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Turbulence is something we all experience at one time or another. In ground school we learned the theoretical explanation: turbulence is an irregular movement of air resulting from eddies and vertical currents; it is produced under four types of conditions. Mechanical turbulence is produced when air passes over the ground, particularly irregular ground, and man made objects.
Thermal turbulence is a result of differential ground heating. Frontal turbulence is produced along the interface of moving air masses, and wind shear, a shift in wind direction or velocity at altitude, can produce significant turbulence. Regardless the source of turbulence, the experience of pilots, passengers and the aircraft itself is the same. Depending of the severity of the turbulence, we can expect all the delights of discomfort, airsickness, possible injury, and, in severe cases, potential loss of control or structural damage to the aircraft.
It is important to understand the causes of turbulence, to learn to recognise situations that may be difficult or hazardous and to understand the limitations and procedures to ensure safe flight in your aircraft. Or is there more to it? The POH for the aircraft you fly will provide specific information regarding load limits. The vast majority of aircraft flying with a Certificate of Airworthiness C of A are classified in either the Normal or Utility category. Some aircraft may be flown in either category, the C, for example, depending on how it is loaded with passengers, baggage and fuel.
Most aircraft are designed to exceed these minimums. Applying a slightly greater load to the aircraft will not immediately result in the loss of vital surfaces or components due to the built-in safety factor.
However, we all know the story about old pilots and bold pilots. Knowingly stressing an aircraft above its limits can lead to serious and possibly fatal consequences down the line, if not sooner.
A g is a measure of acceleration. In normal, unaccelerated flight, we experience a 1g load: the lift produced by the airfoils is equal to the weight of the aircraft, including any tail loading being produced. We can say, under these conditions, that the load factor, the ratio between the dead load, the weight of the aircraft, and the live load, the increased weight of the aircraft caused by acceleration is 1 2.
Acceleration increases the live load. Increasing the live load increases the load factor. The greater the acceleration we produce -- the greater the difference between weight and lift -- the greater the load on the aircraft. The formula tells us that lift is equal to one-half the product of four factors:. Increasing the angle of attack increases the amount of lift and reaches its maximum value at the Critical Angle of Attack. Increasing the angle of attack beyond the critical angle of attack results in a significant reduction of lift: a stall.
The point at which the stall is reached depends on the shape of the wing profile. This is expressed in the formula for lift by the Coefficient of Lift CL ; the critical angle of attack, the angle at which maximum lift is produced, is represented as CLmax. For our purposes, we can assume that the CL, S and r remain constant.
Once our aircraft is loaded and in the air we have little ability to change the coefficient of lift except as a function of angle of attack , the surface area of our wings or the air density. Lift is equal to K times the square of the velocity. This lets us see a bit more clearly the relationship between lift and air speed. Lift increases with the square of the air speed. If we slow the down to just at stall speed, flying at our maximum angle of attack CLmax , and pull the control column full back, the aircraft will stall; we and the aircraft will experience a 1g load.
The wings are incapable of producing more than 1 g at that speed. If we pull our control column full back while flying at a higher speed and smaller angle of attack, we will produce more than a 1g load; at the moment of stall, we will be accelerating. As we just discussed, the load applied to our aircraft is increased by the square of the speed. If we increase our speed to say 1. If we were to double the speed at which we stall the aircraft, say from 50 knots to knots, we increase the load by four times 4 being the square of 2.
If we apply full control inputs at knots and stall the aircraft, instead of the mild, rather pleasant 1g stall, we and the aircraft will experience a 4g dose of acceleration. How do we determine the maximum speed at which we can safely apply full control inputs without exceeding our load limits? We work backwards from the correct answer.
We can look these values up and save ourselves some effort, or we can work them out ourselves. We can derive them either mathematically or geometrically.
Geometrically we can draw the triangles representing vertical and horizontal lift, measure the lengths of the sides and work out the ratio between weight and lift.
To calculate the maximum stall speed that will keep us on the sunny side of our load factor, we should ask ourselves this question: at what stall speed will the lift produce a load factor of 3.
Since stall speed increases with the square root of the load factor we can say the square root of our maximum load factor 3. At any airspeed less than Vs times the square root of our maximum load factor, our aircraft will stall before more load is imposed on it than it is designed to handle.
The wings cannot produce more load than the aircraft is designed to deal with. Enter the concept of Manoeuvring Speed Va. As student pilots, we all learn Manoeuvring Speed Va ; it is one of the three speeds that must be memorised for the Private Pilot flight test. We also learn that Va is the maximum recommended speed for turbulent air penetration.
This is good information and will, most probably, help us earn pass marks on the private pilot flight test. As we grow and develop as pilots, we begin to take on new challenges. The bottom line: Va is the speed below which our aircraft will stall rather than bend or break when we impose or have imposed on us—as in the event of a vertical gust—an increased load.
But, as with most things, the devil is in the details. If we go back to the POH for our , we read that the Manoeuvring speed is given for three weights:.
This will give us an actual Va Vs x vmax load factor of between Cessna, appropriately, chooses to err on the conservative side in its published figures. As pilots who intend to become old pilots, we should be encouraged to do the same.
Why, we might ask, does Va decrease with weight? This is a key point and a very interesting one. As we discussed earlier, the stress or load factor imposed on the aircraft when it is accelerated is a function of the lift to weight ratio. At full weight, lbs for our in the Normal category, we can impose 3.
We can generate a total of lbs of lift by sudden control input x 3. The designers of the aircraft have structured the machine to be robust enough to withstand the 3. If we toss out our passengers and burn off some of the fuel, we might achieve a gross weight of lbs. At full weight, lbs, we happily stalled our aircraft at At our new and lighter weight, we can develop the same amount of lift at the same airspeed.
Remember the formula: aircraft weight is not a factor in the production of lift. If we could manage to lighten our aircraft to half its full weight, lbs, we would find the abrupt stall would now produce a very undesirable 7. First, in plain English:. We know that a wing stalls just past the angle at which it is providing maximum lift, so we can refer to our CL in terms of its maximum value, CLmax 4.
In this case it is 0. We know, from the POH, that stall speed at gross weight, depending on the location of the C of G, is approximately 50 Knots.
To calculate the stall speed for a lighter weight, we pop the new weight into our handy, little formula. In this case we find that our 1g stall speed at lbs would be Working back up to calculate our Va, remember we multiply the stall speed by the square root of the load factor.
In this case So, here we are, ready to go flying. Not too many of us are willing to do all the calculations when we are actually flying, especially if the air is a bit turbulent. Nor do we need to do them if we have some reasonable rules of thumb to follow that will keep our passengers and us safe and sound. Here are the rules handed down to me by my mentor, Captain John Spronk:. In Smooth flying conditions:. In Light Turbulence:. In Moderate Turbulence:. In Severe Turbulence:.
Kershner, William K. Macdonald, A. Limited, Ontario, Canada, , pg. Kershner, pg. Ibid, pg. Phone: Email: info principalair. Unit D Liberator Ave. The Square of the Airspeed V2. Increasing airspeed results in a significant increase in lift. We can see from the formula that lift increases with the Square of the Velocity.
If we double the airspeed, we increase lift four fold. Air Density r.
Aftermath: Remember 19?
Over the years, we've had more requests for aircraft flight manuals than any other product. We're happy to continue offering Aircraft Information Manuals, similar to the type sold by the airplane manufacturers. They include the same data and limitations contained in the original aircraft Pilot's Operating Handbook. Our reproductions are all done on state of the art Canon digital printers, after being meticulously cleaned up and scanned.
Cessna 172M Skyhawk 1976 Pilot's Operating Handbook (D1057-13)
Cessna 172M Manual (1976) D1057-13