Front Side v. “Back Side” v. Back Side
We should note our “
Fundamental Maneuvers” of Straight and Level, Climb/Descend, Accelerate/Decelerate, and Turn are an incomplete list. We also have Zoom and Dive. To Climb or Descend, we change Power thus giving excess to Climb or letting Drag dominate to Descend. To Accel/Decel, we change Pitch typically while also changing Power so as to stay Level or stay on a Constant Glide Path. Turning isn’t relevant to this discussion.
Zooming and Diving, however,
are Trades between Kinetic and Potential Energy.Zoom and Dive differ from Climb and Descend in this as Climbs and Descents involve trades in Chemical Energy for Potential Energy while Accelerations and Decelerations are really Chemical for Kinetic. When we Climb or Descend, we maintain our Trim Setting maintaining a Constant AOA.
Our fixes for those Distributive Errors are really miniature Zooms or Dives. Shallowing or steepening VVI at constant AOA (or speed) are miniature Climbs and Descents.
For the Front Side, we use Pitch to Control Glide Path and Power to Manage Speed. These are Zooms or Dives connected with Accelerations or Decelerations.
For a moment, imagine flying Front Side but at zero Power. If we’re High, we can Nose Down as we will Accelerate per this Dive trading Potential Energy for Kinetic Energy and this adds Drag as we move up the Parasitic side of the curve. Added drag helps against the over-energy state of being High. There’re limits to this as we cannot Dive through the Ground and we have a Vne. If we’re Low, we can Nose Up reducing Parasitic Drag to stretch. There’s a limit to this as we’ll eventually bleed through L/Dmax to the Back Side and thus as we’d continue to bleed adding Drag from the Induced side of curve.
Use Pitch (trim) to control AOA (or speed as proxy) and Power for Glide Path in what some mistakenly consider Back Side though what are really Power Techniques. These work on any plane in any position of the curve. To prove this, consider Cruise flying obviously Front Side. What happens if you add Power with no other changes (and no autopilot engaged)?
Note: Gliders use “Power Techniques” too though by simultaneously adding Drag and reducing Lift through Spoilers. Spoilers are mechanized to act with the same effect as a Throttle or Power Lever with more Spoiler Control Lever Back less Spoiler Control Lever Forward. Nominal Spoiler is midrange allowing for adjustments in both directions. Trim for AOA (or speed), Spoiler in lieu of Power for Glide Path. If excessively high beyond means of maximum spoiler, use both spoiler and forward slip.
Speaking of gliders, we should make a caveat regarding “If Low and Slow, Fix the Low then the Slow.” As you can imagine, such won’t work well for a glider and is a recipe to end up significantly shorter out of energy perhaps unintentionally stalling while near the ground. But we’re not going to fix the slow first either, rather instead we’ll fix both simultaneously. Step one — Spoilers In. The immediate effect will be to shallow out starting to fix the low though such also better enables “acceleration” via mini-dive. Now push for L/Dmax. AOA is ideal though most gliders only have airspeed indicators, this is ok. You’re simultaneously fixing both speed and as best you can altitude as you’ll be moving forward from the back side of the drag (thrust required) curve to its low point. (Glider drag polars typically are expressed as sink rate vs speed hence L/Dmax is the tangent from origin as you’d see on the power required curve. Here we’re thinking forces of drag vs speed.) Note for headwind you’ll want to push a knot or two faster than L/Dmax (maybe even five knots in strong headwinds) while for tail winds, you can subtract similarly but no slower than maximum endurance glide time.
So, with gliders, you fix both simultaneously, but it will probably feel like fixing the slow then the low and unlike with powered flight, you should probably think of it this way. This feeling of fixing the slow then the low in gliders (or airplanes with lost power) is especially true as you’ll finish fixing the slow before you finish fixing the low even as the act of fixing the slow also fixes what low may be fixed. Maybe this is why the FAA answers the low and slow differently than the Navy? In the glider or without power, you’re better thinking fix the slow first if slow and low. Fixing the slow helps resolve the low but better to think it sequentially.
What is true Back Side? Consider again flying with zero Power though this time
on the Back Side. If you’re High, Pull Up thus Slowing and Increasing Induced Drag thus Sinking then Dump the Nose to Accelerate reducing Drag and Catch desired Glide Path.
If Low, Push the Nose gaining Speed Reducing Drag to Float further. Obviously, these are uncomfortable, hence we prefer the Power Techniques. They’re also more susceptible to mistakes, hence again, we prefer Power Techniques. Note with the back side power unavailable case, the FAA’s answer to Low and Slow is the only viable answer though it is a case of least bad not best. Use Power techniques and with them use the Navy’s LSO Rules. If you’re pulling or pushing while low to stretch a glide you may want to consider a spot with less buildings and trees short of the desired runway. If you were on best glide, push and pull both fall short.
Where is the Back Side? For jets, gliders, and (all) engine(s) out propellers, it is the left portion of the drag curve aka thrust required curve where induced drag is dominant. For propellers with operating propulsion, it is the left side of the power required curve hence the expression “back side of the power curve.” This means propellers fly on the front side for significantly more of their landing conditions than do other fixed wing aircraft except for power out. Yet piston (and electric) propellers handle much better with power techniques! See further below and think response time. For both jets and propellers, these transition points are coincident with maximum endurance flight time.
Note: I like to think of these transitions as “Reverse Demand” not ‘reverse command.’ To me, “Reverse Command” is the difference between front side and back side techniques with regards to what results when you pull or push. Reverse Demand is the increased need for throttle be it thrust or power to fly slower. See the difference? Reverse Command tells you how to pitch should you need to control altitude by speed. Reverse Demand tells you what is required to control altitude with power, thrust, or spoilers.