Ideas about refinements to the flight model

Some more additions which I did not post over at the MSFS forum.

From pure theory I would assume that in the real Osprey a lot of the "magic" happens in the flight control computer … and it is a good (safe) assumption that the throttle is

a) more of a "logical" concept … e.g. target (airflow) power
b) and not a "mechanical" … fuel flow valve remote control

I would assume (a) and that also should make the Osprey flight model code a lot easier.

c) However, there will alway be a delay between the throttle position (=target value) and the actual rotor (shaft) power.

Contrary to a jet (airliner) turbine a heli (=V-22) turbine has a rotor with an adjustable angle = adjustable "drag".

d) So the flight control can keep the turbine RPM at an optimal level and it then simply adjusts the blade angles in such a way that the displaced airflow equals a certain "target power" (as defined by the throttle position).

Now I have no idea in what range a real Osprey will (have to) adjust the RPM during flight … but I would assume there is some "optimal power point" RPM and the flight control would try to keep the RPM at this optimal point for as long as possible (lower RPM = reduced wear and tear) then … once the blade angle has reached its maximum the RPM will also have to start to increase until both are at their max value.

… but I think the game Osprey can simple define some fictional RPM range … as it does not really matter … if the flight model simply assumes that:

e) airflow power = fuel flow rate (which is energy) … reduced by conversion losses due to rotor angle combined with rpm

So while I would assume a linear relationship between throttle position (a) and the airflow power (e) … in order to reduce mechanical stress on the equipment there will always be a gradual (linear?) delay between throttle adjustments and the reaction of airflow power.

And … as the original post on the MSFS forum pointed out … it is the instantaneous reaction to the throttle position change in the new flight model which does feel unrealistic (because such behaviour can not exist if an object has actual mass … as mass needs to be accelerated)

I never really tried to "reverse guess" the parameters of the flight model. But in order to base my impressions on some "facts" I just checked v1.2.1.

Given that …

• the Osprey is 100% fly-by-wire technology
• and that it requires (most likely) very high precision control from the pilots
• in order to perform the tricky tasks it was designed for

I always found it unrealistic, that the actual useable throttle range is very very small.

Here is what I see with v1.2.1

A) In level flight (ca 16000 kg weight) at 4000 ft (clear skies) I get:

• 130 ktas at 35% throttle
• 290 ktas at 50% throttle

So the entire "realistic" flight envelop must be controlled by (fine tuning) a throttle position within 15% of its actual range. So one or two millimeters of adjustment results in a large speed change. The lower 35% are basically not usable, because the flight control computer should prevent the pilot from entering those positions. For other aircraft masses the ranges most likely will vary (but I have not yet mapped out the related throttle positions). The upper 50% could be used for high climb rates … but I also have not tested how the actual mapping works out.

B) Even for the lightest Osprey config … around 16000 kg … I need 52% throttle for a VTOL lift off … which basically says that the (sim) Osprey needs 160000 N of effective proprotor thrust in order to cruise at 290 ktas.

So in a certain way I wonder if the lower 35% of the throttle maybe could be mapped differently in a fly-by-wire system.

I understand that taxiing requires slow speeds … but here thrust vectoring allows a very fine grained power (vector) control … so a reduction (non linear mapping) of the lower 35% to say just 5% would perhaps also allow proper handling on the ground.

I have to map out some other weight-to-trust configurations but it think there is some room for tuning in the future.

The above, however, does not really matter in the context of the initial MSFS topic which was about the lack of (mass) inertia when it comes to rapid throttle adjustments in the hover case.

PS: … and I clearly am not an aircraft engineer and obviously am not aware of the real scientific terms. So forgive me if something is labeled in a confusing way.

One more piece of feedback … and please remember I am not an aircraft designer … just a simple goose.

I did a quick "rule of thumb" calculation for the VTOL performance that I see in my crude experiments … and it feels like the overall performance is in the proper "ball park". I see:

• "hover lift efficiency" (kg/kW) at around … 4
  • … which matches the expected range given by … https://en.wikipedia.org/wiki/Disk_loading
• a "Figure of Merit" (kW_actual_lift/kW_powershaft) at around 0,5
  • … which also seems the proper range as explained here … https://aviation.stackexchange.com/questions/45802/how-do-i-compute-thrust-of-a-rotor-disk-and-the-necessary-power-to-drive-it