation is 1900' above sea level? Then you'll be practicing these manoeuvres 100' off the ground?

Option B is better, but the Flight Test Standards (as well as many POHs), call for sufficient altitude so that you can recover from an inadvertent stall by 2000' AGL, which means you should be practicing above that altitude. How much higher? Well, depending on the type and severity of the stall and the promptness of recovery, you may lose anywhere between a couple and a thousand feet (of course, in theory, you could lose all altitude in a stall, if you don't initiate a recovery at all, but obviously you won't do that). So give yourself 500-1000' extra feet.

So, if your practice area elevation is 1000' above sea level, you want to add 2000' + (at least) 500'. So what you should see on your altimeter is 3500'.

And, by the way, how do you know your practice area ground elevation? Pull out your VNC and examine the colour-coding on it, as well as the figures next to major obstacles (refer to the legend and/or your instructor to make sense of them).

Airspeed: too fast = raise nose, too slow = lower nose.
Altitude: climbing = reduce power, descending = add power
Yaw: use rudder to keep the ball centred (which rudder will you need? review question 4!)
Heading: generally ailerons are used to keep the plane on heading when it strays, but in slow flight you may need to help yourself with rudder because ailerons become mushy and ineffective at low airspeeds.

A. Just look at the power curve. At airspeeds above best endurance higher speed requires more power; below best endurance (i.e., in slow flight), lower speed requires more power. Question 3 probes into why.

B. As we just mentioned, during slow flight a *higher* power setting is required for lower airspeed.

C. Would be a correct answer if "best range" was replaced by "best endurance".

D. This is only true in slow flight.

A. An extremely high rpm is not necessary for practicing slow flight. Most of the time students practice slow flight while maintaining altitude -- the rpm required for that is typically close to normal cruise rpm.

B. Due to the factors in answer B, the engine may in fact overheat during prolonged practice of slow flight.

C. Straight and level slow flight requires a more nose-up attitude than cruise flight, so forward visibility is indeed impaired.

D. Slow flight is a stable flight regime. It is not terribly efficient due to lots of drag, but if you maintain your attitude and power setting, the plane will remain in slow flight.

A. Correct! To understand this effect, visualize the propeller rotating clockwise (from the point of view of the pilot) and swirling the air around the airplane and pushing it back. As that clockwise-rotating air reaches the tail, it passes under it without a problem, but strikes the vertical stabilizer from the left side on the way up. This yaws the plane to the left.

B. Gyroscopic precession occurs during pitch changes. Visualize where you would have to push on the propeller in order to raise the nose up -- you'd have to push on the bottom of the prop disk. The gyroscopic action then translates that 90 degrees clockwise, pushing on the left side of the disk (from the pilot's perspective), so the plane yaws to the right. Therefore, during entry into slow flight, gyroscopic precession results in a RIGHT-yawing tendecy, though it is likely to be not very noticeable if the nose is raised gradually.

C. Yes, asymmetric thrust (a.k.a. p-factor) is a major reason you have to step on the right rudder pedal in slow flight! If you look at the propeller blades, you'll see that they are not flat, but rather shaped like wings. So, just as wings, they have an angle of attack, an angle at which they "bite" the air and create a force -- it's just that this force (thrust) is directed forward, not up as is the case with wings. Well, when the PLANE is at a higher angle of attack, the two propeller blades have different angles of attack. The downgoing blade (on the right) "bites" into more air, producing more thrust than the upgoing blade on the left. This pulls the plane to the left.

D. When the propeller rotates clockwise, it causes the plane to rotate counterclockwise (much like when you push on a wall, the wall pushes you in the opposite direction). This causes a ROLL to the left, which the pilot corrects with a roll to the right, which, as we know from Exercise 9, produces adverse yaw to the left.

A. Yes, flight at low airspeed (between stall and endurance) is the very definition of slow flight.

B. If you are trying to maintain altitude during slow flight, you will likely need considerable amount of power, close to cruise power, but techincally speaking slow flight can occur at any power setting. You can have little to no power and be descending at low airspeed, or full power and climbing at low airspeed. As long as you are doing those at low airspeeds, you are in slow flight.

C. First of all, not all planes are equipped with a stall horn warning. However, even if you plane is, it won't go off until 5-10 knots prior to a stall. Now, when practicing slow flight, you will try to get to the edge of a stall, to truly push the envelope and develop skills and coordination necessary for precise flying at this regime, but, strictly speaking, flying even a couple of knots below the speed for best endurance is slow flight -- and your stall horn won't go off then.

D. Yes. When you slow the plane down, there will be less air going over control surfaces, so the plane will always feel more sluggish than during normal cruise.

E. You could climb, descend or fly straigh and level in slow flight.

F. When flying straight and level in slow flight, the plane will probably be in a pretty nose-up attitude, but it's possible to descend in slow flight so that your attitude is not particularly nose up, yet the plane is descending very slowly. This is because you need a high angle of attack during slow flight, which is measured in relation to the relative wind, not a nose-up attitude (which is measured with relation to the horizon).