When does Va decrease with increased weight?

[Superpilot]

We all know (I hope) that Va (Max manouvering speed) usually increases with increased weight, however the place I currently work owns Cessna C208Bs (Grand Caravans) with a Payload Extender kit (STC). The kit simply introduces a couple of wing fences and reinforces the landing gear legs. This increases MTOW by about 300lb, however the Va is lower than the original of 148. With the kit, it is listed as being 143.

Apparently there is a very simple answer to this! Anyone? ;)

[john_tullamarine]

Va looks at several things but one of the main drivers is the intersection of limit load factor and the stall line.

If, say with a mod, the stall speed reduces, then one would expect the Va to reduce on the new stall line. Depending on the two lines, a modest weight increase post-mod may still result in a lower Va.

One presumes that there is no design change to limit load factor ..

So what is the pre-mod and post-mod stall speed variation ?

[Massey058]

There is no change to the stall speed but the load factor limit is reduced to +3.36, -1.34g from +3.8, -1.52g flaps up. Flaps down it is reduced to+2g from +2.4g.

As stated by Superpilot the STC adds stall fences and stronger gear axles and also the larger tyres. There are no other material changes.

[john_tullamarine]

the load factor limit is reduced to +3.36

I shall repair to the TCDS to consider. Presumably, the STC presumed no growth capability.

However, just changes the flavour of the earlier story. If the stall speed reduces OR the limit load factor reduces, then the intersection of the two lines will occur at a lower speed ...

Any other bets ?

[Checkboard]

http://www.rainierflightservice.com/blog/wp-content/uploads/2011/01/vg-diagram.jpg

You can see the vertical axis is the load factor, and the horizontal axis speed. The stall speed at various loads is the curve marked "accelerated stall", and it runs up until meets the load limit for the particular airframe.

The idea is: Pull back on the stick below this meeting point (the manoeuvering speed), and the wing will stall, thus unloading it, before it over-stresses. Pull back on the stick above this speed, and you will overload the wing before it can stall ...

If you lower the limit load, then the intersection runs down the curved stall line to a lower speed.

[Superpilot]

So the answer is that you will over-load the wing before you stall the aircraft and therefore Va is limited by wing loading primarily?

[john_tullamarine]

That's about the size of it .. sort of.

If the aircraft doesn't have any growth capability, or such is not investigated, one can look at the notional maximum strength of the wing in terms of gross weight x limit load factor and then divide that by the revised, higher gross to figure a reduced limit load.

However, it doesn't work the other way for mods. If the gross weight is reduced, one can't increase the limit load .. as all the bits and pieces of things which are bolted or sticky taped to the airframe will have been designed to the original spec requirements.

If one increases the limit load, the wing might be OK but lumpy bits might fall off when you pull the higher limit load.

[Massey058]

And that is what worries me a little. While the increase in MTOW is only 312 lbs it does eat into the buffer up to the ultimate design load.

Obviously the reduction in MAUW Va correspondent with the reduced load factor addresses that to an extent but there is still that reduced margin in normal flight regimes.

[KiloB]

Back in the dim and distant days of my training we were told very specifically that Va was the speed below which we could not break the A/C "with the use of the controls". I clearly understood that this was a sort of pact between us and the Designers.

Then came AA587!

Would anyone like to comment on why Fins and Rudders are not required to be designed to the same standards of strength?

KB

[Mad (Flt) Scientist]

They are designed to the same standard.

When it was said "you cannot break the aircraft with use of controls below Va" there was an expectation of reasonableness - or, if you prefer, good airmanship - in that sentence. Do something extreme - in ANY axis - and you'll end up with a nasty result.

All the FAR requirements for control inputs pretty much assume a single full application, and then the removal of that application. I don't believe any of the axes assume repeated reversals.

[Checkboard]

Roll and yaw limits are around 2/3rds of the pitch limit, as I understand it - no I don't have a reference for that.

[KiloB]

Ah yes; I forgot Rule No 2. If you break it it's your fault!! I can't remember anyone pulling the rudder off a DC3, Britannia or similar however.
I sometimes wonder if we chase the 'efficiency' God too hard. My other bugbear is underslung thrust lines which give wonderful cruise efficiency but can bite badly if you get too slow. Again, it was the drivers fault. :\

[Coool Hand Luke]

Va "generally" increases as weight increases. This is due to the increase in AOA relative to structural load limit, which as was mentioned, is because of the aircraft's higher AOA. With that, the aircraft will now stall sooner that before in relation to the certification load limit. However, with the modification mentioned perviously, this may have a material effect on the AOA, such that for a given weight the AOA with the modification may have effectively reduced AOA and not increased AOA. So again, in summary, it is all about the AOA. Clear as mud?

[zzuf]

Am I loosing my marbles or is Va misunderstood by many pilots. A review of FAR 25 or FAR 23 doesn't show any requirement that Va must be somehow associated with stall at the limit load factor.
Quoting just FAR 23, but FAR 25 is similar.
1. FAR 23.335(C) shows that Va can be selected from a range of speeds, the simplified design load criteria in Appendix A seems to allow an even lower Va than the full requirement.
2. Va is a design speed, it doesn't change with weight unless the manufacturer actually provides a range of Va v's weight.
3. The structural requirements for maximum control deflections at Va are directed to the "horizontal tail surface", "vertical tail surfaces", and "ailerons". FAR 23.423, FAR 23.441 and FAR 23.445 are specific.
4. There is no FAR 25 or FAR 23 requirement which relates limit load factor with stall speed and Va.
I understood that this apparent misunderstanding of Va led to the development of the "new" FAR 23 speed of Vo - the "Operating Maneuvreing Speed" which is related to limit load factor and stall speed.
This seems to be also covered in the changes to Va placards in FAR 25 aircraft:
Design Maneuvering Speed Limitation Statement (http://rgl.faa.gov/Regulatory_and_Guidance_Library%5CrgNPRM.nsf/0/8AB55A430816ADE786257627004F06C6?OpenDocument)