✈️ Exclusive Benefits for Readers
Verified codes to save on your next trip.
In this installment we will talk about one of the most famous phrases of the aviation «Speed and height they preserve their teeth» How true or not is this saying? Foto The A320 Study Guide
Post written by: William Casalins
Aeronautic engineer
FIA Foundation
Do speed and height preserve teeth?
NOT ALWAYS
It is the creed of the student pilot and the veteran pilot knows that he has reached seniority by using it.
Alejandro Torres, a young co-pilot Boeing 727 In the middle of the cruise he turns around and asks me about the Coffin Corner, I start by clearing my voice (to start the conference) the flight technician, Hernan Buitrago, He tunes his ears and gets ready to see where I'm wrong and Captain Ricardo Barragán looks at me out of the corner of his eye. (You know I'm going to contradict everything) and he checks it when I tell the co-pilot: Do you know that the plane does not stall due to low speed and that on the contrary it can do so due to high speed??
Loss of Sustain
Many times we have heard a pilot, when you give the approach or takeoff to your copilot, or an instructor to his student “increase your speed, you are going to stall.” How am I going to tell veteran and new pilots?, that the plane does not stall due to low speed but rather due to a high angle of attack; but on the contrary, it can stall due to high speed.? is it a paradox? If we look at the support formulas it seems so, that I am wrong, that speed defines lift and that, yes I duplicate it, lift is multiplied by four, what, if the speed drops, the lift also does it in that same proportion. Don't even talk about Casalins this time if you're wrong, and in what way.
S=Cs*a*p/2*V´´
But how is it really??
Let's start from back to front:
How can you stall due to high speed?? Okay, Let's define what a stall is and differentiate the stall with the decrease in lift., When you decrease the lift it forces you to descend if it is below the current weight, but the stall forces you to reduce the angle of attack by lowering the nose of the plane, there is the difference, You may be stalled if your angle of attack is above a critical angle, it doesn't matter what speed you have. That's why; to fix the hight speed buffet, It is necessary to lower the speed and to correct the low speed buffet you have to lower the nose first (to get him out of the loss) and then increase speed if you don't want to go down.
High speed stall is promoted because, after a certain air speed, This becomes a fluid with compressible characteristics (what the air is really like) while at lower speeds it is considered incompressible (like the water) density and other factors are considered constant; later (at high speeds) the formula we mentioned before does not work at all (there are many factors that prevent it). The same as with a wing angle of attack beyond critical, you will be in a stall regardless of whether you have a higher speed, not to mention that you are doing a maneuver with some additional Gs that would obviously aggravate the situation.
High speed stall has been improved, for decades, with the arrow of the wing, So, even though the plane is flying faster, the wing makes the air “believe” that it is going at a lower speed and behaves as such, reducing the separation of the boundary layer and the formation of shock waves than those that would be produced in a straight wing with the same profile. However, this is not free and at the time of takeoff and landing, complicated hyperlift mechanisms are also needed., because the air continues to “believe” that it is going slower than it is supposed to and would need a higher approach or takeoff speed, with the necessary extra runway length to achieve sufficient lift value.
Nevertheless, the expression speed of loss is still used. Let's look at the formula (S=Cs*a*p/2*V´´) again for a given weight, with a constant area, the same height, We will see that to have a minimum speed we need a maximum CL, and that is achieved with a certain angle of attack (for a particular profile); Then, If there is a relationship between speed and loss, but what really happens is that if the speed decreases below that value and the pilot wants to maintain altitude, instinctively he will raise his nose without realizing that doing so will complicate the maneuver, because everything he wanted to avoid is going to happen and with often disastrous consequences if you don't have the necessary height to recover.. But this (to use a stall speed) it has its advantages, The speedometer is the instrument that a pilot watches the most because it is the most important in the flight of the airplane., some aircraft also have angle of attack indicator, but most have rather alarms so that the pilot knows that he is exceeding (the next to exceed) that value. Other airplanes go even further and apply a forward force to the elevator column. (to lower the angle of attack). We can see that in these devices it has no relationship with the speed or weight of the plane and is only connected to an A.O.M sensor.. which is activated a few degrees before it reaches the critical point and thus alerts the pilot a few knots before this loss of lift becomes effective.
Coffin Corner
The coffin corner is as horrible as its name suggests., as weight and altitude increase, those speeds change for the worse. The low one increases and the high one decreases and they try to get closer, reaches an altitude where they are very close (each other) and the pilot has a dilemma, If it accelerates it stalls and if it slows down too, It is already clear that there is a height limit for a given weight.
Then it is even clearer, that speed and height do not always preserve teeth.
✈️ Exclusive Benefits for Readers
Search here for hotels ALL over the world at the best price.
