Technical Interview 1 Flashcards
Why does a swept wing increase Mcrit?
On a straight wing airplane, all of the airflow over the wing travels parallel to the aircraft’s chord line. But,on a swept wing, only some of the air flows parallel to the chord line. The other part flows perpendicular to the chord - this is calledspanwise flow.
Only the component of airflow flowing parallel to the chord line accelerates. So, by reducing the amount of airflow flowing parallel to the chord line, you’ve reduced the amount of acceleration - and increased your critical Mach number.
What is the maximum operating ceiling of a typical jet?
41.000ft
Maximum Operating Altitude.
Typically, maximum Operating Altitude is the highest the plane can go for structural/pressurization reasons. For instance, FL370 is our Maximum Operating Altitude in the ERJ because that’s the highest we can go will maintaining a 7.8 psi cabin differential pressure while also maintaining an acceptably low cabin altitude (8000 ft), even though the engines and aiframe would have no problem climbing higher if the cabin altitude was not an issue.
Service ceiling, on the other hand, as flyguy said is the maximum altitude at which the airplane can maintain at least a 100 fpm rate of climb with all engines operating.
Absolute altitude is the highest altitude the airplane can reach, period…..in other words you’re hanging on at Vy with a 0 fpm rate of climb. Maximum Operating altitude is usally not the same number….MOA is more of an equipment limitation than a performance limitation.
Are swept wings effective?
At low speed a swept wing is not effective, at high speed they are effective.
When you reduce the amount of air flowing parallel to the chord line, you reduce the amount of lift the wing creates. At high speed, this isn’t a problem - your high airspeed requires a small angle of attack to create lift.However, at slow speeds, you’re at a high angle of attack, and sweeping the wing can force a very high angle of attack - nearing your stalling angle of attack.
What is the difference between angle of attack and angle of incidence?
Angle of attack is the angle between the chord of the aerofoil and the relative airflow, it changes in flight with attitude.
Angle of incidence is the fixed angle between the chord and the aircrafts longitudinal axis. During your takeoff roll, the relative wind is parallel to your runway. The angle of incidence defines your angle of attack until you have enough speed (and tail down force) to lift your nose off.
What is a Mach trimmer?
Its an automatic system, that adjusts the elevators with respect to the stabilizer as speed increases in the case of the 737 or decrease the incidence of the variable incidence trimming tailplane in other aircrafts, to automatically compensates for Mach tuck.
Diferente del Stabilizer Trim (FCOM 737)
What is the screen height for a single engine?
The height of an imaginary screen placed at the end of the Take-Off Distance Required (where the aircraft reaches 35 ft or 50 ft, not 35 ft or 50 ft at the end of TODA) and at the beginning of the Landing Distance Required. The aircraft needs to reach the screen height at the end of its take off distance
For a single engine is 50 ft (15 m)
At what speed does Mach tuck occur?
Mcrit
What increases Mcrit?
Swept, thin wings
What are the advantages of an all moving/variable incidence tailplane?
- As the elevator and the horizontal stabiliser remains in line when trimmed, it produces a lot less drag.
- It is very powerful and gives an increased ability to trim for larger CG and speed range.
- Trimming does not reduce the effective range of pitch control, as the elevator remains approximately neutral when the aircraft is trimmed.
Can you explain to me what Mach Tuck is?
Mach Tuck is a nose-down pitching moment experienced as an aircraft passes Mcrit.
As speed increases beyond the Critical Mach number (MCRIT), shock waves start to form, they move aft and get stronger as the airplane accelerates. The CP also moves aft and this increases a moment of force that makes the aircraft to pitch down. If the aircraft is allowed to continue to accelerate beyond the limiting mach number, the centre of pressure may move so far rearward that there is insufficient elevator authority available to counteract the nose down moment
Another reason for Mach Tuck is reduced downwash on the tailplane. As the airplane increases speed, the changing airflow from the wing reduces the tailplane effect, and promotes a stronger nose down moment
What is Mcrit?
The speed where airflow somewhere on the aircraft reaches the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This creates a shock wave. The critical Mach number varies from wing to wing. In general a thicker wing will have a lower critical Mach number
Explain what coffin corner is and what happens when reaching the coffin corner?
The coffin corner is the altitude at which an aircraft’s stall speed is equal to the critical Mach number, at a given gross weight and G-force loading. At this altitude it is very difficult to keep the airplane in stable flight. Since the stall speed is the minimum speed required to maintain level flight, any reduction in speed will cause the airplane to stall and lose altitude. Since the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any increase in speed will cause the airplane to lose lift, or to pitch heavily nose-down, and lose altitude. The “corner” refers to the triangular shape of a flight envelope chart where the stall speed and critical Mach number lines come together
What do airplanes have to guard against reaching Mcrit?
Barber pole on the ASI, flying at a Mach number at altitude and audible warnings.
How do you prevent an aircraft reaching Mcrit?
Decreasing speed
Why do jet aircraft fly as high and fast as possible?
Because the higher a jet aircraft flies, the more efficient it will be, the lower the fuel flow, the greater the range.
Fuel flow is a function of the fuel used per unit of thrust, multiplied by the total number of thrust units. Obviously if it were possible to reduce the fuel used per unit of thrust and the total number of thrust units required then the total fuel flow would be reduced. Fuel used per unit of thrust is most commonly known as specific fuel consumption. The specific fuel consumption needs to be small, in other words, the aim is to reduce the amount of fuel to produce each thrust unit. For a jet engine this occurs when ambient temperature is very low and engine rpm is very high. This can only occur at high altitude. So flying at high altitude, minimises the fuel used per unit of thrust.
At high altitudes, the air is lower in density and therefore easier for the engine to process and turn into thrust (less drag is encountered). Less fuel is therefore required.
Regarding OAT, the air at higher altitudes is cooler. Cooler air entering the engine expands within it more than warm air does (it’s air expansion that provides the thrust). The result is greater thrust output for a given fuel burn.
Having minimised the fuel used to produce each unit of thrust, the aim is now to fly the aeroplane using minimum possible total thrust because each unit of thrust requires fuel to be consumed. This problem is comparatively simple to solve. In level flight the forward acting force of thrust is controlled and balanced by the rearward acting force of drag. If drag is small the aeroplane need fly with only a small amount of thrust.
To minimise drag, the jet aeroplane simply flies at the velocity for minimum drag. Therefore VMD is the speed to fly for maximum endurance for a jet aeroplane.
In summary then, for a jet aeroplane to maximise its endurance by minimising its fuel flow, the pilot would fly the aeroplane at VMD and fly as high as possible.
Jet engines work better at high altitude because the air is cooler, cool air expands more when heated than warm air, it is the expansion of the air that drives the combustion engines. Also the air is less dense at altitude therefore the aircraft flies much faster at high altitude than low altitude at the same thrust.
