Forces in Transonic Flight Flashcards
What is the first effect of the onset of compressibility with increasing M?
what is the result?
the air gets less warning of the approaching wing, and the point where the airflow divides moves closer to the leading edge
This increases the angle of upwash, and Cl increases
Where does the shockwave first form on the upper surface and what causes it to form there?
a normal shockwave first forms at about 70% chord on the upper surface where the normal separated flow is seen as a compressive corner
what is the lambda foot and what causes it?
The thicker foot on the shockwave compared to the top
caused by the pressure waves migrating forward through the subsonic part of the boundary layer
What causes the high levels of drag when the normal showckwave forms on the upper suface?
As the airflow from the leading edge meets the very high pressure in the shockwave, this massive adverse pressure gradient causes immediate separation, and subsequently high levels of drag
what happens to lift behind the normal shockwave on the upper surface?
lift is lost due to the separation of flow as it passes through the shockwave
what causes Mach buffet / high-speed buffet?
the separated flow behing the shockwave
what happens to CL as speed increases in the transonic regime?
CL increases up to twice its low speed value due to the massively increased speed of flow over the upper surface
what happens to CP as speed increases through the transonic regime?
starts at about 20% chord and then varies position as pressure differences change as Mfs increases
eventually settles at about the 50% chord position at about Mfs 0.98 because the upper and lower shockwaves have moved aft to the trailing edge and the pressure patterns over the upper and lower surfaces are similar, with rather less pressure above because of the 2-degree positive alpha
what is shock stall?
occurs when CL, as a function of Mach no, reaches its max value (for a given alpha)
This diagram shows how CL changes with Mach number and how CL has been increasing as we have been accelerating. It is at its peak at our speed of M0.81
If we increase speed further we encounter a rapid loss of lift known as shock stall and this can occur at any angle of attack
what is the drag divergence number?
As we continue to accelerate the shockwaves grow and spread across the span, giving a sharp increase in Cd at a higher speed than Mcrit
Why does Cd start to decrease again beyond Mfs 1.0?
because both shockwaves have attached to the trailing edge, there is no longer any separated flow over the wing and this component of wave drag disappears
what are the two components of wave drag?
Energy drag – caused by a loss of energy by the temp rise through the shockwave
Separation drag – caused by separation of the boundary layer behind the shockwaves
Why does Cd remain at a higher level in supersonic flight than in subsonic flight once it decreases again from peak value?
when the upper and lower shockwaves have moved to the trailing edge, the separation drag lowers but the the energy drag remains
what causes tuck under?
The final movement of the CP aft to about 50% chord actually increases stability but this is a massive aft movement and unless the aircraft is specifically designed to cope with it the aircraft will pitch down uncontrollably
(the lift/weight couple becomes rapidly unbalanced as the CP moves aft causing the pitch down moment)
To resist this pitch down at tuck under the pilot will pull back on the controls while accelerating but in subsonic flight these actions are required when decelerating. This action is described as a loss of both … and …, and is not acceptable in civil aircraft
‘stick force stability’
‘stick force stability’
what is the supercritical wing shape designed for?
Designed to overcome the loss of lift that occurs at the speed when the top shockwave remains at 70% chord, while the bottom shockwave moves back to the trailing edge
With the flatter top the acceleration of the flow is not as high and the top shockwave does not form until a higher Mfs, delaying Mcrit, and when it does form it is less intense, giving less drag, and it moves quickly to the trailing edge
It has an S-shaped camber line to recover some of the lift lost by having a flatter top surface
The thicker section also allows for greater strength, allowing the use of higher aspect ratios to reduce induced drag
Sweeping the wings is another way of increasing Mcrit and by doing so, drag is reduced and the onset of drag rise is delayed
how is this achieved?
If this wing was straight, the direction of airflow would be straight over the wing, leading edge to trailing edge as shown by X
But by sweeping the wing the distance travelled is greatly increased as shown at Y. This means that, as with a thinner wing, the air is parted more gently and its acceleration is gentler as it rises over the thickest part of the wing
A lower local airspeed is the result, which means that the Mfs will be greater before this flow becomes sonic, increasing Mcrit. The improvement increases the more we sweep the wing, and the drag divergence Mach number will also increase
once supersonic, does a straight wing or a swept wing have a higher Cd?
This diagram illustrates the theoretical changes in Cd for our standard 12% T/C wing at 2 degrees alpha, straight, and with 40 and 60-degree sweeps showing the delayed drag rise and reduced drag
However, swept wings do have a higher Cd at supersonic speeds than straight wings
what effectively happens to aspect ratio as wing sweep is increased?
Swept wings reduce the distance between the wing tips, which reduces the wing span. This reduces aspect ratio as AR = span/mean chord
So swept wings have the same characteristics as a low aspect ratio wing
However, it does come with the benefit of a gentle, progressive stall
