High Speed Flight
Subsonic .7 or less
Transonic .7 to 1.3
Supersonic 1.3 to 5
Hypersonic above 5
The Speed of Sound
is effected by temperature alone and no other variable.
It decreases with lower temp. and increases with higher temp
The speed of sound is 661.48 kts at 59 F
Slowing to 602.0 kts at -30.0 F
Slowing even more to 573.6 kts at -69.7 F
Mach number is the ratio of the true airspeed to the speed of sound or TAS/Speed of Sound.
If aircraft TAS is used to compute Mach number it is the Airplane Mach #.
If a velocity at some point on the airplane is used, the resultant value is referred to as Local Mach Number.
Some parts of the airplane may experience compressibility because of higher Mach even though the plane may be going less than mach 1.
The speed at which the first part of the plane goes mach 1 is called the critical Mach #.
Attainment of speeds over Mach 2 are very difficult because of the temp problem.
At mach2 the air temp may reach 250F even at OAT of -66F at 35000 feet.
At mach3, temp shoots up to 640F.
Since aluminum looses its strength at 250F you can see the problem.
Compressibility drag forms when a shock wave forms.
The shock wave is nothing more than the boundary between supersonic air and subsonic air.
Sharp increases in static pressure and density occur as well as a loss in energy to the airstream and possible airflow separation.
The transonic region is the place where most problems occur.
The shock wave may jump back and forth across the surface disrupting airflow and causing a buffet.
With proper design this has all been eliminated.
Wave drag is the drag caused by this shock wave and it gets worse the faster you go.
High Speed Drag
There are ways to delay the onset of this shock wave:
Keep this principle in mind: Wave drag rise is roughly proportional to the square of the thickness to chord ratio.
So a thin skinny wing with a long chord is better than a short fat one.
Use thinner airfoils. The air is not sped up so fast so lower local mach #
High Speed Drag
Use wing sweep forward or backward. The ratio of thickness to chord is reduced
Low aspect ratio. Substantial increase in the Mach # occur with a wing using less than an aspect ratio of 4.
If the boundary layer can be eliminated or bled off this reduces or eliminates the shock interaction between sub and supersonic flow.
High Speed Drag
Likewise if the flow can be reenergized, separation is not as likely to occur and a favorable pressure gradient is established in the area of the shock wave.
Use the Supercritical wing.
This airfoil has a flattened upper surface which delays the formation and strength of the shocks to a point close the the trailing edge.
Additionally the shock induced separation is greatly decreased.
The critical mach# may be delayed up to .99
Since lift is developed by camber they moved the camber to the aft edge of the wing
There are 2 main advantages by using the same thickness to chord ratio
It permits near mach 1 cruise before the transonic drag rise.
The second way is because of lower drag numbers a larger thickness wing may be used with less drag penalty.
A mach cone or bow shock forms off the front of the plane if the wings are swept back they remain in the subsonic flow region behind the shock wave.
This shock wave occurs after the normal shock wave has progressed to the trailing edge of the surface.
Wherever there is a bend in the flow an oblique shock wave will result.
An expansion wave forms where there is a outside corner to turn.
When this happens, there is no loss due to pressure or temp changes.
In fact lift is developed and this is a good kind of shock wave.
One big problem with theses types of planes is the large pitching down moment because of the shift in AC aft causing mach tuck to occur.
Boeing proposed a swing wing to counteract.
Lockheed originally proposed canards to counteract the nose down pitch.
The final design was the double delta.
The Double Delta
This wing is really two wings in one.
The forward delta begins to generate lift supersonically negating the pitch down problem.
Also at slow speeds it generates a vortices that causes extra lift over the rest of the wing behind it.