Ever heard of Bruce Artwick's Flight Simulator? Geoff Cramond's Aviator? Chuck Yeager's Advanced Flight Trainer? These are some of the very first desktop flight simulators, and believe it or not, flight simulation is a quarter of a century old. Flight Sims certainly have come a long way sine then. The graphics in modern flight sims are simply stunning - you sit in extremely realistic cockpits flying over extremely realistic landscapes. But graphics isn't all what makes a flight sim, the underlying flight dynamics is what makes a virtual aircraft behave like its real counterpart - or not. After 25 years of flight simulation, we think it's time to have a closer look under the hood of two very popular ones: Microsoft’s Flight Simulator X and Laminar Research’s X-Plane 8.50.

While FSX and X-Plane are both flight sims, they both work completely different when it comes to flight dynamics. FSX uses the so called parametric approach. The flight model is supplied by a set of parameters, which are completely independent of the visual model. For example, you could have the visual model of a Cessna 172 and tell it to fly like a Boeing 767. There are many parameters that FSX needs - basic ones like weight, number of engines, wing area, and more complex ones that determine inertia, forces, and torque. X-Plane in contrast uses the geometric approach; it determines the flight dynamics from the visual model directly. Build a C172 correctly and it's going to fly like a C172. This makes creating new aircraft very intuitive, because you only need to get the geometry right, but don't have to bother about empiric parameters.

It is a myth that X-Plane's flight dynamics is better then FSX' based on the fact that X-Plane uses a geometric model. Flight dynamics can only be as good as the quality of the underlying model.

The common consensus is that because of the geometric model X-Plane's flight dynamics is better then FSX'. However, this is a myth. Very small geometry variances, such as dents in propellers, or seams where the wings join the fuselage have an impact on flying behaviour. To model flight dynamics accurately, X-Plane would need very detailed models of 100,000+ polygons. Even modern desktop computers can't calculate physics for such a detailed model in real-time - maybe in ten years time, but certainly not now. In contrast, a very good parametric model could model these fine geometric nuances - if there are relevant parameters to be tweaked. This really emphasises one important point: flight dynamics can only be as good as the quality of the underlying model. If we have a very sophisticated model, the flight dynamics will be good; if the model is poor, the flight dynamics will be shoddy. This is true whether the model is parametric or geometric.

The quality of a flight dynamics model is probably one of the most controversial topics and passionately discussed in the flight sim community. People argue about general feel, speeds, and roll rates - even if they have never sat in a real plane! However, there is little point arguing digits behind the decimal point for roll rates. Typically, an aircraft model goes through changes over time. There are about 20 different models for the C172 alone, including structure changes and engine upgrades. Even if you have the same model, different age and wear and tear will affect performance. On top of that, there is no perfect way to recreate controller input at home, so the feel on the desktop PC will never match the real thing. In our review, we are going to take a very simple approach. All we want to know is if we change certain controller inputs, does the aircraft do the right thing. The results are very interesting and we have to admit very surprising.

There is little point arguing digits behind the decimal point for roll rates. Typically, an aircraft model goes through changes over time. There are about 20 different models for the C172 alone.

In this review we're going to have a closer look at the default Cessna 172 both in FSX and X-Plane. To make things a bit more interesting, we'll also explore Rob Young's flight dynamics from the RealAir C172 in FS9. Rob has a reputation of being a flight dynamics wizard, and we want to explore if Rob could push the envelope to create more realism. We chose the C172 as it is a simple aircraft and available in FSX and X-Plane. Sim aircraft vary in quality, and as Randy Witt from Laminar Research admits, the C172 is not necessarily X-Plane’s strongest competitor. Additionally, the RealAir C172 is several years old and the commercial RealAir offerings are of much higher quality. We used the FS9 version, because it's not compatible with FSX as yet and will spin and side slip in FS9 but not in FSX.

As every pilot knows, an aircraft has several control inputs. We’ll be exploring the elevator, the ailerons, the rudder, the throttle, and the flaps. We designed a simple set of tests that you can easily recreate at home. These tests cover what a student pilot learns and don’t delve into the more complex issues. When flying the C172 in FSX and X-Plane it is amazing how differently the plane feels in each sim. Probably, this is partly due to controller settings which are hard to match between both sims, but there are also subtle differences in flying behavior and one can easily tell both sims use different algorithms to calculate flight dynamics. One point that is worth mentioning here is the fluidity of X-Plane’s instruments. In FSX the instruments have a low refresh rate and at times seem jerky and not very responsive. X-Plane’s instruments in contrast feel very life like and are a joy to use. Microsoft has some work to do in that department.

Let’s get into our flight dynamics test…

Elevators:

The elevators control the pitch of on airplane. If we move the elevators, the aircraft should pitch up or down. A change in pitch also results in a change of altitude and speed.

the elevator changes pitch

Both FSX and X-Plane respond very similar to elevator changes, with the resulting change in altitude and speed. Same for the RealAir C172. This is really very basic and we would have expected both sims to pass – and they do.

FSX X-Plane RealAir

Aileron:

The ailerons control the roll of the plane. The plane should start to roll around its longitudinal axis and also start turning. With the aileron deflected, we should experience a yaw into the opposite direction of turn – the so called adverse yaw. The C172 doesn’t feature much adverse yaw, but it should be visible at low speeds. Once a turn is established, we should experience yaw into the direction of turn. If we continue the turn, the nose should drop and we should start a spiral descent.

the ailerons cause roll and adverse yaw, then yaw

When we deflect the ailerons in FSX, the plane rolls, and the balance ball indicates yaw into the turn direction. The plane starts the spiral descent and recovers when wings are levelled. X-Plane behaves similar, but with some differences. X-Plane seems a bit more unstable when the bank angle is less the 30 degrees. While it starts to roll, the airspeed increases, resulting in a pitch up movement, which then develops a roll into the opposite direction. It’s possible to induce a prolonged oscillation between 10 and 30 degrees of roll, with the pitch and airspeed oscillating accordingly, but the yaw staying constant. In this situation a spiral dive never develops fully. Unfortunately, this behaviour is completely unrealistic. The C172 is a very stable plane which will either gently settle into a spiral dive or gently roll back to level, depending on aileron input. The RealAir C172 behaves very similar to the default FSX C172. However, none of the sims show adverse yaw at low speed, which is disappointing.

FSX X-Plane RealAir

Rudder:

The rudder controls the yaw of the plane. When applied the aircraft should yaw into the direction the rudder is applied. The plane will become imbalanced which is indicated by the balance ball. Applying the rudder also causes the aircraft to roll. The plane first yaws, then rolls, followed by a pause and then more roll. The aircraft again ends up in a spiral descent.

the rudder causes yaw, then roll

When applying the rudder in FSX, the plane yaws and becomes imbalanced. We then observe the roll, pause, more roll and spiral descent. However, the pause is more of a violent jerk, as if the aircraft has been hit by a giant sledge hammer, and the plane swings into the opposite direction for a brief moment! X-Plane behaves exactly the same, for all the differences between FSX and X-Plane it’s amazing how similar this behaviour is. The C172 is a very stable plane and such a sudden jerk nor the yaw into opposite direction is even remotely realistic. The RealAir C172 behaves differently than both FSX and X-Plane and in fact very realistically. The pause is very gentle, as one would expect, it feels like the airplane is smoothly continuing its flight without the major disruption in the default FSX.

FSX X-Plane RealAir

Throttle:

Changing throttle will change the pitch of the plane and as a consequence altitude. However, the slipstream of the propeller also causes yaw. In a C172 we should observe a yaw to the left when adding power at low speed.

The change in pitch and attitude are both observable in FSX and X-Plane, however, more interesting is the sims feature the yaw behaviour. In FSX at a low airspeed we can indeed observe the yaw effect when adding power. On the other hand, X-Plane doesn’t show any yaw tendencies what so ever, the aircraft simply flies straight ahead. The RealAir C172 shows exactly the same behaviour as FSX.

FSX X-Plane RealAir

Flaps:

When lowering the flaps, the plane will pitch up - this effect is called ballooning, and conversely a plane will sink when the flaps are raised. To maintain speed after the flaps are lowered, a lower pitch attitude is required.

change of flaps causes ballooning and sinking

FSX simulates that behaviour very well, lowering the flaps pitches the plane up and the speed decreases. X-Plane works very similar to FSX, the differences are pretty negligible. Again, the RealAir C172 matches the FSX one.

FSX X-Plane RealAir

Stall:

When approaching a stall, the airspeed should decrease and the airframe should start to shudder slightly for a brief moment. This is followed by a high rate of descent and the plane tipping forwards.

FSX does the basic stall behaviour beautifully, the rate of decent gets very high as the plane tips forward. In X-Plane the stall feels a little different and is a slightly more difficult to trigger, but all in all it also works very well. What we’re missing though in both sims is the airframe shudder that precedes the stall. FS add-on airplanes quite nicely simulate vibrating airframes these days, so we would really like to see that. Having said that, the RealAir C172 doesn’t feature the shudder either.

FSX X-Plane RealAir

Spin:

In a spin, one wing of the aircraft is stalled and therefore the aircraft establishes roll and yaw. It looses height rapidly, but the airspeed is low. As the plane spirals downwards, rate of rotation can be very fast. At higher nose attitude, the rate of rotation and loss of altitude will be less. However it is difficult to maintain a spin in a C172.

This is where FSX and X-Plane differ a lot. In FSX we can get into a semi-spin with the aircraft yawing and rolling while sinking fast, but keeping a low airspeed. However, it’s impossible to achieve a real spin, the plane prefers the spiral dive. Since it is indeed hard to spin a C172, we accept this behaviour. X-Plane works like the opposite. The C172 gets to a high rotation speed very fast and also sinks very fast. The speed increases rapidly and the plane shoots towards the earth – not what one would observe in a real spin. In contrast, the RealAir C172 is a true master at spins. Seeing a very nicely developed spin with low airspeed is certainly an experience, and the RealAir does a fantastic job. However, it is probably a bit unrealistic to get a well sustained spin in a real C172, but we’ll let that one slip true.

FSX X-Plane RealAir

Sideslipping:

Deliberate sideslipping is used in airplanes to achieve a steeper glide path. Nowadays, this is the purpose of flaps, but nevertheless, sideslipping should still work in a flight sim. Sideslipping is achieved by crossing the controls, i.e. ailerons and rudders are applied in different directions. The effect of sideslipping is a high sink rate. While this maneuver is commonly used in older aircraft that don’t have flaps, you need to perform it even in modern GA aircraft when your flaps have failed you.

FSX X-Plane RealAir

FSX does a really poor job of sideslipping. Crossing the controls is virtually impossible. If you bank the plane slightly, it wants to turn, even if you apply full counter rudder. Achieving a high sinkrate is impossible. X-Plane does a slightly better job. It is actually possible to achieve a higher sinkrate with applied rudder, however the counter bank angle is very small. The whole behaviour is a bit odd, as you simply only need to bank slightly, and then while holding the bank angle constant, rudder pressure controls sinkrate. In a real C172 you need to compensate the rudder with increasing bank angle. Again the RealAir C172 is a true master at this. It’s simply pure joy to cross the rudder and ailerons and watch the sinkrate increase, all the while the C172 continues a straight patch. This maneuver can be done in the RealAir very smoothly.

That concludes our exploration into the world of flight dynamics. The results have certainly been surprising. The FSX flight model is certainly anything from perfect, but X-Plane doesn’t exactly wallow in glory either. Since flight sims are a quarter of a century old, I have to say I would expect better than what we have seen here. Processors are certainly up to the task to handle more complex physics. The last couple of years has seen a new advent of physics modeling in the video game industry, somehow it seems flight sims have avoided this trend and this is not good news for simmers. As far as I remember, the flight dynamics of the default C172 doesn’t feel much different than what FS2000 had to offer and that was seven years ago.

This test has revealed some crucial weaknesses in both sims, especially in the rudder department. I sincerely hope that both Microsoft and Laminar Research do some neglected homework and pay more attention to flight dynamics in their sequels.

The RealAir C172 shows what is achievable in Microsoft’s engine and it seems that the Aces team is left behind in modern add-on aircraft. One often heard argument is that more complex flight dynamics are too complicated for the average gamer. Well, I differ in opinion there. When it comes to rudder control, the default C172 handles almost like a dead brick compared to the RealAir C172. If I would be a complete newbie, I would prefer the RealAir any time, it’s a pure joy to fly.

In light of our basic flight tests with the C172, the myth that X-Plane has superior flight dynamics than FSX appears to have been mercilessly shattered. Yes, it handles differently, yet in our tests it exposes the same shortcomings that plague FSX. It even seems that FSX has a slight edge over X-Plane. To be fair, X-Plane does offer some things that FSX doesn’t offer, such as undulated runways (compared to the forced flat planes in FSX), actual wheel physics on the ground (not modeled at all in FSX), and a true 3D atmosphere which allows for up and down moving air (FSX only allows for rising or sinking air columns, but doesn’t offer a continuous 3D atmosphere). Of course, this is not a comprehensive test and only focuses on the C172 which may not be representative for the overall quality of either sim. Nevertheless, this test has revealed some crucial weaknesses in both sims, especially in the rudder department. I sincerely hope that both Microsoft and Laminar Research do some neglected homework and pay more attention to flight dynamics in their sequels. After 25 years, simmers certainly deserve better.

Christian Stock

About the author:

Christian Stock has been a keen flight simmer since FS2000. He is one of the leading scenery designers in the MSFS scene and has published several scenery creation manuals, ranging from scenery coding over terrain scenery to weather theme creation. He has also written occasional opinion piece and several flying tours. He and his young family currenty reside in Melbourne.