Short Field Landings
First you have really got to know this
Most people (and even some pilots), probably from watching movie heroes wrestling with the controls, think that the control stick (the elevator control) in an aeroplane controls the height of the aeroplane and the throttle (the power control) makes you go slower or faster.
It is in fact exactly the other way round and one of the best books on flying will tell you this and plenty of other things that fall in to the non logical category.
The book is called
and Rudder: An Explanation of the Art of Flying"
The correct speed and descent angle
Perfect landings on a short runway involve flying your 3 axis small light aircraft, ultralight or microlight in an apparently unstable condition, close to the edge of disaster in fact. It is often referred to as flying on the back side of the power curve and it is also said to be in ‘the region of reversed reaction’, as we shall see later.
There are two things of interest during a short field landing, the speed and the descent angle.
Speed is important, you need speed for safety but any excess speed will soon convert to floating and a longer landing distance.
Descent angle, as any pilot knows, is varied by altering the speed during a glide approach, any fluctuation from the best approach speed, either faster or slower, will result in a steeper glide. Diving to make the approach steeper is obviously not a good idea as again the extra speed will become a major embarrassment as you serenely float down the runway at five feet never touching it with the hedge looming ever closer. That leaves the instinctive selection for short landing, a slower than normal approach. This is the point where the big problems begin.
Why it's a drag.
All aircraft are plagued with various types of drag but broadly speaking, they can be divided up into two sorts. We have parasitic drag and induced drag and the total drag is both added together.
Parasitic drag is due to skin friction, sheep droppings, the undercarriage ,all those plastic ties and your badly fitted wing covering. It is the grand total of the pressure difference between the front and back of the aeroplane. It is the effect of trying to push air round all these obstructions. Parasitic Drag increases by the square of the speed, i.e. double the speed and the drag quadruples. It only really affects the cruise region of flight, and is of little interest to us when discussing the problems of short field landings.
Induced drag is rather more subtle, it varies inversely with the square of the speed, i.e. halve the airspeed and the induced drag quadruples, go faster and it reduces. If you consult your Aerodynamic Theory books (perhaps long forgotten) you will see it is caused by a rearward component of the lift produced by the wing. Lift acts in a perpendicular direction to the airstream over the wing. With an angle of attack one component acts vertically and keeps the aircraft in the air but the second and rearward component tries to slow it down. It is induced by the lifting effects of the aerofoil shape.
What is important to realise is that as parasitic drag increases the induced drag decreases, and vice versa. The slower the aircraft and the higher the angle of attack the higher the induced drag and this is the major effect. In a steep climb you need the same power as you would need to get VNE (Velocity Never Exceed) in straight and level. Just to add to all your problems neither engine output nor prop efficiencies are particularly good at low speeds due to a fixed pitch prop. It is just where you might need plenty of thrust to get out of a sticky situation.
The point where parasitic and induced drag are equal (the bottom of the total drag curve) just happens to be the point where the aeroplane flies at its most efficient, the best glide speed for maximum distance. It is often very close to the normal approach speed of an aircraft - about 1.3 times Vs (30% above the stall speed). Flying faster than that is the 'front side' - the region of normal command, and slower is the 'back side', the region of reversed command.
So much for the theory but what does it all mean in practice!
The back side of the curve
Consider an ultralight flying at its normal cruise speed of 50 to 55 mph, trimmed and maintaining a constant altitude now apply slight forward pressure on the stick, and as altitude is lost, the speed increases. Release the stick, and in a matter of seconds, the speed will reduce back to its trimmed value. It is said to be positively speed stable, always tending to return to its original condition. If you want to accelerate to a higher speed of, say, 65 mph, without losing altitude, you must increase the power as you lower the nose.
|Drag is high at a high angle of attack. Aircraft may not accelerate even under full power Take the same aeroplane, hanging on the prop, flying at 40mph - somewhere between Vs (stall) and VMIN D (lowest drag speed) also trimmed and maintaining constant altitude. A slight forward pressure will also increase the speed, but this time, as the total drag decreases by quite a large amount, a bonus occurs, the aircraft will climb as well as accelerate unless you reduce the throttle. Less drag reduces the power requirement to maintain altitude.|
It seems paradoxical to lower the nose to make the aeroplane climb (‘the region of reversed reaction’), but that's the way it is when you are flying the back side of the curve. (Remember this next time you are still heading for those trees after pulling back on the stick on climb out. Just grit your teeth be brave and stick the nose down)
This is the reason that pilots are taught, when landing, to use pitch (the elevators) to control airspeed during an approach to landing and power to control the descent rate. The danger comes when you raise the nose at slow speeds, as most pilots will naturally do when they want to reduce the sink rate. The drag shoots up so fast that a great deal of additional power is then needed to maintain speed.. Power is something that microlight aircraft are not exactly over endowed with and a power shortfall results in a further speed reduction, We now get more drag and then a further increase in the rate of descent and so on in a vicious circle until a stall inevitably occurs.
Doing all this at and above the threshold, results in that heavy drop onto the ground that will most probably damage your undercarriage, and not least, your pride.
Doing it for real
Picking a speed for final approach for a short field landing is critical. and every mile per hour in excess speed to that needed to complete the landing reduces this performance. The figure most often used for short field landings is 1.2 times Stall speed (VS). Various factors influence normal approach speeds, and one to consider is weight. A decrease in the weight of aircraft due to less fuel at the end of a flight will result in a helpful decrease in the stalling speed. The weight of the passenger will of course be less helpful and increase the stalling speed.
As a short field landing is a critical situation and the margins are tight it is necessary to be pernickety about exactly what speed you will use. .Without resorting to handbooks of technical data it is easy to work this out from a simple test.
Next time your aircraft is in the landing configuration with regard to weight and you are at a safe height of course, get into wind and stall the aircraft with power off and the ball in the middle. Note the ASI reading that the break occurs or sink sets in, and then add 20%. This is VREF and is the final approach speed for a short field landing at that particular weight.
While you are at it put the power on and bring the nose up as it is worth spending a bit of time getting the feel of the aeroplane flying slow with a lot of power. The balance of the control pressures and the reactions can be quite different to normal cruising flight.
Another factor to consider is wind-shear. Flying so close to the edge even a small reduction in airspeed over the wing will have a large effect. If it's gusty, adding half the gust value to VREF can be a lifesaver. You should also be aware that many short strips are sited in areas of relative calm with wind gradients due to obstructions upwind on short final. You were always taught to keep a hand on the throttle so if you suddenly sink you will need to get power on and fairly quickly
Aim to keep the runway threshold in that part of the screen you marked as the horizon when you first learnt to fly and view it all the way down, It's better than floating over the top and then diving for the threshold. Speed control is critical and as we have said arriving too fast will end with you floating above the runway, but arrive too slowly and you risk an uncontrolled landing.
Trimming accurately, and having the approach well stabilised in good time is imperative. If you're chasing the ASI or trying to hold the threshold in the windscreen without much success put the power on and go around and start again. Short field landings are definitely an area where the decision to go around should be made rather sooner than later. Mess up an approach on a 5,000ft beach and you have plenty of room and time to spare to sort it out. In short landings the concentration needed is higher than normal so don't stack the odds against yourself. The same goes for being accurately aligned with the runway in good time as 'short' usually means 'narrow' when it comes to farm strips. Coarse use of the rudder and ailerons at low airspeeds with a lot of power is a good recipe for an incipient spin. The chances are that it will all be over before you have started to work out what is happening. If you are too low on the approach by more than a few feet which can be dealt with by lowering (yes lowering not raising) the nose slightly for a second, is sufficient reason to go around. Pulling the nose up even very slightly in such a situation puts you much further into the danger zone. There is also a danger in being too high on your approach with all the power off then sticking the nose down. This just gets you there faster and in fact higher than you would have done. Side slipping to shed the excess height using opposite aileron and rudder is the answer here but this needs a fair amount of skill and practice and never near the ground.
Assuming a fair amount of power is being used the nose attitude is going to be higher and, the round out needs a gentler touch than normal. When you remove power the aeroplane will sink very soon after you close the throttle so you need to be in the landing attitude and very close to the ground. When you are safely on keep the stick back to present as much area of wing to slow you down and if you have the luxury of brakes apply these gently and amaze yourself by using only half of a very short runway.
In the end, the biggest single variable factor in performing short field landings is the pilot. As my Instructor used to say, “You know all the Aerodynamic Theory but you still can’t do it can you !”
Gradually working your way towards finer margins is the way to do it and just when you are absolutely sure you finally have it to perfection it suddenly all goes wrong again.
That’s Flying of course !