The most obvious and universally important sweet spot occurs while landing. Here, it is almost easier to define what keeps the airplane out of the sweet spot than it is to say what puts it there. When an airplane is landing, the ideal situation is to slowly bleed off speed during the last 30 feet of the final approach in such a way that, when you hit ground effect, the airplane has just enough energy left that it will float - but when you attempt to hold it off and cushion the touchdown, it neither balloons back up nor drops like an anvil. With the right speed at the right height, it'll just sit there and let you gradually play with the ground, doing your very best to let the main gear kiss on first. That's the sweet spot. Since a good portion of the sweet spot is directly coupled to the speed at that point, how do you get the correct speed, and how critical is it that the speed be right on the money?
Answering the last question first, setting the speed on an exact number as you enter ground effect really isn't that critical as long as it is within certain parameters. Without doing empirical testing, we'd guess there is at least a three-knot window, maybe more, in which the airplane will gradually settle into the appropriate groove. You don't need to know the exact speed because, as you are setting up the flare, the airplane tells you when you are doing the right - or wrong - thing.
The flare actually starts when you initially break the glide and start bringing the nose up. However, the important part of the search for the sweet spot occurs just as you're trying to bring the airplane level in preparation for the final part of the flare. As you're bringing the nose up to level the airplane over the runway, the airplane's reaction to your back pressure tells you immediately whether you're at the proper speed or traveling too fast or too slowly.
If, as you start to bring the nose up, you sense that the airplane is eager to gain altitude, you are obviously too fast. Either you weren't observing the recommended glide speed or you were late in starting to break the glide and bring the nose level, so that you arrived on the feathery top of ground effect with too many knots. The secret to getting the airplane into the sweet spot begins with holding the proper glide speed during the approach. However, even if you're holding the right speed throughout the approach, it is still possible to arrive at ground effect with too much or too little speed simply because of the way in which you broke the glide and initiated the flare.
If you wait too long to break the glide and come whistling into ground effect without slowing down at least a little, you'll ricochet off of ground effect like a flat rock skipping across a still pond. This is definitely not the sweet spot. Conversely, if you initiate the speed reduction too high or simply pull the nose up too rapidly, the airplane will hit ground effect without enough speed and will settle right through it, just as the flat rock sinks into the water when it runs out of energy.
Ground effect could be visualized as a layer of dense air that materializes as the wing gets close to the ground. The air really doesn't change, but a byproduct of aerodynamics makes the airplane perform as if the air is thicker - in other words, the airplane requires less energy to stay airborne in ground effect than it would just a few feet up. It must slow down before it will settle through ground effect. If the pilot leaves the pitch attitude alone, the airplane will lose its speed and settle onto the ground with the nose slightly lower than it was when ground effect was encountered. The nose is slightly lower because, as the airplane slows down, the tail becomes less efficient and the amount of elevator being held when ground effect is first encountered is not enough to hold the nose up. This is a bad situation because the nose gear could be the first to find the ground.
The correct way to set it up in the sweet spot is to level off in ground effect and then try to hold the airplane off the ground as long as possible, as low as possible. The airplane will settle through ground effect and touch down as the speed dissipates.
It's right there, just as you start to hold the airplane off the ground, that you gain the sense of being in the sweet spot. If it gradually slows and gently settles toward the ground, giving you plenty of time to increase back pressure and hold the nose up, you've found the sweet spot. If you have to toggle back and forth with the elevator, trying to get it to slow down while fighting its urge to climb, you're too fast. If, as you try to pull to level, the nose comes up but the rate of descent doesn't decrease and the airplane settles onto the ground (usually hard), you're slow.
Incidentally, a basic rule right here is that the yoke doesn't move back unless the airplane is moving down. If the airplane isn't settling, it still has too much speed, and applying back pressure will only set you up to balloon. Just wait until it slows enough to start settling toward the ground before you apply back pressure. If, however, it's settling fast and the airspeed is too low, bringing the nose up will only make the situation worse. That's where a healthy dose of power and a go-around may be necessary.
As you're sitting there in the sweet spot, there are really only three things you have to worry about. First, you must try to hold the airplane in a slightly (repeat slightly) nose-high attitude. Second, you want the tail to be directly behind the nose and traveling on the same line (no yaw). Third, you need to be constantly looking for any sideways drift that may occur because of crosswinds.
If you're in the sweet spot, the airplane will just hang there, fat and happy, while you work at holding it off and watch for drift. If everything is in harmony, as you try to hold the airplane off the nose will gradually come up, but the airplane won't have any tendency to move away from the ground. Instead, it will stay at almost exactly the same altitude until the speed deteriorates enough that it will begin settling even though you're increasing the angle of attack. At that point, you should be aware that the nose angle doesn't need to come any higher. If you try to hold it off too long, it will stall and drop you the remaining distance. Don't rush it. Just make sure that you're aware of the nose attitude and, as it begins to touch down, you make an effort to hold the nosewheel off the runway.
What about other sweet spots in other flight regimes? Yep, every phase of flight from climb to cruise to approach has a sweet spot to be discovered.
The sweet spot on climb is initially defined by the best-rate-of-climb speed given in the pilot's operating handbook (POH). However, what the POH doesn't mention is that simply holding the right speed doesn't guarantee maximum performance. For one thing, best-rate speeds change slightly with density altitude, so if that's a factor, the right speed has to be located on the pertinent chart in the POH and matched to your operating environment. Not all POHs have a chart for best-rate speeds. If yours is among them, don't worry about it because using the same best-rate speed at higher altitudes results in only slightly lower climb rates. The POH doesn't mention the most important thing for putting the airplane in the sweet spot for climb. The key factor is getting your feet into the act to keep the ball in the center. If the ball on the turn coordinator isn't centered, the airplane is yawing; it won't settle into a sweet spot; and you'll get less-than-optimum performance.
There is also a sweet spot for cruise. It would be easy to assume that simply using the right power settings results in the fastest cruise speed. This is sometimes true, but it's often false. Some airplanes demonstrate a characteristic in which you can set the proper power settings for cruise, then gradually bring the nose up and force the airplane to settle into a nose-high cruise at a lower-than-normal speed. Most often this occurs while leveling out from a climb: The nose is pushed over and the power brought back before the airplane is going fast enough. The pilot then brings the nose up slightly to compensate for the airplane's tendency to lose altitude. This is a very subtle thing, but it's a technique that's sometimes good for three to five kt of cruise speed. You may hear this technique referred to as "operating in the drag bucket," while getting the airplane up to speed is called getting it "up on the step." While there is a lot of argument as to whether the "step" actually exists, there is no doubt that some airplanes settle into a sweet spot for cruise much more easily and more quickly if the initial climb is carried 50 feet above the intended altitude and the airplane is allowed to gain speed while the pilot purposely loses that unneeded 50 feet. It's worth trying in your airplane to see if it settles into a slightly higher cruise speed more quickly or not. The one universal benefit to letting the airplane settle down to its cruising altitude is that it will come up to cruise speed more quickly than by just pushing the nose to level and letting the engine pull it up to speed.
And then there is the sweet spot on final. As with the sweet spot in cruise, some airplanes appear to have it while others don't. The reality is that the sweet spot on final is actually defined by the airplane's aerodynamics. One of the major differences between airplanes is how speed-stable they are on final. Some airplanes appear to have a distinct sweet spot on final, where you can trim it hands-off and it sits there like a rock, even in turbulence. When on final, this airplane's speed is unlikely to change more than a couple of knots, and the airplane will always return to trim speed after being disturbed. This characteristic appears as a distinct, easily defined sweet spot. Other airplanes won't hold a trim speed within 10 kt without help from the driver. This airplane also has a sweet spot, but it is harder to find and hold onto.
The biggest ingredient in establishing a sweet spot on final is holding the best-glide speed. However, the balance is further refined by correct trimming and attention to keeping the ball centered. In some airplanes, keeping the ball centered during a gliding final is no big deal because the ball is barely affected by P-factor or slip-stream yaw. In others, just reducing the power to start the glide sends the ball dashing for the end of the tube. If the ball isn't centered, the airplane will never settle into its sweet spot because it will be flying down final in a yawed condition, making it aerodynamically dirty.
The only sweet spot that is clearly seen from one airplane to the next is the one that occurs on landing. So, is "sweet spot or go" good advice for all airplanes? Some folks may see it as overkill, but one thing is absolutely certain: If you land an airplane when it is in the sweet spot, you'll never get in trouble. So, yeah, maybe there is something to the old adage.
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