I’m really talking about in the air. On the ground there usually seems to be some of the expected left turning tendency, but after takeoff there seems to be more right turning tendency.
This is sometimes worse than at other times, which lead me to wonder if it was related to wind conditions - maybe an exaggerated weathervaning effect.
No, I talking about take off. Once in the air, high power high angle of attack gives me left turning tendency. But on the take of elk it doesn’t.
That’s a common misconception. (Steady) wind has absolutely no effect on an aircraft in flight.
It’s only when ground (navigation) is being added that wind becomes a factor, since it affects ground speed and the ground track, but concerning aerodynamics/performance, wind doesn’t exist.
Well, as I said I wondered if it were something to do with the wind in the sim, and wasn’t suggesting that it was necessarily realistic. Whatever the cause I do experience a tendency for the C172 to roll right once off the ground, sometimes more noticable than other times.
That should be easy to test. Turn around 180deg an the check if the rolling tendency reverses.
I’ve done that. The rolling seems to lessen somewhat, but it doesn’t seem symmetrical. Between that and the fact that it is an intermittent thing, it made me think of wind as a possibility.
But to be honest, it isn’t a huge deal for me at this point. As I mentioned above it seems to happen less since the USA update where they made adjustments to the aerodynamic models of various planes.
So many flight instructors in this conversation.
The Cessna 172 still has no slipstream effect, unless you consider that one tiny hair to the right on the slip indicator slipstream. You can see after take-off there is no rudder required at all to fly coordinated. Also when cycling power between zero and full close to stall speed there is no yaw at all:
In real life you would see the nose moving all over the place when not correcting power increase / decrease with rudder. Here the rudder is not touched at any point. I did need to apply constant right rudder during take-off roll, so the right rudder, left rudder, back to right rudder issue during take-off roll seems to be fixed (no wind).
I listened to the Developer Q&A today and heard the developers talking about propeller drag. The developers were talking about drag of sorts (friction from engine components, pistons, and the like). That was really great to hear those effects are modeled, but that’s not the propeller drag people are talking about.
With a variable pitch propeller: When the prop lever is moved forward, the blades flatten… increasing their RPMs and also increasing the frontal area of the blades to the oncoming airflow from the forward movement of the aircraft. This increased frontal area increases total aerodynamic drag on the aircraft, and when the engine power setting is low, you can feel a noticeable aircraft deceleration, even though prop RPM has increased.
The reverse is also true… when the prop level is pulled full back, the blade angle changes, reducing the frontal area of the blades that are being hit by the air from the forward motion of the aircraft. Even though RPMs decrease, at low power settings, you can feel a slight acceleration as the overall aircraft drag is reduced and the aircraft accelerates slightly.
This is the propeller drag that does not seem to be modeled in the simulator. This effect is fairly slight for small, single-engine GA aircraft as the propeller blade angle range is only a couple degrees. It is more evident in small GA twins. If a small GA twin has an engine failure on one said and the prop is just windmilling in the air, that’s like a massive airbrake that will adversely affect controllability and the ability for sustained flight. Prop feathering is required to minimize as much propeller drag as possible.
For high-speed turboprop aircraft, there is a fairly significant blade angle range available due to the greater airspeed range. This blade angle range has the ability to provide significant braking action in flight if the blade angle is controllable by the aircrew.
Most of my time has been in large multi-engine turboprops with FADEC, so I could not control the blade angle in-flight. When the computer decided to make significant blade angle changes automatically, you really felt it in the aircraft as the drag either substantially increased or decreased based on the computer logic deciding what was needed for that phase of flight.
Exactly what I was thinking, although the drag of the engine has an influence on the windmilling speed of the propeller which in turns has somewhat an effect on drag, I was thinking he was mentioning that maybe?
I will see if I can write something more comprehensive today, but in simple terms a propeller is an aerofoil, and when engine power gets too low the RPM drops, the propeller AOA becomes negative and the propeller starts driving the engine instead of vice versa. This negative AOA produces negative thrust (its almost a kind of reverse thrust if you will but without a negative blade angle).
The drag from internal engine components has an influence on propeller windmilling speed and therefore the drag produced by the propeller but I doubt this has a large effect (on the PT-6 it has NO effect since it is a free turbine).
Yes, Seb was talking about the drag affecting the engine (windmilling) and the amount of windmilling considering the engine drag. Not the aerodynamic drag on the plane caused by the propeller blades.
Feathering the propeller should reduce both the windmilling effect and the aerodynamic drag caused by the propeller and I believe this is not yet simulated.
Not entirely, windmilling effect and aerodynamic drag from the propeller are not two separate things. They are not two separate kind of drags, a windmilling propeller is creating negative thrust (drag), and therefore a windmilling propeller basically acts as a thrust reverse.
This “discing” or creates frontal surface with propeller blades in fine pitch is an oversimplification, it doesn’t really work like that. Therefore it is not true a propeller is creating drag by increasing frontal area AND by causing aerodynamic drag due to windmilling.
Agreed. I’m not an expert on this subject at all but in the basics I think we sort of say and/or mean the same thing.
Found this video that explains the subject quite well I think.
Concerning the last Developer Q&A, I’m not entirely sure the developers understand the problem of “missing propeller drag” in the sim. Seb was talking about drag from internal engine components instead of aerodynamic propeller drag. In all fairness, the drag of internal engine components had an effect on propeller windmilling speed and therefore aerodynamic drag produced by the propeller. I’m not a developer to be clear but I doubt this is the real problem. Also it would not explain the lack of drag on turboprops in MSFS as the PT-6 for example is a free turbine engine and therefore not influenced by drag of the engine itself, sure it has some drag from the reduction gearbox and power turbine, but I don’t think this is the real problem.
Edit: after doing some testing it seems like there is propeller drag modelled on certain aircraft. The single engine piston aircraft I have tested seem fairly accurate, the Baron isn’t accurate at all, the landing gear on the Baron does not produce any noticeable drag & able to fly way below Vmca without departure from controlled flight. All three turbo-props have no propeller drag modelled at all. None of the MSFS aircraft have the ability to feather the propellers although on the Kingair and TBM the RPM does drop significantly when selecting feather (but with no change in aerodynamic drag). In short: multi-engine piston aircraft and all three turboprops are significantly flawed.
First it is important to understand the following definitions when talking about propellers:
A propeller is an aero foil and essentially behaves like a aircraft wing orientated in the vertical plane. The working principle of a propeller is the same as for a wing, the only difference is that the resultant force is acting mostly vertical on a wing and can be divided into a vertical component (lift) and a horizontal component opposing the direction of movement (Induced Drag), on a propeller this resultant is acting mostly horizontal and can be divided into a horizontal component (Thrust) and a vertical component (Drag).
Very important, this is not the drag we are talking about in regard to the missing aerodynamic drag from (windmilling) propellers!
The engine power has to overcome the drag produced by the propeller, if the two are in balance the propeller RPM is constant, if the engine is producing more power than the propeller is absorbing, the RPM increases, increasing the thrust (and drag) until the two forces are in balance again and vice versa when reducing power.
Regarding constant speed propellers, this becomes a little more complex as there is an extra variable introduced. The propeller blade angle is variable, a constant speed propeller is able to MAINTAIN a certain RPM with an increase or decrease in engine power by modifying the blade angle, therefore changing the power absorbed by the propeller (higher blade angle is more drag and vice versa). For example if engine power is increased, the propeller tends to speed up but the blade angle is then increased producing more thrust (and drag), the increase of power is absorbed by the propeller with no increase in RPM! There is a limit to this, when power is reduced far enough the propeller will eventually reach the minimum blade angle, if power is reduced further the RPM will drop, if power is reduced further then at some point the propeller will start to windmill and drive the engine instead.
The angle of attack on a propeller is based on two components, the tangential velocity of the propeller and the True Airspeed. The tangential velocity of the propeller is depending on the propeller RPM, higher RPM is higher velocity and vice versa. We can’t call it “RPM” because the RPM is the same for the entire propeller, whereas the tangential velocity is not, it is depending on the location of the propeller, the tip of the propeller having the highest tangential velocity and the root having the lowest (zero velocity). To keep it simple we will call this component RPM, but technically this isn’t correct. I hope the picture below also illustrates clearly how we are looking at the propeller cross section in further examples.
In the first example the aircraft is stationary (TAS = 0), the propeller has a high angle of attack (AOA = blade angle) creating high drag on the engine. If we now release the brakes and start rolling the TAS increases which reduces the angle of attack, the reduced angle of attack causes a reduction in drag. The engine is now producing more power than the propeller is absorbing and therefore the RPM increases. This increases the angle of attack again to bring it back in balance, this is the constant force balance in play during flight. It is also the reason why on a fixed pitch propeller (e.g. Cessna 172) the RPM increases with increasing airspeed for a given engine power setting and vice versa.
Now the core of the issue. What happens if the we reduce power all the way to idle in flight? If the engine is not producing any power why doesn’t the propeller (and therefore engine) stop rotating? As soon as the engine fails or power is reduced to idle, the RPM drops significantly as the propeller absorbs more power than the engine is producing, eventually the angle of attack on the propeller becomes negative and the forces acting on the propeller will basically flip. Positive thrust becomes negative thrust (THE DRAG WE ARE MISSING IN MSFS) and drag becomes the driving force, keeping the propeller and engine rotating. This drag is very significant in real life and cuts the glide ratio short by a significant amount, the deceleration in real life is therefore also significantly higher than in MSFS and it is possible to make steeper approaches without picking up speed.
Note that a windmilling propeller basically acts like a thrust reverser, but at a positive blade angle.
Without diving deep into the working principle of a Constant Speed Propeller, Propeller Governors etc. There is a noticeable difference between single engine and multi-engine aircraft. The principles are all the same independent if we are talking about single-engine, multi-engine, fixed pitch or constant speed propellers. Also on a constant speed propeller the engine will windmill, the reason is simple, a constant speed propeller (it is in the name) wants to maintain a constant propeller RPM. If an engine fails or power is reduced to idle the constant speed propeller will drive the propeller to the fine pitch stop and from there it will essentially behave like a fixed pitch propeller.
Belief it or not, windmilling on a single engine aircraft is actually beneficial. From an aerodynamic and glide range standpoint, windmilling obviously isn’t a great benefit. But only having one engine, the main priority should be to restart the engine, windmilling of the propeller keeps the engine turning and increases the possibility of restarting the engine in flight. This is the principle difference between single and multi-engine aircraft. On a multi-engine aircraft an engine failure will not only cause asymmetric thrust, on top of this the windmilling propeller will cause a load of drag. On a multi-engine aircraft the priority is therefore to reduce the drag, secure the engine and continue flight on the remaining engine.
Reducing the drag of the propeller and stopping the engine is done by “feathering” the propeller. The blade angle is increased to a near 90 degrees which results in zero resulting force on the propeller blade, the propeller stops rotating and drag from a windmilling propeller is canceled. Feathering can be accomplished manually or automatically by the aircraft (i.e. auto-feather). Nice detail, the propeller blade angle when feathered is not exactly 90 degrees (few degrees lower in fact), if the propeller blade angle would be increased to 90 degrees the propeller blade would start to act like a wing and will produce a slight lift force (and therefore drag), the angle of attack is therefore kept slightly negative.
This is exactly the next problem in MSFS, last time I checked propellers are not able to feather, neither does the propeller produce drag as we know so the propeller performance in MSFS is basically a feathered prop anyway. When propeller drag becomes implemented feathering would become very important. At this moment, accurate simulation of asymmetric thrust and drag on multi-engine aircraft is unavailable.
In all cases below, flight model set to MODERN, assistance set to HARD, community folder completely emptied, ISA conditions, no wind.
Maximum weight, 120 kts glide speed, flown with FLC on autopilot, prop first feathered and then windmilling. I cut the engine (by closing fuel selector) overhead an airport at 3000 ft, ISA, no wind, glided down to 0 feet, paused the sim, took the drone camera to check my location, looked it up on Bing maps and measured the distance.
Prop. Feathered:
Propeller speed = 260 RPM
Glide range: 17.5 km = 57400 ft
Glide ratio = 57400 / 3000 = 1:19,13
Prop. windmilling:
Propeller speed = 960 RPM
Glide range: 17.1 km = 56088 ft
Glide ratio = 56088 / 3000 = 1:18,70
My methods are a little bit mickey mouse and completely accurate, the autopilot started to level off slightly when doing the glide ratio test with feathered propeller because I put 0 ft in the altitude preselect so thats were the extra 400 m likely comes from. It is safe to assume that there is absolutely no drag modeled on the TBM…
Next test, FLC at 120kt in descend, measuring vertical speed:
Flight idle: -700 ft/min
Windmilling: -650 ft/min
Feathered: -700 ft/min
Full reverse: -900 ft/min
Conclusion, no windmilling drag at all. In all situations the vertical rate is fluctuating between 650 and 700 ft/min.
Speed 110 kts at FLC, max. weight:
Power idle: -1100 ft/min
Engine off: -1300 ft/min (mixture cut-off)
Low RPM: -1100 ft/min (mixture cut-off and prop full back)
Below, same test as TBM, power cut at 3000 ft overhead an aerodrome. Mixture cut-off, prop in low RPM, 110 kts at full weight according to the POH it should result in a glide ratio of 1.7 nm per 1000 ft.
Glide range: 4.9 nm
Glide ratio: 4.9 / 3 = 1,64 nm per 1000 ft
Conclusion, the G36 is pretty accurate! Maybe the problem only concerns turboprops?
Speed 110 kts at FLC, max. weight:
Power idle: -1000 ft/min
Engines off: -1450 ft/min (mixtures cut-off)
Props feathered: -1450 ft/min
Conclusion, not sure if the increase in rate if descent is because of losing idle thrust or if it is because of propeller drag. The props don’t feather, absolutely no response to propeller lever (RPM stays the same when feathering). Interestingly I accidentally left the gear down during this test so I rerun the test with gear-up with same results! The landing gear on the Baron does not seem to create a noticeable increase in drag.
Beechcraft Baron
One engine full power, the critical engine windmilling
Take-off flaps, gear-up
ISA conditions
Max. TOW, aft CG
Seems the aircraft stalls before losing control due to asymmetric thrust and drag. I haven’t flown the Baron myself but I have heard stories about how it is handling single engine go-arounds and I even know someone who died practising single-engine go-around on a Baron. Can anybody with experience on the Baron confirm whether this behaviour is correct? Is it still climbing this well on one engine with flaps at take-off at speeds way below Vyse?
By the way the rudder axis on my joystick is seriously borked so I’m lacking precise control over the rudder…
Edit: I noticed I shutdown the wrong engine, I had the right engine in my head as being critical, forgot that the aircraft I used to fly has counter rotating props, oops…
Second attempt: now with the critical engine failed, 50% payload, gear down for some extra drag, first one flaps take-off, second one flaps landing:
The Beechcraft Kingair is an absolute joke. I have flown this aircraft in real life, I can see it is seriously flawed.
Max. take-off weight, speed 140 kts on autopilot using FLC:
Power idle: -300 ft/min
Full reverse: -1150 ft/min
Engines shutdown, props full forward: -850 ft/min
Props feathered: -850 ft/min
I didn’t note the RPMs, the RPM does drop significantly when selecting feather, but with no effect on the aircraft drag. The difference between flight idle and engines shutdown is likely not an increase in drag but rather a way too high flight idle power setting…
When shutting down the critical engine (windmilling) and adding full power on the life engine, the aircraft stalls way before running out of rudder to counter the asymmetric thrust and drag. Second try with flaps in landing position to lower the stall speed, able to fly just 77 kts with the joke full back, rudder full in just keeping the slip indicator centered, again no departure from controlled flight below Vmca.
Max. take-off weight, 110 kts on FLC (no idea what best glide speed is, I just picked a speed).
Power idle: -650 ft/min
Full reverse: -1050 ft/min
Engine shutdown, prop full forward: -900 ft/min
Prop feathered: -900 ft/min
On the Cessna Caravan the prop RPM does not respond when selecting feather. RPM remains the same. Conclusion, the three turbo props seem to suffer the most from incorrect (completely missing) propeller drag…
That would have saved me some work, just seeing your post now… 
Wow, you really put in a lot of work there!
Excellent crash course and it leaves no more room for interpretation. Hope the developers pick this up!
Test of glide ratio on the TBM 930:
Maximum weight, 120 kts glide speed, flown with FLC on autopilot, prop feathered and windmilling. I cut the engine (by closing fuel selector) overhead an airport at 3000 ft, ISA, no wind, glided down to 0 feet, paused the sim, took the drone camera to check my location, looked it up on Bing maps and measured the distance.
Prop. Feathered:
Propeller speed = 260 RPM
Glide range: 17.5 km = 57400 ft
Glide ratio = 57400 / 3000 = 1:19,13
Prop. windmilling:
Propeller speed = 960 RPM
Glide range: 17.1 km = 56088 ft
Glide ratio = 56088 / 3000 = 1:18,70
My methods are a little bit mickey mouse and the autopilot leveled off slightly when doing the glide ratio test with feathered propeller because I put 0 ft in the altitude preselect so thats were the extra 400 m likely comes from. It is safe to assume that there is absolutely no drag modeled on the TBM at least…
Next test, FLC at 120kt in descend, measuring vertical speed:
Flight idle: -700 ft/min
Windmilling: -650 ft/min
Feathered: -700 ft/min
Full reverse: -900 ft/min
Conclusion, no windmilling drag at all. In all situations its fluctuating between 650 and 700 ft/min.
Beechcraft G36 Bonanza:
Speed 110 kts at FLC, max. weight:
Power idle: -1100 ft/min
Engine off: -1300 ft/min
Low RPM: -1100 ft/min
At least some effect. Same test as TBM, power cut at 3000 ft overhead an aerodrome. Mixture cut-off, prop in low RPM, 110 kts at full weight according to the POH it should result in a glide ratio of 1.7 nm per 1000 ft.
Glide range: 4.9 nm
Glide ratio: 4.9 / 3 = 1,64 nm per 1000 ft
Conclusion, the G36 is pretty accurate! Maybe the problem only concerns turboprops?
To be honest this doesn’t fully surprise me, the G36 seems to have gotten the most thorough post release TLC outside of the A320.
Beechcraft Baron:
Speed 110 kts at FLC, max. weight:
Power idle: -1000 ft/min
Engines off: -1450 ft/min
Props feathered: -1450 ft/min
Conclusion, the props create drag when windmilling but props don’t feather (RPM stays the same when feathering). Interestingly I left the gear down during this test so I rerun the test with gear-up with same results! The landing gear on the Baron does not create a noticeable increase in drag.
