What is "aircraft stability" - IN VIDEO

[bongodriver]

Originally Posted by Crumpp

Really?....youre really going to ignore item 1 in that same page?....the one that states 'CLEARLY' that the 'short-period longitudinal oscillations were satisfactorily heavily damped in all conditions tested'

[Crumpp]

Are you really going to ignore the second part, the stick fixed longitudinal instability was unsatisfactory in all conditions?

Are you going to ignore the fact the game characteristics are exactly the opposite of the real thing???

The game results....




The RAE results for the same test.....

[bongodriver]

Originally Posted by Crumpp

Are you really going to ignore the second part, the stick fixed longitudinal instability was unsatisfactory in all conditions?

Are you going to ignore the fact the game characteristics are exactly the opposite of the real thing???

The game results....




The RAE results for the same test.....

Yes I am going to ignore it because you are highlighting stick-free behaviour in game and highlighting stick-fixed in the NACA report and your in game results don't point out what configuration the aircraft was in, you may notice that not all of the stick-free oscillations in the second graph are divergent.

[ACE-OF-ACES]

That and I am pretty sure that klem, the guy that did that ingame stick test, told Crumpp that it was not something he (klem) would consider a full fledged test.. Let alone one that relates to the topic.. Yet Crumpp still keeps posting references to that ingrame graph as if it was some 1C approved 'golden' test.. Also note how Crummp fails to give klem any credit as the one who actully did the test.. Which may falsely lead some to belive that Crumpp is actully doing his own ingame testing

[bongodriver]

That and the fact that both graphs illustrate long period oscillations which even Crumpp can't argue we're not tested by NACA and he also can't argue the fact that long period oscillations matter squat.

Note:

Long period oscillations are measured over minutes

Short period oscillations are measured over seconds

[Christop55her]

It takes more controlling on the part of the pilot.

[Crumpp]

Yes I am going to ignore it because you are highlighting stick-free behaviour in game

Wow,

Nobody has compared anything in the game regarding the NACA measurements yet.

The test for the game is stick free, just like the RAE measurements!!!!!

They do not match. The RAE found the Spitfire to be longitudinal neutral or unstable Stick Free.

Take a second and digest the data in both test's before frothing at the mouth I am wrong.

[bongodriver]

Originally Posted by Crumpp

Wow,

Nobody has compared anything in the game regarding the NACA measurements yet..

Then what the hell are you even contributing here?

Originally Posted by Crumpp

The test for the game is stick free, just like the RAE measurements!!!!!

it is for both, fixed values are on the left column and free on the right, and in both cases some stability was recorded, you said it yourself, long period oscillations are irrelevant, if you can't sort out a bit of divergence after 4 minutes then youre either dead or paralysed.

Originally Posted by Crumpp

They do not match. The RAE found the Spitfire to be longitudinal neutral or unstable Stick Free..

not in all conditions tested, you took the time to put it in red yourself, both columns show some element of stability, engine off with flap and gear up is stable in bot fixed and free.

Originally Posted by Crumpp

Take a second and digest the data in both test's before frothing at the mouth I am wrong.

Look pal, take a second yourself and read from post 68, you are the one that mentioned long period oscillations were not measured, all I did was say 'exactly' and pointed out that NACA concluded short period longitudinal oscillations were satisfactorily heavily damped. you are the one 'frothing' at the mouth in a desparate attempt to give no quarter, you really should just give in because you 'are' wrong.

[bongodriver]

We went down this road on the last Crumpp crusade against the Spit and a point was raised on just exactly how do we simulate stick-free in a computer game given the constraints of current game controllers, how exactly will a computer determine you don't have your hand on the stick? all it will do is sense no input from the controller and assume the stick is being held in place....in essence it will always be stick-fixed.

[SlipBall]

Some interesting thoughts from a man who flew these aircraft in modern times...maybe off topic a bit, but an enjoyable read.
Flying Old Aeroplanes in the 21st Century;
The Handling Qualities of World War II Fighters

by Dave Southwood

A lecture given to the Flight Test Group of the Royal Aeronautical Society
18 March 2004, 4 Hamilton Place, London W1

Introduction
One hundred years after mans' first powered flight, there is still a great fascination for the aviation history of the century of unparalleled technological advances that has just past. For pilots, it is fortuitous that this interest extends to maintaining flying examples of aircraft types from throughout the whole period. The willingness to build flying replicas of the earliest machines has been another indication of the commitment to preserve our aviation heritage for posterity. However, the skills required to fly these early machines, and indeed aircraft from all but the last one or two decades, are considerably different from those needed to fly the modern glass cockpit, fly-by-wire, highly automated airliners and combat aircraft of the early twenty-first century. Flying training has evolved such that only the skills needed to fly the current generation of aircraft are taught. Teaching skills that are required in order to fly older types of aircraft, such as those with a tailwheel or powerful piston engines, when these skills will be redundant after completing training is a luxury that the military, commercial employers and indeed most private pilots cannot afford.

Therefore at the start of the 21st century it must be appreciated that the scope of the training and experience of many of the current generation of pilots, both military and civilian, almost certainly will not have prepared them to fly such aircraft. Their flying ability will be as high as that of previous generations of pilots, but ability only gives an individual the potential to learn the skills required to fly. It does not automatically install those skills; they have to be specifically learnt. In this article I aim to discuss some of the handling qualities from one specific group of vintage aircraft, WWII single piston-engined tailwheel fighters. Hopefully this will provide an appreciation of some of the skills that have to be learnt in order to fly such aircraft safely, competently, and to enjoy the experience. I freely acknowledge that this is only a brief summary of the flying qualities of a few types of aircraft from one generic class that I have been fortunate enough to fly over the last 15 years. Every other class and type of vintage aircraft will have its own individual, and often unique, qualities of which I have little or no experience.

Scope
This article draws on my experiences flying the following aircraft types: Spitfire (Marks V, VIII, IX and XVI - all Merlin-engined), Hurricane Mk XII, Bf109G-2, HA 1112 (Merlin-engined Me109), P-51D Mustang, P-40M Kittyhawk, Corsair (FG-1D and F4U-5) and F6F-5 Hellcat. The flight envelope considered is that pertaining to display flying i.e. up to maximum continuous power, altitudes below 10,000 ft, normal accelerations from +5g to 0g, standard level looping and rolling aerobatics, close formation flying and tailchasing. Aspects covered will be those where the greatest differences from modern aircraft occur; ground handling, take-off, general up-and-away handling qualities, accelerated power-on stalling, high power low speed controllability, engine handling and landing.

But first, it is worth discussing some of the basic characteristics relating to the configuration of this class of aircraft that have a profound effect upon the handling qualities that will be discussed.

Powerful Piston-Engined and Tailwheel Characteristics
The power produced by a piston engine is transmitted to the propeller via torque on the propeller shaft. The propeller then transfers the power to the air flowing through it, resulting in both thrust and, due to the drag on the propeller blades and friction, energy loss. However, if the propeller is not able to transfer to the air all of the power produced by the engine, the torque on the propeller shaft produces a rolling moment on the engine in the opposite direction to the rotation of the propeller. If and when this situation occurs, it is at a combination of high power and low forward airspeed. Note that in piston-engined light aircraft torque effects are not usually seen except in aerobatic aircraft during prolonged upward vertical manoeuvres. At full power our subject aircraft can produce typically 1500 to 2300 horsepower and even though maximum continuous power is somewhat less, torque effects are still seen and will be discussed later.

The nature of the airflow behind the propeller, normally referred to as "propwash", is a helical flow whose velocity and helix angle are largely a function of engine power setting and of the forward airspeed of the aircraft. Obviously, the airflow within the propwash over the left and right sides of the aircraft is not symmetrical, in particular over the fin and rudder. In single-engined propeller driven aircraft this is the main cause of the directional trim changes with variations in airspeed and power. Aircraft with a high-powered engine and a large operating speed range (VNE of the Mustang is 505 mph) potentially will be affected by these trim changes much more than low powered, low speed light aircraft. In addition, any sideslip will displace the propwash laterally, varying the downwash angle experienced by each tailplane and possibly resulting in the blanking of one tailplane by the fin and rudder. Thus, sideslip may give rise to significant pitching moments.

The centre of gravity (c.g.) position of tailwheel aircraft is behind the mainwheels. This situation is inherently directionally unstable on the ground. One way to visualise the mechanism for this instability is to imagine an aircraft landing with right drift applied (i.e. tracking down the runway with the nose pointing to the left of the centreline). The friction on the mainwheels will have a lateral component to the left, but inertia will keep the c.g. travelling along the runway, leading to a yawing moment to the left. The left lateral frictional force on the mainwheels will also result in a right rolling moment of the c.g. about the right mainwheel due to inertia. This will increase the loading on the right mainwheel thus exacerbating the left yawing tendency. This uncommanded directional divergence is commonly referred to as a "groundloop". The tailwheel opposes such a yawing motion, with a locked tailwheel reducing groundloop tendencies considerably compared to a castoring tailwheel.

Ground Handling
The aircraft being considered have some markedly different ground handling characteristics, largely due to the longitudinal distance of the c.g from the mainwheels and the design of the tailwheel.

Probably the greatest vice of the Spitfire is that it is very "tail light" due to a short longitudinal moment arm of the c.g. from the mainwheels. Except when a tail wind prevails, the stick must be kept fully aft and any brake applications must be made very smoothly and progressively. Sharp brake inputs, or large power increases without full aft stick, inevitably cause the tail to leave the ground. Unfortunately, the Spitfire has very little clearance between the propeller tips and the ground, and very expensive ground strikes by the propeller are not unknown! A particular problem can occur during the pre take-off engine checks at high power. The thrust line is above the mainwheels and produces a powerful nose down pitching moment that is opposed by the moment of the c.g. about the mainwheels and the aerodynamic down force on the tailplane and elevator due to propwash and any headwind component. If the tail should rise, closing the throttle will reduce the problematical nose down moment due to the thrust. However, it will also reduce the propwash over the tailplane and elevators, thus reducing the aerodynamic tail down moment, and often making the tail rise even further. Unfortunately, once the tail has started to rise in this situation there is often no recovery. If the stick is held fully back in tailwind conditions, the wind may overcome the propwash resulting in reverse flow over the tail. This causes an aerodynamic upforce on the tail and thus a nose down pitching moment that could cause the tail to rise. Therefore, the elevators are held neutral when taxying in a tailwind. Because the Spitfire is so tail light, taxying in more than 25 kts of wind is performed with a person sitting on the tailplane. During WWII a pilot actually took off with a WAAF still sitting on the tail! He thought that the aircraft was handling somewhat strangely so landed immediately - still with a somewhat shaken young lady adversely affecting his c.g. position.

The Messerscmitt 109, conversely, is very tail heavy and this causes a different problem. Even when the tailwheel is unlocked, it is still held in a fore-and-aft position by a spring. To make a tight turn on the ground, the tailwheel must be made to castor by braking it out from the spring. Due to the narrow undercarriage track, the yawing moment generated by differential braking is small, and it is difficult to break out the tailwheel due to the high weight upon it. The required technique is, whilst applying differential braking, to apply full forward stick and increase power in order to generate upward lift on the tailplane and thus unload the tailwheel - anathema to a Spitfire pilot! Such a manoeuvre on wet grass is still ineffective as any brake application locks the wheel and the aircraft just slides forward in a straight line. Ground handlers have to hold the wingtip on the inside of the turn in order to generate sufficient yawing moment to break out the tailwheel, after which normal taxying is possible.

Take-off
It is worth considering why it is preferable for aircraft with difficult directional control characteristics to take-off and land on grass rather than asphalt or tarmac runways. When an aircraft is drifting during take-off or landing, the magnitude of the lateral force on the mainwheels, and thus the tendency for the aircraft to yaw, is a function of the coefficient of friction of the runway surface. Grass has a lower friction coefficient than hard runways and thus for a given amount of drift, grass surfaces produce less yawing moment. In simple terms, take-offs and landings from grass result in fewer directional control problems.

The power response to throttle inputs is essentially instantaneous with these engines. Therefore, very rapid large power increases at the start of the take-off roll will produce a large yawing moment due to propwash. Note that all of the aircraft being discussed have propellers that rotate clockwise from behind and thus will yaw to the left. The Mustang and Corsair in particular have very little rudder power at the start of the take-off roll, and even full right rudder is insufficient to stop the left yaw if the throttle is opened too rapidly. The only recovery is then to throttle back until the rudder becomes effective and corrects the swing. This problem is exacerbated by crosswinds from the left, and if a significant crosswind exists it is preferable to take-off such that it is from the right whenever possible (tailwind component and runway permitting). Note that all of these aircraft except for the Bf109 have a rudder trim tab. The setting of this tab theoretically has no effect on the rudder authority available at the very start of the take-off roll. However, if it is mis-set, as speed increases the rudder forces may become too great to achieve full rudder, effectively reducing rudder authority and potentially causing a loss of directional control.

The Spitfire needs about half right aileron at the start of the take-off roll to counteract the torque of the Merlin engine at low speed and high power. The torque effect reduces as speed increases, and aileron power increase with speed also. Therefore, the ailerons are effectively back to neutral when unstick occurs. It is easy and quite natural to make this aileron input although somewhat unusual. Sometimes the left undercarriage leg oleo is charged to a greater pressure than the right one to help reduce this phenomenon.

I will now describe the complete take-off technique for the Messerschmitt 109, the worst case aircraft of this selection for take-off and landing. Allegedly, approximately one third of the 33,000 Bf109s built were badly damaged or destroyed in take-off and landing accidents. A prudent pilot tries whenever possible only to operate from grass surfaces and with no more than a 10 kt crosswind component, certainly never with a tailwind. The aircraft is lined up pointing down the runway and the tailwheel lock, which gives the major positive contribution to directional stability during take-off and landing, is engaged. The power is slowly increased to approximately 1.15 atm (34" Hg) manifold pressure (1.3 atm being the maximum setting for the engine) and the aircraft kept straight using about one third to one half right rudder. Once running straight with the power set, the tail is raised very slowly until just clear of the ground. The elevators are effective at very low speeds with take-off power set, but if the tail is raised at an excessively high rate the gyroscopic moment from the propeller generates a very rapid left yaw. Once stabilised on just the mainwheels, the 109 is directionally unstable but luckily the rudder is very powerful. However, any yaw that occurs must be stopped very rapidly or a roll will develop in the opposite sense to the yaw and a catastrophic groundloop becomes a distinctive possibility. Also, once the yaw has been contained it is best to just maintain the heading that the aircraft is on. Any attempt to correct back to the runway bearing may cause a groundloop in the other direction unless the correction is made very slowly. Unfortunately, the field of view from the cockpit on the ground is very poor and thus it is hard to detect when the aircraft yaws off the desired heading. The ability to sense and detect any yaw and then to make the required correction very rapidly are the skills that must be mastered in order to take-off safely in this difficult aircraft.

General Handling Qualities
All of these aircraft have reversible (manual, unpowered) elevators, ailerons and rudders that, over the relatively large speed range available, give some markedly differing flying qualities. The Bf109's flight control system is slightly different from the others in that it has a variable incidence tailplane for pitch trimming and only a fixed tab for rudder trimming. It also has independent free-floating leading edge slats. All of the other aircraft have at least cockpit adjustable elevator and rudder trim tabs.

One of the most striking characteristics for a pilot who is familiar with modern fighters is the relatively poor roll performance. The P-40 has the best roll performance of this group, although qualitatively it is similar to that of a clipped wing Spitfire. A 1g 360¢ª full stick roll in a clipped wing Spitfire IX at 250 KIAS and 5000 ft takes 3 seconds. However, the same manoeuvre in the Hurricane at 200 KIAS takes 6 seconds. The other aircraft lie between these two extremes. Any aileron rolls flown at low level must be entered on a positive climbing line in order to complete the manoeuvre at a height that is not below that of entry, and all require rudder co-ordination.

The next interesting characteristic is apparent manoeuvre stability (stick force per g). The Mustang, Bf109 and P-40 are all very heavy in pitch, requiring approximately 20 lbf/g at around 250 KIAS albeit with gradients that appear to be linear. Thus, two hands are required on the stick for manoeuvres above approximately 3g. For singleton displays this is not a problem as a constant power setting is used. However, close formation manoeuvres, particularly loops and level turns, are a great strain on the forearm and it helps to be trimmed slightly nose up when running in for a loop such that you are holding a push force. Also, a coarse handful of nose up trim during a level hard turn helps to reduce the excessive force, especially when speed has reduced from that at trim. At the other end of the spectrum are the Spitfire and Hurricane, both of which are manoeuvre unstable at aft c.g. positions. The degree of instability increases as a function of g or AoA in that a slight pull is required for low g manoeuvres, changing to zero stick force and then a push as g increases. However, at a more normal mid c.g. position, a clipped wing Spitfire IX with 2650 RPM and +6 lbs boost set, at 250 KIAS and 5000 ft in a steady left turn requires a 9 lbf pull at 2g and a 22 lbf pull at 5g (26 lbf in a right turn). This shows a marked reduction in apparent manoeuvre stability as g increases. However, these overall light forces make the Spitfire delightful to fly in pitch and all required manoeuvres can be flown with one hand. Even at aft c.g. when it is manoeuvre unstable it is still possible to fly aerobatics in the Spitfire although it takes care to avoid excessive g.

The Hurricane has unusual apparent longitudinal static stability characteristics (pitch trim change with speed). Depending on the c.g. position and the type of propeller fitted it varies from slightly stable to slightly unstable. However, at higher power settings (2400 RPM, +4 lbs boost), left sideslip (which occurs as you accelerate) generates a strong nose down pitching moment and right sideslip (seen on deceleration) a moderate nose up pitching moment. Combine this with essentially neutral lateral static stability (rolling moment due to sideslip) and very low sideforce characteristics (lateral acceleration due to sideslip), and it is easy for the sideslip that occurs during speed changes to appear as longitudinal static instability.

To complete the picture, control force harmony varies considerably amongst these aircraft. The P-40 is very heavy in pitch and light in roll. The Spitfire is light in pitch and very heavy in roll. The Mustang and Bf109 have well matched aileron and elevator forces but are very heavy overall. The most pleasant aircraft to fly in terms of elevator and aileron harmonisation and overall control forces are the Corsair and, in particular, the Hellcat.

High Power Accelerated Stalling
I shall just discuss the high power accelerated stall characteristics of the Mustang, as it is indicative of what may occur in this class of aircraft. Note that it has a laminar flow wing. With a typical display power setting of 2700 RPM and 45" Hg MAP, decelerating turns at 3g in either direction eventually, at approximately 125 KIAS, result in a very rapid flick to the right accompanied by a harsh snatch of the stick, also to the right. The word departure is very apt for this manoeuvre! There is essentially no warning of approaching the stall under these conditions, although a very slight burble may just be felt through the elevators. Checking the stick forward breaks the stall quickly, although the ailerons then have to be returned to neutral and inertia continues the roll. Bank angle excursions of around 180¢ª are normal during the stall and can be more. If such a stall occurred at low level, or in combat, the outcome could be disastrous. In a display I always fly the Mustang with sufficient speed that I keep well clear of the stall. Perhaps the very high stick force per g is beneficial in helping to prevent inadvertent stalls. Basically, the Mustang has the most vicious high power accelerated stall characteristics of any of the aircraft that we are discussing.

Low Speed, High Power Handling
At the top of looping manoeuvres, a significant right rudder deflection is needed in all of the aircraft in order to counter the yawing moment due to propwash and thus to keep straight. If the manoeuvre entry speed has been correct, then speed at the top of the loop is high enough that rudder authority is sufficient and the pedal force is easy to apply. However, if a looping manoeuvre has been entered too slowly, the first indication of this error to the pilot is that an excessive right rudder deflection is needed, with a much higher than normal pedal force. It is even possible to reach full rudder and still be unable to control the yaw if the speed is very low, and right aileron may be needed simultaneously to counter the torque. Achieving such a low speed manoeuvre is possible due to the high power of the engine compensating for the lack of kinetic energy at the start of the loop and the reduction in stall speed due to the high-energy propwash over the wing roots. It is a potentially hazardous situation as, if the aircraft experiences either a positive or negative angle of attack (AoA) stall, a very rapid departure may occur due to the sideslip present and the rudder and aileron deflections; this may culminate in a spin. The Mustang and Corsair are probably the worst of these aircraft for encountering this situation. Recovery must be effected by smoothly reducing power whilst maintaining a low AoA, attempting to keep straight with rudder and gently rolling back to wings level. If this occurs during a display, the aircraft's height will be around 2500 ft agl, and great care must be taken to try to avoid getting into an irrecoverable low speed, steep nose low attitude. The moral is do not enter looping manoeuvres too slowly.

Engine Handling
This could be the subject of a complete lecture on its own. Large, powerful piston engines require a great deal of management by the pilot in order to get the best performance from them, and perhaps more significantly when flying them today, conserve engine life and prevent damage which could lead to an engine failure. Oil and coolant/cylinder head temperatures need to be maintained within limits, usually by manual operation of some type of flap or gill. It is very important on the ground not to exceed around 1000 to 1200 RPM, depending on the type of engine, until an oil temperature of 40ºC is reached. This invariably means that you cannot taxy on grass until the oil has warmed up. Conversely, some of the liquid cooled engines, especially in the Spitfire V and Bf109, will overheat and boil quickly on the ground (about 10 minutes from a cold start on a cool day, 5 minutes if the aircraft has been flown previously and a quick turnaround performed). Modern ATC does not understand the need for these aircraft to get airborne expeditiously after engine start!

It is easily possible to overboost all of these engines (exceed the maximum MAP/boost for a given RPM), causing a significant reduction in engine life and increasing the chances of a mechanical failure. On take-off, only two-thirds to three-quarters throttle will be needed to give maximum permitted MAP. Also, the large radial engines have no altitude boost compensation, and neither do some of the in-line ones. Therefore, if the MAP is set at, say, 2000 ft running in for a display, it will increase significantly when diving to display height, possibly resulting in overboosting.

Underboosting is a major concern with the large radial engines such as the PW R2800 in the Corsair and Hellcat. If the MAP is reduced to the point where the propeller is no longer producing thrust and the airflow is driving the propeller, the reverse loading on the crankshaft results in great stress and potential mechanical failure. These engines must not be throttled back below "square power" i.e. 24" MAP at 2400 RPM, 20" at 2000 RPM etc. This makes it difficult to drop back in formation and to decelerate during the circuit to land, particularly in the Hellcat, which does not have much drag from the flaps and little drag in sideslip. Formation flying requires a careful choice of RPM to allow a good operating range of MAP without over- or underboosting the engine.

Fortunately, with these engines no pilot selection of carburettor or other engine anti-icing or de-icing controls is required within today's operating envelope. Mixture control is also simple, with typically just AUTO LEAN, AUTO RICH and EMERGENCY RICH positions.

One big issue with handling these engines is the rate of throttle movement, especially when increasing power. A large and rapid forward throttle movement gives an almost instantaneous increase of power and, at low speeds, this can lead to yaw and/or roll which may not be controllable. Late go-arounds must be handled with great care and with careful throttle handling.

Thermal shock is another potential source of damaging an engine and reducing its life, especially with the radial engines. If a sudden power reduction is made after operating at a high power setting and high cylinder head and oil temperatures, this will result in differential cooling rates for different engine components and thus inconsistent contraction of adjacent components. To prevent this, we aim not to go directly from display power to a very low power setting for a circuit to land. We try to have a few minutes at an intermediate cruise power setting between these two extremes. Also, it is important to let the engine temperatures stabilise by setting a low RPM for a minute before shutdown.

Landing
The relative merits of "3-point" and "wheeled" landings in a given aircraft type are determined by several different flying qualities. To show this, we will examine some of the aircraft that are easiest to land in a 3-point attitude, and some that are best wheeled onto the runway.

The Merlin-engined Spitfire has a threshold speed of 70 KIAS, with a landing configuration idle power stalling speed of 50 KIAS, and controls that are all effective down to almost taxy speed. If a wheeled landing is attempted, the slightest bump on the runway causes the aircraft to become airborne again as the wing is still generating significant lift. Likewise, any attempt to lower the tail prematurely results in a further lift-off. Therefore, a 3-point landing is easier than a "wheeler". But after touchdown, even from a good "3-pointer", full back stick and only very gentle braking must be employed to prevent the tail from lifting and the propeller striking the ground.

The Bf109 has a threshold speed of 175 kph (95 KIAS). The float after flaring is quite prolonged prior to touchdown in a 3-point attitude. However, using a lower threshold speed to shorten the float sometimes results in no reduction in the rate of descent when flaring and a heavy landing. Also, aileron and rudder effectiveness appear to be lost at about touchdown speed. Unfortunately, at idle power the aircraft is markedly directionally unstable on the mainwheels alone and this, combined with an almost ineffective rudder, can easily lead to severe directional control problems if a wheeled landing is attempted. However, by touching down in a 3-point attitude the lockable tailwheel gives a significant contribution to directional stability and a much straighter rollout occurs. If ever I touch down on the mainwheels in the Bf109 I gently ease back on the stick to get airborne and continue the flare for a subsequent 3-point touchdown. The fairly weak brakes on the narrow track main landing gear make directional control after touchdown poor, and the excessive use of the brakes will lead to them becoming less effective as the rollout progresses. The Bf109 is not an easy aeroplane to land, but an understanding of its characteristics does enable it to be flown safely.

Both the P-40 and Hurricane can, on a bumpy grass runway, develop a longitudinal porpoising motion if the stick is not held fully back after touchdown. A "3-pointer" is easier for achieving full back stick quickly although a wheeled landing followed by a gentle lowering of the tail is feasible on smooth runways.

For heavier aircraft such as the Mustang and Corsair, most of the above mentioned difficulties such as bouncing airborne do not occur. However, they both have very little aileron power after a 3-point touchdown and any lateral disturbance can be difficult to control. The higher touchdown speed of a wheeled landing gives greater aileron control power to counter such upsets, and there is sufficient elevator power to maintain the touchdown attitude until well below touchdown speed. Maintaining this flatter attitude also helps maintain the weathercock stability and rudder power as the fin and rudder are not blanked by the fuselage. Note that, particularly in the Corsair, some swing often occurs when the tail is lowered, possibly due to the fin blanking or to gyroscopic moments from the propeller.

Crosswind landing limits for these aircraft are low by modern standards, 10 kts for the Bf109 and 15 kts for the others being sensible. Remember that they were designed in the days of circular grass airfields (or for aircraft carriers) so crosswinds were less important than they are with single runway airfields. It is interesting that the RAF Pilots' Notes for most WWII aircraft advocated a crabbed approach with the drift removed by using rudder just before touchdown. The problem with this technique is that the aircraft may start to drift away from the runway track during the flare, and touching down with drift present will excite the directional instability discussed earlier. The preferred technique of most pilots who fly these aircraft nowadays is to approach wing low, pointing straight down the runway. If the crosswind is not too strong the wings may be levelled using aileron just before touchdown. However, it is often better to touch down with the bank still applied on the into-wind mainwheel (and simultaneously on the tailwheel if a "3-pointer" is essential). Directional control power is the limiting factor of the Bf109, but for many of the aircraft the crosswind limit is determined by available roll control to prevent the into-wind wing lifting due to lateral stability. For this reason, the Mustang needs to be wheeled on in a flat attitude in a strong crosswind to give increased aileron power at the higher touchdown speed. In the Corsair it is advisable to land using only a half flap setting and then to raise the flaps before lowering the tail.

Conclusions
This lecture has given only a very brief overview of some of the more significant flying qualities of a few types of WWII fighter. My intention was to try to show that many of the flying skills needed to fly these aircraft safely are not required in modern aircraft and so have not been learnt by today's pilots. However, flying ability has not changed over the years so modern pilots have the same potential as earlier generations to learn these skills. I hope that I have not made these aircraft sound daunting to fly; they are not. But they are different and require different skills. So saying, they obey the same equations of motion as any other aircraft with reversible (unpowered) flying controls.

As well as handling qualities, there will always be perhaps an even greater interest in the relative effectiveness of these aircraft as fighting machines, a quality that is determined largely by performance. The handling qualities had only to be good enough to realise this performance. However, in the interests of conserving both engine and airframe life today we do not explore the limits of a vintage aircraft's performance. There is a significant amount of documentary evidence on performance from wartime trials and there are books available containing such data to satisfy modern curiosity and thus negate the need for such flying.

As a final thought, please note that it is easy to concentrate on the deficiencies of an aircraft and gloss over its other possibly excellent characteristics. The fact that the Spitfire has poor control force harmonisation and is easy to tip on its nose does not detract from its outstanding turn performance, superb low IAS and high Mach number controllability, and benign stall characteristics. It is a very charismatic and pleasant aircraft to fly overall. And the challenge of trying to master the Bf109 on take-off and landing makes it a very satisfying machine to fly once you have gained some confidence in it.
In my opinion, the nicest pure flying machine of all of those discussed in this lecture is the one that, perhaps, I have mentioned the least - the Hellcat. But then military test pilots never talk much about the good characteristics of an aircraft.