B58, C90, P-38 constant speed prop bug / flap pitch moment reversed

  • Finally a sim where the sound pitch correctly changes with RPM and not with MP like FSX/P3D :)
    Unfortunately the governing range is way too small and RPM starts to drop already at medium to low power settings, even with the prop at high RPM/low pitch settings.
    It's worst with the C90 where this happens at rather high power settings.


    Flap pitch moment on all planes except the C172 reversed, up instead of down.
    On the 172 there's also a pitch up moment, but in that case this is correct.

  • No no. It's perfect on the 172. Basically all other planes have the pitch moment reversed.
    E.g. if you extend the flaps on the 737, Macchi, P-38 etc., they all pitch up quite strong but IRL flap extension causes only a mild pitch down moment.

  • Concerning engine sound on the C90...has there anything changed in the last few patches because presently the sound/prop pitch relationship is now even worse.
    When I started this thread I wrote that the this finally a sim where the engine/prop sound is correctly tied to engine RPM and not power.
    Just tried the C90 and now it's as wrong as in FSX and P3D. The sound is tied to torque instead of RPM!
    Strange enough once on the ground the sound suddenly changes and it's tied to RPM instead of torque.
    This is very noticable as after touchdown for some reason the prop suddenly severly overspeeds >3200RPM.
    On the other hand, if I leave the power levers at idle, the prop governer maintains 1900RPM down to approx 40kts which shouldn't be the case either.
    Also the power levers can be pulled into ground fine in flight which should be impossible as idle is a mechanical stop.


    Another point: Why has to Aerofly C90 so many flap positions?

  • The high rpm is caused by our governor being to slow to rapidly transition from flight idle to flat disc without allowing intermediate wind-milling conditions. Ergo the propeller is in the negative torque section for too long, speeds up and as a result the sound pitch is also too high.
    I have not seen any code that changes from torque relationship to rpm relationship, the sound pitch of the propeller should mainly be influenced by the prop rpm, but there are also influences caused by the blade angle of attack which was probably attempted here...


    Is there really a mechanical stop that inhibits you to pull the levers aft or could you lift the levers below flight idle theoretically even in flight? (there is a difference between lever position and actual used input - ground fine being inhibited in flight but lever displacement allowed)


    What kind of airspeed would you expect when the governor is no longer able to maintain rpm?

  • By 'wind-milling' do you mean feather? That doesn't make sense because on e.g. C-90 idle is +13° pitch and ground fine is +3°.
    Torque (and blade angle) is never negative.
    You can lift the power levers into ground fine and even into reverse in flight....but this is what happens in this case as the loss of lift is very large....https://www.youtube.com/watch?v=jFK5fnBQdBE
    Suggest to take a look at the Carenado C90GTx as the drop in RPM (and general RPM behaviour) works very nice.


    edit: the prop governer isn't too slow to react because even when the C90 is sitting static on the runway at ground fine, RPM is around 2700RPM while at idle it's around 1600.

  • no I mean windmilling as of negative torque, increasing rpm... like autorotation in a helicopter... not feathered...
    very high blade angle = feathered, least aerodynamic drag when the engine is failed, propeller can be stopped in this position savely
    medium high blade angle to flight idle = flight range, governor keeps rpm, propeller creates thrust, blade angle of attack is positive and a function of the propeller rpm
    beta range = blade angle is a function of power lever angle AND some sort of protection that the Aerofly C90 aircraft currently lacks.
    disc = blades are angled 90deg to the flight direction, when still moving forward the effective blade angle of attack is negative.


    The region I am talking about is between the disc and the flight idle, through which the propeller has to transition when you touch down and either go to "disc" / ground fine or even further into reverse with the power lever (beta range). The blades are rotated from a positive angle of attack where they produce forward thrust towards the negative angle of attack at flat disc position where their pitch is 90deg to the flight direction. If I am not mistaken there must be a certain region where the propeller blades are able to extract energy from the incoming air flow because the lift vector is pointing in the direction that they are turning, thus speeding up the blades and creating a negative torque situation. If you transition fast enough this won't be an issue but if you are slow or maintain a blade angle in that range then you'll have an overspeeding propeller and this is currently the problem that you describe. From what I read on the internet there are negative torque protections that avoid the windmilling range. There must be some device that inhibits the propeller blades from staying in this dangerous region, because they do transition from flight idle all the way to disc or reverse... In videos that I have seen the transition is very fast, less than say 0.7 seconds and its also audible with the very typical whoosh sound, where you can hear a slight rpm increase followed by a very strong stall of the blades I assume and a large rpm drop. I don't know if this "device" is just an electronic circuit that tests for negative torque and either drives the blade angle towards flight idle or disc, both of which would be safe on the ground or if its just some asymmetric mass on the blades themselves. (blades can be feathered in the full reverse too as far as I know, would create a bit more drag and should spin the propeller the wrong way if there is no engine torque left I think...)


    For example:
    In the majestic Q400 Dash 8 the blades sometimes oscillate between flight idle and disc when you are driving about 80kts and set the power levers between flight idle and beta. So they definitely programmed some protection system...

  • There's no special device needed to keep the prop from overspeeding. As you already mentioned, beta only defines the range where the blade angle is being controlled by the power levers and not the prop levers.
    Below idle until reaching reverse, power is always idle and it's impossible for the prop to overspeed at idle!
    Note that even if disc means approx a 90deg angle, this angle is just a single point on the prop blade. Even at 90deg 99% of the propeller blade are not at an actual 90deg angle, hence they are creating drag.
    Also on an P&W turboprop negative torque can be rather high during flight. E.g. at idle and high speed.
    That's one of the very few advantages of the free turbine vs the fixed turbine as the fixed one, like the Garrett must never be operated at negative torque settings in flight and hence approaches with fixed turbine planes are much more shallow.
    The 'woosh' sound isn't the blades that are stalling but the prop blades going below disc RPM into reverse.
    Don't understand how or why a propeller can be feathered in reverse, however the resulting blade angle in feather is always the same.

  • I'm looking forward to a nice geared turbine propellor noise from the Dash 8. It will be a change from the free turbine in the King-Air.


    There are some effects which we expect to be significant but which in real life are practically unnoticeable.


    We expect a significant increase in noise from big power changes but it doesn't seem to happen, a mixture of a heavy headset and being seated close to the propellor tips masking much of the more distant and quieter exhaust noise.


    Another is the piston engine manifold pressure increase with slowing the prop rpm, with presumably less air being consumed an increase in manifold pressure might be anticipated but I never really noticed it although it is a strong effect in Aerofly simulation. It might happen in some planes and I've no idea why it doesn't happen in many, perhaps longer open inlet valve time volumetric-efficiency at a lower rpm matches the reduced strokes rate?


    Regarding the roll rates in the sim Corsair, it would be too high with joystick sensitivity anything other than dialled way down. It is possibly too high at the minute but reduced joystick movement is do-able and it would be worse to have control rates too low.
    In real life, WW2 era fighter roll rates reduced very significantly with high speed especially with manually powered fabric covered control surfaces as in the Corsair. Most of them had fabric covering behind the outer wing's mid chord so it might have had peculiarities from high speed ballooning of both the ailerons and the rear of the wing.
    The Corsair had a single stage supercharger and would not have been at its best at very high levels so extreme high speed dives might not have been tried all that often. Compressibility at altitude would have been a consideration with the carrier landing friendly wing thickness.

  • 1.I'm looking forward to a nice geared turbine propellor noise from the Dash 8. It will be a change from the free turbine in the King-Air.
    2.Regarding the roll rates in the sim Corsair, it would be too high with joystick sensitivity anything other than dialled way down. .


    1. ? The Q400 PW150 has the same free turbine engine design as the King Airs PT6.
    2. Was testing at low altitude and 250kias. Sensitivity is already down to 30%

  • Yes the q400 has a free turbine as well. The propeller is connected to the low pressure turbine with a gear box but the low pressure turbine has no connection to the high pressure turbine, ergo it is called free spinning turbine.
    But the twist in the propeller blades isn't that high and the disc power requirement seams to be all right, if you just look at the torque. The high pressure part currently creates to much gas in idle (60% NH/N1) ergo the prop also speeds up too much in flat disc position.


    Sensitivity of the control inputs does not change the maximum deflection as far as I know, I don't use it as I have explained in other threads before. Mach effects are not yet implemented but planned, so don't worry about them quite yet. It is quite easy to reduce the roll rate of the F4U, all you need to do is decrease the control surface deflection. But from what I remember we already use quite authentic control deflections and the lift increase per flap deflection is also known from wind tunnel tests (2.1 delta CL per rad of deflection). Of course we could decrease that value to account for leakage in the flap hinge etc. But honestly I think none of us can really judge the current roll rate. It may be impossible to deflect the ailerons to 100% deflection in the real aircraft due to the control forces, who knows?

  • ... from what I remember we already use quite authentic control deflections and the lift increase per flap deflection is also known from wind tunnel tests (2.1 delta CL per rad of deflection)...But honestly I think none of us can really judge the current roll rate.


    Wind tunnel tests alone aren't sufficient for a realistic simulation IMO but given the fact that the P-39, P-40 (one of the fastest rolling planes without boosted ailerons), P-47, P-51 and F-4F-3, F-6F-3 all have a roll rate between 70 and 95deg/sec in a speed range of 220-270kias, the >180deg/sec of the F-4U is definitely way too high.

  • Oops about direct drive, wrong gain. The presumed offset reduction gearbox threw me, I'd presumed free turbines used epicyclic as in the PT-6, never thought rear exhaust co-axial shaft.
    I read that most WW2 fighters got higher and higher stick forces with speed and sometimes it eventually would not move at all. The P-38 roll rate increased enormously in late models with boosted ailerons.
    Perhaps the Aero' Corsair aileron deflection could be made inversely proportional to true (?) airspeed, i.e. less restricted at low level and more so higher up?

  • Yes the q400 has a free turbine as well. The propeller is connected to the low pressure turbine with a gear box but the low pressure turbine has no connection to the high pressure turbine, ergo it is called free spinning turbine

    Just read that there are 3 spools in that PW150A engine, a single stage hp turbine spool driving a single stage hp centrifugal compressor, a single stage lp turbine spool driving the lp axial flow compressor and the two stage power turbine spool so that the gas flow drives the gearbox independent of the core or gas generator spools. The centre spool/connecting driveshaft runs freely inside the entire length of the engine to connect the power turbine with the gearbox at the front.

    The accessory drive is taken off the hp spool.

    Anyway there shouldn’t be much energy and therefore noise in the final exhaust. It is all prop noise and the cabin is quite loud.