Aeroplanlære - Kap 5 (Stall) Flashcards
What causes a plane too stall?
Stall is caused by an excessive angle of attack combined with a low kinetic energy in the boundary layer and a high energy in the adverse pressure gradient which causes separation closer to the leading edge
How do we unstall an airplane?
By reducing angle of attack
Describe the 4 different parts on the model
Can a cambered aerofoil with negative angle of attack still generate a positive lift coefficient?
Yes
How does the stall angle change with wing aspect ratio?
High aspect ratio stalls at a lower angle of attack
Why is highly tapered and/or swept wings dangerous to stall with?
Where does a rectangular wing stall first and last?
- Wing root
- Mid span
- Wing tip
Where does a tapered wing stall first and last?
- Mid span
- Wing root
- Wing tip
Where does an elliptical planform wing stall first and last?
- Wing root and mid span
- Wing tip
Where does a tapered and swept wing stall first and last?
- Wing tip
- Mid span
- Wing root
What can we install on the wing, to reduce tip stalling tendencies?
We can install a wing fence or vortillion
What can we do to the wing, to reduce tip stalling tendencies?
Use notch or washout
What is the best place on the wing to stall first?
Wing root since stalling the tip affects our ailerons and our ability to manoeuvre
How do we change the aerofoil shape so that the wing root stalls first?
Increase the sharpness of the leading edge
How does wing loading affect stalls?
Higher wing loading = Higher adverse pressure gradient
This means that flow energy is lost earlier and thereby earlier stall onset
Does slat and slots increase or decrease the AoA which we stall?
Increase
Does trailing edge flaps increase or decrease the AoA which we stall?
Decrease
Does leading edge flaps increase or decrease the AoA which we stall?
Increase but produce less lift than trailing edge flaps does
Which wing stalls first at high angles of attack in a turn?
The side with the down going aileron
Which effect does the weight have on stalling?
At higher weights the speed must increase to increase lift or we need a higher angle of attack
This results in stalling speed being higher for heavier aircrafts
How do we calculate the stalling speed on a heavier aircraft than our old one, and why is this possible?
The wing always stall at the same AoA which is Clmax
Therefore we can calculate the new stalling speed the following way:
What is the new stalling speed if:
Old stall speed = 60 kts
New weight = 1300 kg
Old weight = 950
70 kts
What does slats and flaps do to our Clmax?
They increase it
What does slats and flaps do to our stalling tendencies?
Slats and flaps increase lift which means:
1. The critical angle occurs at slower speeds
2. Lowering flaps reduce stalling speed
What is Vsi?
What is Vs0?
Which effect does landing gear have on stalling speed?
Drag is increased from gear this cause nose pitch down
Downforce on tail result in lift increase
Lower stall speed
Which effect does propellers have on lift?
There is increased lift in the area with propeller slipstream
Which effect does jets have on lift?
Lift is reduced by vertical component of thrust
Which effect does CG have on lift?
Forward CG increase lift since you need more taildown movement and therefore also have more induced drag
Which type of wing is easiest to define stall on?
1. Straight wing
2. Swept wing
Straight wing since it occurs more rapidly
Which effect does contamination of wings have on stalling speed?
How do we recover from an accelerated stall?
Reduce angle of attack
How do we calculate stall speed in turns?
Calculate square root of load factor and multiply it with the old stall speed
Example with old stall speed at 60 and a 45 degree turn:
What is the stall speed in a 30 degree turn with a normal stall speed at 50 kts?
53,7 kts
How do we unstall a wing?
Reduce angle of attack
When does a wing stall?
When it exceeds its stalling angle of attack
What will an aircraft keep doing in a full developed spin?
It keeps rolling, pitching and yawing at a stable low speed, if no control inputs are made from the pilot
Which effect does CG have on spin steepness?
A forward CG will make a steeper spin
A aft CG will result in a flatter spin
What kind of spin is the hardest to get out of?
A flat spin, since the angle of attack is greater and it has a rapid rate of yaw
What kind of mass distribution has the most dominant factor in a spin?
Lateral distribution of mass since it results in more roll and less yaw therefore a steeper pitch attitude
How do you prevent a spin?
If stalling characteristics are felt reduce angle of attack since a spin is a stalled condition
Which phases does a spin consist of?
Three phases:
1. Incipient spin - Start of spin - Aircraft starts to auto-rotate
2. Fully developed spin - Spin becomes steady
3. Spin recovery
How do you recover from a spin?
- Close the power
- Oppose yaw with full opposite rudder
- Unstall the wings by pushing the control stick centrally forwards
Is a spiral dive a stalled condition?
No
How do you recover from a spiral dive?
- Close power lever
- Level wings
- Gently ease out of the dive
What increase in a spiral dive?
Airspeed and load factor
Which stall warnings are there in a level flight?
- Controls become sluggish and unresponsive
- A high nose Attitude is seen
- Might occur buffeting depending on wing profile (Rectangular)
What are the classic aerodynamic indications of a stall (Symptoms)?
On a swept wing, what are the stall warnings?
Which artificial stall warning systems do we have?
What can help a pilot decide the angle of attack?
An angle of attack sensor
What do the T-17 have to indicate a stall and how does it activate?
It has a warning light and a sound to indicate stall
It’s activated by a flapper switch when angle of attack is too high and therefore the stagnation point moves behind the flapper switch
Name 3 AoA indicators:
- The AoA vane
- AoA probe
- Rotating probe
What do almost all aircrafts have to warn of a stall?
An audio warning
What do a stick shaker and stick pusher do when stalling?
Stick shaker - Shakes stick
Stick pusher - Pushes stick forward unless yoke is pulled hard back or system is switched off
How do you recover after the first indication of a stall?
How do you recover from a stall?
What is Vsw?
Velocity for stall warning
Greater than 5 kts or 5%
