Aircraft Overstressing



  • Flying level with a bumpy following wind at 4,000ft MSL, just under 140 kts IAS and a ground speed of 168 kts, I got dumped out with the "You overstressed the aircraft" message. That has happened twice now when I know I have not reached the yellow arc. (Using v0.3.0)

    Is that what would be expected or do the aircraft overstressing parameters need to be adjusted? I don't want to raise a ticket unnecessarily, so would appreciate some feedback from other users first.



  • I've had this also. I'm sending a ticket now for this and another thing that can hopefully get put in the next update.

    This one can be fixed for now. In the flight_model.cfg file, under the [AIRPLANE_GEOMETRY] section, near the bottom are

    negative_g_limit_flaps_up =0
    negative_g_limit_flaps_down=0

    Change those 0's to -1.5



  • There is no published negative G rating for this aircraft. This is from the POH...

    Screenshot 2021-04-14 134337.jpg



  • @RetiredMan93231 I wasn't inverted. Honest guv ๐Ÿ˜‰



  • @RetiredMan93231 I can't find any design specifications pertaining to Arrow negative G limits, but to be certified in the normal category it must be capable of not less than .4 X positive load limit. That works out to at least 1.52 negative G's for the Arrow.

    Reference: https://www.govinfo.gov/content/pkg/CFR-2011-title14-vol1/pdf/CFR-2011-title14-vol1-sec23-337.pdf



  • We had the same problem with the Avro Vulcan in FSX / P3D - it is also not stressed for negative G manouevering, 0G being the maximum (from memory) which allows for a slight push on the stick, nothing more. It easily overstresses. The problem occurs in the sim where transient G-forces (generally down to weather) instantly register an overstress situation because the aircraft is seen by the sim to be a perfectly rigid object and is hit by the weather at that point in its entirity (hammer hitting nail), whereas in real life there is a bit more flexibility as weather is not an 'on / off' situation and the aircraft will, to an extent, ride the movement.

    It's just another of those no-win situations, we could set much more tolerant acceleration levels and then be picked up on the discrepancy from the flight manual! It'll remain as the POH states, however there is a line "load_safety_factor =1.2" which, provided it works as in the previous sims, should give a bit of leeway anyway and is probably the better one to adjust.



  • @Sender46 said in Aircraft Overstressing:

    @RetiredMan93231 I wasn't inverted. Honest guv ๐Ÿ˜‰

    You don't have to be inverted to get negative G's... just push the yoke forward fast enough and you will get a roller coaster ride! Also, in rough air a sudden downdraft can do it. But, I'm sure there is some negative G safety margin engineered into the structural design of the aircraft. If you search online you can find an accident report for an Arrow III that crashed in Florida because it used too heavily for normal commercial pilot training maneuvers and developed stress fractures and ultimately a wing failure.



  • @Delta558 said in Aircraft Overstressing:

    there is a line "load_safety_factor =1.2" which, provided it works as in the previous sims, should give a bit of leeway anyway and is probably the better one to adjust.

    Yes, but if the load safety factor is 1.2 and the limit is 0... 0*1.2=0, so... Hopefully it doesn't work like that, but you never know.

    Also, while the aircraft is not approved for inverted manoeuvres, therefore nothing below 0G, I don't think that means it will break apart below 0G. So i would imagine there needs to be a limit there lower than 0G.



  • @RetiredMan93231 said in Aircraft Overstressing:

    You don't have to be inverted to get negative G's...

    Yeah, I know that, I was joking. Hence the ๐Ÿ˜‰



  • And this is exactly what I was trying to say. If I put a limit of - 1, - 2, or anything which will negate the blunt effect of the sim there will be complaints that it is not accurate because the aircraft is not cleared to that.

    Also, 0g is a difference of 1g from standard loading (we are all subject to 1 x the force of gravity unless disturbed) , so not 1.2 x 0, rather 1.2 x1.



  • @Delta558 The POH doesn't say that the Piper Arrow is not approved for any negative load, only that inverted flight is not approved. In the absence of specific data on negative G (other than inverted flight) for the Piper Arrow, it would appear to be reasonable to me to use what Bernie has referred to.

    Looking at what Bernie referrred to, the 0.4 x positive load limit he referred to is a maneuvering load limit.

    However the reference he linked to also includes gust load factors (separately from maneuvering load factors). Gust load factors would appear to be relevant as well, particularly as I was getting bumped around by the wind at the time in my case. Calculating that appears to be complex but maybe somebody more knowledgeable than me knows what to do with that?

    If gust load factor isn't included in the flight modelling, maybe it would be reasonable to make some allowance for that in the negative G maneuvering load limit?



  • Response from JF to my ticket says: "I would recommend turning off Aircraft Stress Damage and Engine Stress Damage in the Assistance menu of MSFS when using all third party add ons."

    I'm surprised at this response. I purchased the Piper Arrow for realism so I balk at turning realism off. I'm going with the following changes in the flight_model.cfg file (as per the Bonanza):

    negative_g_limit_flaps_up =-2
    negative_g_limit_flaps_down=-1

    I'm no expert by any means but given that there are bound to be some negative G loads on the aircraft in normal flight, those figures make a lot more sense to me than 0 for both of them.



  • Recalling old study material, but the figures BernieV quoted are correct for both CS23 and FAR 23 in the Normal category (to which the Arrow will have been built), stating +3.8g and -1.52g with a 1.5x safety factor. That means the aircraft has to be designed not to fail structurally below 5.7g or -2.28g. Permanent deformation may occur between these figures.

    With respect to the dev team, setting 0 for the negative limit doesn't make any sense.



  • @ShadowSix My opinion is that the G limits in the config file should reflect the structural limits, not the approved limits. After all, the message you get not only says that you have overstressed the aircraft (overstressed, not "gone over the approved limit"), but it is also an instant end to the flight.



  • If you can find me the structural limits, documented officially rather than just some website that says 'about this', I will happily make the change. Believe me, I have looked since this topic was started and I keep ending up directed back to the POH.

    It is not something which has attracted a lot of complaints, we didn't encounter it during testing but as suggested above, if you want to make the fix yourself feel free to change it to whatever you feel suitable.



  • @Delta558 said in Aircraft Overstressing:

    ... I keep ending up directed back to the POH.

    But with respect, the POH doesn't give any indication what the maximum negative load factor is. It only says that no inverted maneuvers are approved, which doesn't give any basis for setting a limit, so I don't understand the logic of using that to justify setting the limit at 0 when it is clear that the aircraft must be capable of some negative G in normal flight. "No inverted maneuvering" does not mean that the negative load factor limit should be zero.

    In the absence of anything more specific on this particular aircraft it therefore seems perfectly reasonable to me to use information that is available, such as what BernieV and ShadowSix have referred to.

    For me the purpose of starting this post and raising a ticket was to address the issue of unrealistic termination of flights due to supposedly exceeding the aircraft limitations for normal flight when that isn't actually the case. So I don't even think it's critical that the limit be calculated exactly, as long as a reasonable figure(s) is/are used to achieve the desired result. On that basis, the -1.52 calculated by BernieV would appear to be reasonable to me. -2 (flaps up) and -1 (flaps down) as per the Bonanza flight_model.cfg file would also appear to be reasonable to me.

    If either of those options (or something similar) achieve the desired result, does it really have to be any more complicated than that?



  • @Delta558 said in Aircraft Overstressing:

    If you can find me the structural limits, documented officially rather than just some website that says 'about this', I will happily make the change. Believe me, I have looked since this topic was started and I keep ending up directed back to the POH.

    It is not something which has attracted a lot of complaints, we didn't encounter it during testing but as suggested above, if you want to make the fix yourself feel free to change it to whatever you feel suitable.

    I've not experienced this issue myself, I tend to fly with 0 wind, or 0 gusts because I really don't like the way MSFS moves the aircraft.

    However, these figures aren't just quotes from any old web site, they're really quite specific requirements laid down by the FAA/EASA and other such legislative bodies. I don't have an FAA quote, but this is from EASA CS23 which governs Class B aircraft (like the Arrow)...

    "CS 23.337 Limit manoeuvring load
    factors
    (a) The positive limit manoeuvring load
    factor n may not be less than โ€“
    (1) 2.1+ (24000/W+10000) for normal and
    commuter category aeroplanes (where
    W = design maximum take-off weight lb),
    except that n need not be more than 3ยท8"

    If you're not getting many complaints then of course you have to consider whether or not it's worth your time to make the changes, but the figures are there and the POH really only scratches the surface for a potential owner and pilot.



  • @ShadowSix ??? Maybe I'm missing something here. Your last post refers to the positive maneuvering load limit. But that is stated in the POH anyway. It's the negative maneuvering load limit that is the problem ....



  • I was responding to Delta558 who was asking about documented references to limits, and trying to illustrate that design specification limits exist outside of the PoH.

    As has already been mentioned in previous replies, the negative limit specified by governing bodies for aircraft in this category is -1.52g with a 1.5x safety factor ensuring no structural failure below that.

    Perhaps I should have included this excerpt (although I think it's already been quoted more or less) also from CS 23.337:

    "(b) The negative limit manoeuvring load
    factor may not be less than โ€“
    (1) 0ยท4 times the positive load factor
    for the normal, utility and commuter
    categories"



  • @ShadowSix Apologies, I missed the point of what you were saying. Thanks for explaining.