36. 36
旋回中の荷重倍数と失速速度
FAA Pilot's Handbook of Aeronautical Knowledge
水平飛行中の失速速度を Vs とすると
W = L = ½ ρCLSVs2
Vs = √(2W/ρSCL)
旋回飛行中の失速速度を VSθ とすると
W = L cosθ より
VSθ = √(2W/ρSCLcosθ)
=
√cosθ
1
×Vs
荷重倍数 n = L/W または
n=1/cosθ
2019/2/17
バンクをつけただけでは失速速度は増えない
38. 荷重の増加(突風)
上昇気流
風速 U 迎え角の増加
速度 V
風速 U
相対速度
V
静かな大気中を飛行してきた機体が風速 U の上昇気流に突入すると、
主翼にあたる気流の向きが変わる。
迎え角が増えて主翼の揚力を増し、機体全体は上方へ押し上げられる。
機体各部には正の荷重倍数がかかる。
2019/2/17 38
The objective of this demonstration maneuver is to show the effect of improper control technique and to emphasize the importance of using coordinated control pressures whenever making turns. This type stall occurs with the controls "crossed" - that is, aileron pressure applied in one direction and rudder pressure in the opposite direction.
In addition, when excessive back elevator pressure is applied a "cross control stall" may result (Fig. 11-25).
This is a stall that is most apt to occur during a poorly planned and executed base to final approach turn and often is the result of overshooting the centerline of the runway during that turn. Normally, the proper action to correct for overshooting the runway is to increase the rate of turn by using coordinated aileron and rudder. At the relatively low altitude of a base to final approach turn, however, improperly trained pilots may be apprehensive of steepening the bank to increase the rate of turn. So, rather than steepening the bank, they hold the bank constant and attempt to increase the rate of turn by adding more rudder pressure in an effort to align it with the runway.
The addition of inside rudder pressure will cause the speed of the outer wing to increase and thus create greater lift on that wing. To keep that wing from rising and to maintain a constant angle of bank, opposite aileron pressure would need to be applied. The added inside rudder pressure will also cause the nose to lower in relation to the horizon. Consequently, additional back elevator pressure would be required to maintain a constant pitch attitude. The resulting condition, then, is a turn with rudder applied in one direction, aileron in the opposite direction, and excessive back elevator pressure - a pronounced cross control condition.
Since the airplane is in a skidding turn during the crossed control condition, the wing on the outside of the turn speeds up and produces more lift than the inside wing; thus the airplane starts to increase its bank. The down aileron on the inside of the turn helps drag that wing back, slowing it up and decreasing its lift, which requires more aileron application. This further causes the airplane to roll. The roll may be so fast that it is possible the bank will be vertical or past vertical before it can be stopped. For the demonstration of the maneuver it is important that it be entered at a safe altitude because of the possible extreme nose down attitude and loss of altitude that may result.
Before demonstrating this stall, the pilot should clear the area for other air traffic while slowly retarding the throttle. Then the landing gear (if retractable gear) should be lowered, the throttle closed, and the altitude maintained until the airspeed approaches the normal glide speed. Because of the possibility of exceeding the airplane's limitations, flaps should not be extended. While the gliding attitude and airspeed are being established, the airplane should be retrimmed. When the glide is stabilized, the airplane should be rolled into a medium banked turn to simulate a final approach turn which would overshoot the centerline of the runway. During the turn, excessive rudder pressure should be applied in the direction of the turn but the bank held constant by applying opposite aileron pressure. At the same time increased back elevator pressure is required to keep the nose from lowering.
All of these control pressures should be increased until the airplane stalls. When the stall occurs, recovery is made by releasing the control pressures and increasing power as necessary to recover.
In a cross control stall, the airplane often stalls with little warning. The nose may pitch down, the inside wing may suddenly drop, and the airplane may continue to roll to an inverted position. This is usually the beginning of a spin. It is obvious that close to the ground is no place to allow this to happen.
If recovery can be made before the airplane enters an abnormal attitude (vertical spiral or spin), it is a simple matter to return to straight and level flight by coordinated use of the controls. If a recovery cannot be completed before the airplane reaches an abnormal or inverted attitude, the control pressures must be released to break the stall and the roll allowed to continue until the airplane reaches straight and level flight. Applying power when the nose is pointed toward the ground will result in a greater loss of altitude - with the possibility of impacting the ground if the stall were to occur during an actual landing approach.
The pilot must be able to recognize when this stall is imminent and must take immediate action to prevent a completely stalled condition. It is imperative that this type of stall not occur during an actual approach to a landing, since recovery may be impossible before the airplane strikes the ground.
The Slip
During a slip, the opposite scenario happens. The nose of the aircraft yaws to the outside of the turn, and the aircraft's banked too much for the rate of turn. The outside wing has a higher angle of attack, and you're most likely lowering the aileron on that wing to keep it up.
ou may have heard that a skid during a stall is more dangerous than a slip, and it's true. But, why?
Stall-spin accidents have been a problem since the first days of flight. Most of us are simply taught to keep an aircraft coordinated when stalling. But, the problem is, most stall-spin accidents don't happen during an intentional stall. They usually happen unintentionally and down low - like when you're turning base to final.
Here's a common scenario: You're turning left base to final, but you're going to overshoot the runway. What do you do? Here's what you absolutely shouldn't do: You add left rudder to tighten the turn, but you don't keep the bank and rudder coordinated - putting the airplane into a skid.
As the inside wing exceeds the critical angle of attack, it stalls and drops. The downward deflected aileron on the low wing is still generating drag, which pulls the aircraft's nose further into the turn. And, the aircraft is still yawing into the turn from the rudder, which accelerates the roll. The result is a quick roll into the turn, and your entry into an incipient spin.
There are a few other factors at play, as well. During a skid, the relative wind isn't coming straight down the airplane's nose, it blows crosswise at an angle from the outside of the turn. That causes the relative wind to flow over the wing at an angle, creating "spanwise" flow - a component of the air flows perpendicular to the wing's leading edge, traveling laterally down the wing.
As you move towards the wingtip, you get more and more spanwise flow. And here's the problem - spanwise flow doesn't generate lift. It effectively reduces the airspeed over that portion of the wing. That causes the wing to stall earlier than normal - so the wing with all of the spanwise flow stalls first.
Finally, the fuselage may block some of the airflow over the wing during a skid, further decreasing the airflow over the inside wing and causing the wing to drop during a stall.
All of these factors play out differently on various aircraft designs - but when combined, they make a skid a deadly condition during a stall.
This is a good illustration of ABL. While most of the gradient is quite high, the steepest gradients are right at the bottom next to the ground (where we’re hopefully riding bikes). This means your 10 meter weather station is reading a significantly higher wind speed than your bike is seeing. This also shows the difference in gradients between different types of terrain
What is the situation in gliders?
Why don’t they have stall warning devices?
One reason is that we don’t want a flap valve on the wing as in airplanes because it disturbs the airflow too much. However, a sailplane manufacturer can drill holes in the nose to take pressure measurements and get a stall warning system of sufficient precision.
In contrast to airplanes, we often fly very near stall speed in small thermals. In that case the stall warning would be bleating at us constantly. Do we want that?
This argument is the most common one against a stall warning device.
Stall warning:
The crucial additional safety feature however is the integrated stall warning, which is always activated and looks after your safety. Some of you might know that 5 years ago a very good friend of mine crashed because he apparently didn’t notice that he was flying far too slowly. This additional safety feature is a consequence of his accident and will save lives maybe yours! Only chances are that you would not even notice. You would just speed up a bit as the stall warning goes off, and that would be all…
You only need to calibrate the stall warning once for further information on how to do it please consult the manual. Once that’s done it works correctly, irrespective of speed and wing loading, which distinguishes it from the stall warning you get in on-board computers: Those are always set to a certain wing loading and will therefore have to be reset when you dump your water ballast which is often not done.
The stall warning works on the basis of two sensors which measure the pressure difference in the nose of the glider not with flaps in the wings or other “aerodynamic cruelties”. In the example it shows 0.08. When the difference is more than 0.30 the warning should be activated.
I know that a lot of pilots argue that such a stall warning is superfluous in a glider and just not practical. According to those pilots it’s superfluous because accidents will only happen to “others”; while allegedly it’s not practical because as a pilot you would quite often have to fly close to stall speed.
I won’t waste any time on the first argument, but the second one is also wrong:
If you fly below stall speed in a glider with a modern profile you fly inefficiently and badly! You may not notice it as much as you do in an older model, because the elevator buffet is not quite as noticeable as on an older profile, but you will climb more slowly or not at all.
If the stall warning goes off you are very simply flying too slowly, not optimized and you are in danger. This is exactly what the DEI-NT is designed to show you.
We will decide in the course of the flight tests whether the stall warning will be in the form of a sound alert or through a vibrating stick. Additionally it will show a warning in the display. Of course the really “tough” pilots can deactivate the stall warning by calibrating it higher than 0.50 to go off at a speed when the glider is already in a mushing stall or worse.
There is a long and rather personal article on the subject “stall warning” on our website. Even though it is 5 years old it only needed minimal tweaking to be as relevant as when it was first written. You really ought to read it again!