question

Interesting case, it could be that your CG is seating slightly behind the main gear, and on the other hand, with the CoL in front of the CG, the prop wash induced lift increase and the plane light enough, a nose up moment is created and sits the plane on its tail.

Of course, that is pure conjecture.

@boufogre it is aheadof the main gear.

Still, the torque lever isn't long enough for weight to cause that kind of a problem and the plane is at its max weight when this happens.

@SWS-AlexVletsas, maybe the weight alone is too weak, but the weight plus the lift induced moment might be enough. I'm not saying it happens IRL but maybe the effect of the propwash on lift is too high.

Hello Alex.

We reviewed you aircraft with some Asobo plane devs.
Although we were not able to identify any major issue on the FM side and on your configuration side, here's a list of potential adjustments that could help.

- Your engine seems to be misplaced, the Engine.0 parameter in the GENERALENGINEDATA section does not define the position of the engine CG but the position of its thrust so it should be shifted towards the propeller.
The doc is not very clear about that and will be improved.

- Unrelated but also check your ThrustAnglePitchHeading values, 2° seemed a bit high to us.

- Are you a 100% sure about your empty CG position? It seemed very unusually shifted to the rear to us.

- When the CG is at 40%, the lift center ends up in front of it. Is that the actual aircraft behaviour?

- Last but not least, what does the flight manual say about flying in this configuration? Is going full throttle and instantly releasing the brakes something that is actually done with the real aircraft?

As a last resort, something we can also suggest and we're sometimes using to increase our aircraft handling on ground is to slightly shift the position of the rear gear contact points.

You can check our Cessna Caravan, it has a very similar configuration but without that tailstrike issue.

Let me know if any of this is helpful.

Regards,
Sylvain

There is definitely something going on with the CP and the CG, especially with swept back wing, where the MAC position gets shifted forward with increasing sweep angle when it shouldn't happen. I think that for the sim, the MAC is the wing section at the centre of the fuselage, which is wrong. When sweep is set to 0, then MAC and CG positions make sense and are as defined in the configuration files.

@FlyingRaccoon @boufogre I observed something very strange while debugging the aircraft.

In the weight & balance section debug window, the centres aren't stable but are bouncing:

• Wing Pressure centre: 23-25%
• AeroCentre: 40-70% (too fast to see if it goes elsewhere too)
• Lift Centre: 26-27%
• Aircraft pressure centre: 30-70%

Also, I think I found the source of the problem. Snapshot of aircraft weight debugger attached below.

• MAC length: 64.92 inches (manual), 64.7 inches (sim). Close enough
• Wing Leading edge offset: 51.1 inches. That's 3 inches further forward to what it should be. Probably our problem, working on it.
• Centre of gravity empty: 16%, tipping: 53%
• Centre of gravity at 22% MAC
  • Manual: 0.40*MAC + LEMAC = 0.40*64.92+54.81=69.06 inches aft if datum
  • Simulator: 76.3 inches, which is equivalent to 28%CG and not 22%!!!
• Centre of gravity at 29% MAC
  • Manual: 0.29*MAC + LEMAC= 0.29*64.92+54.81=73.64 inches
  • Simulator: 80.9 inches, which is equivalent to 40.2%, which is slightly past the limit
• Centre of gravity at 40% MAC
  • Manual: 80.904 inches
  • Simulator: 87.6 inches, equivalent to 50.5%, way behind the limit. That might explain why the plane is tipping

The wing dimensions were defined based on the factory 3D model, so we are sure they are correct. There are three possibilities I can see:

1. We misplaced the aero centre by 3", which seems to affect CG calculations. That seems to account for 3" of error
2. the simulator miscalculates CG% in conjunction with MAC length. We are off by 3 inches. Where do the remaining 4.5 inches of error come from?

Also another thing that caught my attention (images 2 & 3) is that as you change the CG, under Aerodynamics debug, the simulator seems to be moving the CP and wing position fore/aft on the aircraft caricature. It may be that you do it on purpose (redraw the wing instead of the vectors), but I am posting it here nonetheless. Watch the wing position relative to the weights.

Lastly, why does this happen only after releasing the brakes and applying power? If it was strictly an aerodynamic/balancing problem, the plane should tailstrike even with the brakes held. Why does it happen only AFTER releasing them? CG bugs aside, it almost seems as if you're forcibly pressing on the nose gear with brakes set (momentum) and when they are released, it springs back and raises the nose.

Edit: After correcting LEMAC position (54.1 inches aft of datum as per the manual), the EWCG of the aircraft now sits at 11% (with the 3" error it was sitting at 16%), making the plane out-of-balance from the get-go! In the aircraft.cfg empty_weight_cg_position hasn't been touched and is 6.027ft behind the datum. In the debugger, the current CG offset is 72.3 inches. That would mean a CG of:

(72.3 - LEMAC) / (MAC length) = 27% and change. While the sim sees the correct distance, its CG% calculation is off by 16%!

Furthermore, now at max gross weight my CG is 34.09%, but the calculation says 87.1 inches, which in reality corresponds to almost 50%CG. Bottom line: the distance of the CG seems to be correct, but the percentage calculation seems to be off and we don't know why.

Furthermore, the CG offset seems to be a factor of weight distribution alone and doesn't take the EWCG and empty aircraft weight into consideration. If I load all stations with 100lbs, I get a CG offset of 82 inches. If I load all stations with 220lbs, I get 87.6 inches of CG behind the datum. While that may is right if we're only talking about cargo, it isn't right when we talk about the entire aircraft. The effect of the cargo to the total CG position should be equivalent to the ratio of the payload weight/total weight. In MSFS, the CG position seems to be taking into account only the cargo weight.

Changes done to flight model for this: aero_center_lift changed from -0.7 to -0.95

@SWS-AlexVletsas, do you have any wing sweep defined in the config file?

Bringing some history into this, back in April and June (SU3 & 4) we weren't having that problem. It seems to have come into play after SU5. To verify this, I ran the following tests to narrow down the issue. Ran tests using:

• April/June flight model and current SWS engine: Tailstrike
• April/June flight model and C208 engine at correct position: Tailstrike
• November flight model and current SWS engine: Tailstrike
• November flight model and C208 engine: Tailstrike

Tailstrike configuration

@FlyingRaccoon What was interesting was the elevator's behaviour on the ground. When I push the yoke forward on the ground, the nose wants to go up and when I pull it wants to pitch down. As soon as you're airbourne, it works normally. I tested the default C208 and it doesn't have such an issue. I also checked the various values that Paul used, in case a + should have been a - and vice versa. All is in order.

So what is up and why is the elevator working backwards on the ground?
As we didn't use 0,0,0 as our reference datum, could there be a miscalculation that causes this? Also an academic question (I'm not an FD guy): what are the three lines to the left of the aircraft figure (red, green, blue vertical)?

The only situation I would think makes sense for the Elevator being inverted in during low speed taxi with a tailwind, for example, which you would need to make opposite movement, yoke forward for nose up, and yoke back for nose down, to keep the airplane stable during heavy wind, as needs done in TailDraggers. Do you have any heavy wind acting on your plane at the time of these testing?

@Alec246 testing is always done on an ICAO standard day with no wind.