For me, this has for some reason been a question that was never really discussed or presented by any CFI during training. Yet, managing a "good" landing seems to rely on getting this right.
There is a lot of discussion and writing about "when" to "flare" and/or "when" to "round-out" but there is little discussion and writing on the optimum descent rate or vertical speed when just a few feet above the runway.
Even the ideal rate of descent during the "stable approach" is a bit foggy other than the traditional 3% slope angle.
In a light plane like a Cub, where landings often occur on short runways, generally a much higher decent angle (more like 5-10%) is desired and, because approach speeds are generally slower, the descent angle is steeper given the same vertical speed as a faster plane. In any case, the sensation of going from a steep decent rate to a shallow one as the plane approaches the runway seems more dramatic and is sometimes harder to finesse.
While "greasing" the touchdown with a zero rate of descent sounds logical and desirable, it is really not.
The reason--attempting to achieve a zero rate at touchdown generally requires inputs starting at around 10-15 feet and an extremely low rate of descent during this last 10 feet can waste a lot of runway. Just for example---assume that the pilot flares at the threshold from an approach descent of 500 feet per minute at 10 feet altitude and achieves a rate of decent of 60 feet per minute (1 foot per second) at 6 feet while traveling 60 miles per hour (88 feet per second). In this example, the "float" would be for six seconds or 528 feet--or more.
Boeing defines a "hard landing" as a touchdown at a vertical speed faster than 240 feet per minute or 4 feet per second and an "ideal" vertical speed of 60-180 feet per minute. (Boeing claims that they design their planes to tolerate up to 10 fps.) My Cub (as do most light singles) seems to tolerate a 4 foot per second decent at touchdown, but "aiming" for 2-3 fps is probably ideal.
Achieving a touchdown close to the desired spot on the runway is an important skill--especially on short runways, so in a small, light plane, a predictable touchdown point usually will require a "firm" grounding or a vertical speed of 120-180 feet per minute (2-3 feet per second.)
Since these light planes often descend at 700 feet per minute on final, the deceleration from 720 to 180 feet per minute (12 to 3 fps) must occur with accurate and precise timing. Too soon and runway is wasted. Also, too soon or too much stick back can prolong the process, and speed falls off enough to begin a phase with an increasing sink rate. Too late may produce a hard landing, and often a bounce. The ground is approaching quickly and inexperienced pilots tend to fixate on it. (That is why it is always important to look out toward the end of the runway to reduce this "Ground Rush" sensation. )
It is often said that most bounces occur due to excess speed---too much horizontal speed or too much vertical descent rate. But, in fact, perhaps the most common cause of a bounce when landing a trail dragger is an attempt to reduce the decent rate by pulling the stick back or adding power just at or just before touchdown at a speed less than stall. (This usually is the result of leveling out too early and can happen when attempting a wheel landing and/or a three point.)
When flaring for a three point landing, the "wasted" runway is less of an issue as speed is lost during the flare and ground roll will be shorter. But wheel landings typically occur at a higher speed, so hitting the runway as early as possible is often important. But, three point and/or wheel landings both require accurate timing as flaring too soon or too fast for a three point can cause a hard landing as the plane stalls high and "falls" hard to the ground.
One of the "lessons" I have learned the hard way is that the most important time for a "stable" approach is the last 6 feet above the ground--to maintain that constant 2-3 fps descent rate. Traveling at 50 mph ground speed (73 feet per second) it takes between 150 to 220 feet to travel that last 6 feet during those 2 or three seconds. A lot can happen in those last 2 seconds.
Another reason for a descent rate greater than zero at touchdown is wind. Theoretically at a zero rate of descent, the angle of attack and airspeed is such that the wing is producing just enough lift to support the aircraft's weight. Even a small wind gust can increase airspeed (or angle of attack) and create enough lift to cause the plane to take off and rise above the runway.
Choosing the best rate of descent during this last 6 feet is dependent on the design of the airplane's "suspension" system and the runway surface. This includes the tires, "springs" and "dampers". The plane's suspension system will determine: A) the "G" force upon touchdown; and B) the amount of "rebound" energy that is released to induce a "bounce". A "soft" suspension system will allow for more time for the deceleration to occur--hence less vertical "G" force deceleration. A suspension system with little damping, can release significant energy, causing the plane to bounce back into the air. A Cub with bungie cord suspension and fully inflated conventional tires requires a lower descent rate at touchdown than a tricked out backcountry plane with supersoft bushwheel tires and shock absorber suspension. A grass field is softer and more forgiving than a paved runway.
Keep in mind that a plane's suspension may tolerate a harder landing with large suspension travel, but for a propeller driven plane, one must remember that the propeller is quite close to the ground, especially during a wheel landing of a plane with a large diameter prop. An especially hard wheel landing with a bit of stick forward nose down movement at touchdown could result in a prop strike.
Finally, tricycle gear planes have a tendency to see a reduction in angle of attack upon touchdown, where trail draggers have a tendency to see an increase in angle of attack upon touchdown. The higher the vertical rate of descent in a tail dragger, the more force is applied to dropping the tail, raising the angle of attack, and re-establishing lift at touch down. This tendency to re-establish lift must be offset with stick forward and/or reduction on throttle immediately upon touchdown with a wheel landing, or touchdown in a three point attitude must occur as a result of a complete stall just a few inches off the ground. (Precise timing and coordination are critical to a good landing!) Below is a well executed steep descent wheel landing, almost perfectly executed.
Here is an actual Sport Cub, like mine--similarly well executed.. Followed by a short field landing by a Super Cub.
Bottom line for me in my Cub: Short final approach with a steep angle of descent, around 50 mph indicated air speed (55 mph or 80 fps ground speed) with full flaps and vertical descent rate of 600-750 per minute (10-12 fps)---around 1100-1200 RPM or 300-400 RPM above idle. At around 10 feet above the ground--pull (pressure) stick back (1/2 to 1 INCH) and hold to reduce descent rate to around 3 fps (average) with level or slight tail down attitude. Indicated air speed will decay and drop to around 40-45 mph (1.3 Vs with flaps) before touchdown. This process will generally take about 2.5 to 3 seconds and the plane will travel 170-220 feet, so you need to place the plane about 10 feet above the runway at 200 feet before the touchdown point. (If obstacles are present in the approach, keep in mind that even the steep descent of 10 fps and 80 fps ground speed (7 degree slope ) will require 700 feet to bleed off 90 feet of altitude. (Flying 100 AGL above obstacle.) You can increase the descent rate in this case by reducing speed to 45 mph indicated (about 70 fps ground speed) and increasing descent to 12 fps (using a slip). This will cause the slope to increase to 10 degrees and will decrease the distance traveled to 520 feet during the 90 foot descent. (Keep in mind that you need to "take out" the slip at 20 feet above the ground to give yourself enough time to stabilize the plane for that stick back movement at 10 feet AGL. Taking out the slip will reduce drag allowing speed to increase and the descent rate to decrease.)
(KTHV York happens to have a 700 foot displaced threshold on both 17 and 35, so it is ideal to practice here with 100 ft AGL altitude passing over the pavement edge and landing on the white line in front of the "numbers" using a slow steep 12 fps 10 degree descent, or pass over the runway at 75 ft AGL with a 10 fps 7 degree descent Perhaps the reason plane performance data is often given to clear a 50 foot obstacle is because most large trees are about 50 feet tall--but to be safe, until I know for sure based on close observation or printed obstacle data, I assume I need to fly at least 100 ft AGL above most trees and wooden telephone poles on approach. Also be aware that since the typical descent rate for these landing is steeper than normal IFR approaches, the PAPI lights will remain white for most of the descent. )
Keep your eyes focused down the runway and be ready to immediately cut power and push stick forward (release pressure) to kill lift at touchdown. (Also move stick increasingly toward cross wind direction as speed is reduced.) Keep feet moving to maintain directional control. Keep stick forward until the tail begins to fall, and then pull stick fully back--keeping cross wind stick position in place during the entire ground roll. (Taking cross wind stick correction out too soon will allow wing to lift---if there is not enough rudder, a ground loop will result--if there IS enough rudder, the plane will skid sideways away from the wind. In some cases, if you apply too much opposite (downwind) rudder, the plane will steer away from rather than into the wind, causing you then to apply upwind rudder which can cause the upwind wing to fall---keep the cross wind aileron in and apply "just right" rudder--keeping in mind that the rudder often needs to move back and forth to stabilize the tail position--holding the rudder in one position too long often causes overcontrol and excess tail and wing wagging.
Here are two videos with good approach, a bit early to level off and either adding power or pulling stick back too close to the ground--resulting in a bounce. The first time, I go around--in the second one, I "recover" by tolerating a less elegant recovery by hopping three times while making a couple mistakes with the rudder. The good news is that I noticed my mistakes and took corrective action despite being a bit "behind" the plane. But practice makes perfect and learning from mistakes can be productive.
Then, here I execute a nice landing on a grass strip.
My mistakes so far have been that I tried to reduce the descent rate just as I was approaching touchdown by either adding throttle or pulling stick back. The result was increased lift, in combination with the "bounce" energy released by the suspension, causing me to take off and rise from the runway--in other words, a bounce with the response then being a go around or a somewhat dangerous "bounce recovery". You must be sure the plane is in " stable touchdown attitude/speed = descending 2-3 fps" the last 12-18 inches of descent and be ready to reduce throttle and push stick forward IMMEDIATELY at the time of touchdown. (If you are moving the stick back or adding throttle at the time of touchdown, your reaction time is not fast enough to reduce lift and overcome the energy released from the suspension.)
The tricky part of this process is the ability of identifying the site picture at 10-12 feet above the ground. This takes practice and sometimes low passes above the runway can help. The other tricky part is accurately controlling the air speed. Too fast and instead of slowing the descent rate, you can stop it and float. Too slow (often because the stick is pulled back too early) and you will see the descent rate accelerate (sink rate increase) close to the ground which will tempt you to add power or stick just as you touchdown. Most pilots are a bit "ground shy" and pull the stick back at 15-20 feet above the runway--practice and focusing eyes more down the runway will cure this bad tendency.
Below is one of the best advisories in how to time the flare. It is taken from https://www.av8n.com/how/htm/landing.html
12.6.4 Timing the Flare
How do you recognize when it is time to begin the flare?
Let us begin by mentioning a few unhelpful answers to this question.
- You could wait until you see the hair on the instructor’s neck stand on end, then begin the flare. This is not good preparation for flying solo.
- Many books suggest beginning the flare at about the height of a typical hangar. This doesn’t work very well if you visit some place that has bigger hangars, smaller hangars, or no hangars at all. It also isn’t very reliable at night.
- Some people like to flare at about half the height of a typical tree. Alas, trees work even worse than hangars, for similar reasons.
- You could wait until the width of the runway subtends a certain angle in your field of vision. This will get you into trouble if you visit some place with a wider or narrower runway.
- You might think of using the perception of the ground rushing past, which does depend on height. Alas, this is hard to perceive, and is unacceptably sensitive to the amount of headwind.
- You could try to use the depth perception that comes from having two eyes. However, human binocular stereopsis is absolutely useless at distances of 20 feet or greater. By the time this depth perception comes into play, it’s too late. Wiley Post was blind in one eye, but that didn’t prevent him from making good landings.
Here is something that actually helps: Use your sense of timing. At each moment on short final, ask yourself how much time t remains until you would, at the current rate, reach zero AGL. When this time t reaches the special value tF (about two seconds), start your flare. (The exact value of tF will depend on what sort of airplane you’re flying, and other factors.)
Of course the actual flare will take longer than tF — roughly twice as long. That’s because tF refers to what would happen if you forgot to flare. During the actual flare, your descent rate is reduced, so you take longer to descend.
This timing technique has some nice properties. It works on wide and narrow runways both. It works during daytime and nighttime both. It causes you to flare at a greater-than-usual height if you have a greater-than-usual vertical speed.
Now all you need is some way to perceive how much time t remains. You don’t need to know the height in feet or the descent rate in feet per second; all you need is some quantity that perceptibly changes as you approach zero AGL. Figure 12.15 shows one such quantity. The left side of the figure is what you should see when you are on final, at a definitely nonzero height. The letters ABCD and WXYZ represent landmarks along the side of the runway. In particular, for night landings you would use the runway lights as landmarks.
To understand the importance of correct timing, one must appreciate that you are "managing energy" in the form of vertical and horizontal speed and you must also understand the basic "Polar Curve" that applies to the lift/drag characteristics of a particular plane. Best Glide will produce the optimum distance traveled; Minimum Sink is a bit slower with less sink rate. Stall Speed is when any decrease in speed results in maximum sink rate.
(See below)
Polar Curves are commonly referred to in reference to gliders. And, when you are landing the plane, essentially you are a glider!
Generally the speed during the final few feet of descent is below Minimum Sink and since speed will decay during the final phase of flight during those last 10 feet, the sink rate tends to increase as the speed decreases. (Due to drag.) If you pull the stick back too soon (like at 15-20 feet above the runway) there is more time to lose speed and the pilot will notice and feel an increase in sink rate, even though speed is still higher than stall. Generally, most pilots feel that the best action to take if you start "falling down" the left side of the Polar Curve is to add a slight amount of power to increase speed without changing the angle of attack by moving the stick. And, if you catch yourself pulling too early, it is possible to correct the error by pushing the stick forward momentarily (briefly) to slightly increase speed and wing efficiency. Best Glide for my Cub with Full Flaps is 49 mph; Stall is 32 mph. Minimum Sink is not published in the POH, but I have guessed that it is around 45 mph. You will note from the videos that when my indicated speed fell below 40-42 mph, I saw an increased sink rate and tried to arrest it by pulling the stick back causing a bounce. In my "good" landings, I touch down before my speed falls below that 40-42 mph. So the optimum wheel landing touch down speed for my Cub with full flaps is probably about 42 mph. (Keep in mind these speeds are for a level or tail low wheel landing---speeds and inputs are different for a three point. (Touchdown speed for a three point would be the 32 mph stall.) Also keep in mind that all of my comments apply to a full flap approach--speeds would be higher with no flaps..for example, Best Glide with no flaps is 68 mph, so with no flaps speeds would be 10-15 mph faster and the descent slope would be less steep. )
Some CFI's will teach using the flaps (removing them) to change the rate of vertical descent near the ground. Essentially this changes the Polar Curve-shifting it to the right, effectively increasing the sink rate. I find changing the flaps during this critical space to be too distracting and I would only consider this strategy if dealing with a very gusty wind condition.
I am a big fan of Boldmethod training. https://www.boldmethod.com/ I get a daily email from them and find the info very helpful. They illustrate how to judge when you are about 10-12 feet above the runway--using a long known and accepted method of watching the growth rate of the apparent runway width. The video illustrates. I encourage you to click in the link to their website to read the entire article.
https://www.boldmethod.com/learn-to-fly/maneuvers/how-to-perfectly-time-your-flare-and-touchdown/
One does not need to always fly the 7-10 degree slope approach in a Cub (especially on long runways) but it easier to stay proficient if you reduce the number of options that you practice and use. That is also the reason why I practice level or "slow" tail low wheel landings and seldom execute a true three point. The other reason I prefer landing with the tail up a bit is that is seems easier when in a cross wind, and in my flying area, it seems there is almost always a cross wind!
If I wanted to emphasize three point landings, I probably would land without flaps and would end up touching down at 40-42 mph, or about the same speed I am landing on two wheels with full flaps. It is good to land slow--the less energy at touchdown, the better, but at speeds below 40 mph, I tend to notice that my ailerons and rudder are much less effective, so for me, the 40-42 mph is the most comfortable.
Taildragger pilots have this ongoing "argument" about the best landing being "Three Point" or "Wheel Landing". What finally made up my mind of which "side" to pick was watching landings of one of the most popular taildraggers ever, the DC-3. Here are two beautiful wheel landings--one of them very short:
And, if anyone ever claims that you can't land short on two wheels--watch this...
And, when you become VERY skilled, you can do THIS!!!!
And this is a pretty good "Cub" wheel landing...
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