Published Va DECREASES with Weight Increase...?

[HighFlyingA380]

I was always taught that VA increased as weight increased, which can be corroborated with the formula: VAnew = MaxGrossWeightVA x √(NewWeight/MaxGrossWeight)

The published VAs, following that formula, for the standard Caravan are:
148 @ 8,750
137 @ 7,500
125 @ 6,250
112 @ 5,000

Now, several of my Caravans have the APE II STC which increases the max gross weight from 8,750 to 9,062. It would then logically follow that according to the formula, the new VA should be 151kts. However, the STC lists the new VA as 143kts, accompanied by a sticker to alter the original panel placard (pictured below).

In talking with folks around the office, I've heard several different theories, but none have seemed much more than a guess... So, anybody have some insight as to what kind of witchcraft is going on here?

 

[TMetzinger]

I'd guess that they either did new testing and calculated forces on surfaces and structures at the new weight and speed, or determined that the strakes themselves warrant those figures.

Sent from my Nexus 7 using Tapatalk

 

[Graueradler]

Apply the 143 to the new gross weight and calculate the rest using the formula???

 

[HighFlyingA380]

Apply the 143 to the new gross weight and calculate the rest using the formula???

Ah, good question... I'll look in the AFM on Tuesday to see if the listed speeds in section 4 has been amended to reflect that.

I have looked for the actual STC paperwork for some insight but couldn't find it, so I'm assuming that's on file and not required to be carried on-board.

 

[sarangan]

I don't have direct experience with Caravans, but I think this has to do with the change in stall speed due to the modification. If the new stall speeds are lower, then your VA's will also be lower. The equation you quoted only applies within the same aircraft. You can't apply the equation to calculate VA for a different aircraft (which is effectively what you have after the wing modification).

I was always taught that VA increased as weight increased, which can be corroborated with the formula: VAnew = MaxGrossWeightVA x √(NewWeight/MaxGrossWeight)

The published VAs, following that formula, for the standard Caravan are:
148 @ 8,750
137 @ 7,500
125 @ 6,250
112 @ 5,000

Now, several of my Caravans have the APE II STC which increases the max gross weight from 8,750 to 9,062. It would then logically follow that according to the formula, the new VA should be 151kts. However, the STC lists the new VA as 143kts, accompanied by a sticker to alter the original panel placard (pictured below).

In talking with folks around the office, I've heard several different theories, but none have seemed much more than a guess... So, anybody have some insight as to what kind of witchcraft is going on here?

 

[HighFlyingA380]

I don't have direct experience with Caravans, but I think this has to do with the change in stall speed due to the modification. If the new stall speeds are lower, then your VA's will also be lower. The equation you quoted only applies within the same aircraft. You can't apply the equation to calculate VA for a different aircraft (which is effectively what you have after the wing modification).

Ah, I haven't thought of that. While I haven't specifically looked, I don't remember seeing any mention of changed stall speeds due to the STC, which I would assume would be reflected with a placard on the panel and a new airspeed indicator...

I did some quick research on calculating VA from stall speed, and the one I keep seeing is: VAnew = Vs x √(n) where n is the load factor. However, if I run the factory numbers in that, I see get: 63 x √(3.8) = 123, and not the published 148kts. Are you seeing a different formula?

 

[poadeleted20]

Apply the 143 to the new gross weight and calculate the rest using the formula???

...or just do what the placard says and read the AFMS for that STC, where the new Va's apparently are given for all weights. But it would not be surprising if an alteration required lowering Va -- any number of reasons the modified airframe would have something which now had a lower structural tolerance or a lower stall speed.

 

[sarangan]

Ah, I haven't thought of that. While I haven't specifically looked, I don't remember seeing any mention of changed stall speeds due to the STC, which I would assume would be reflected with a placard on the panel and a new airspeed indicator...

I did some quick research on calculating VA from stall speed, and the one I keep seeing is: VAnew = Vs x √(n) where n is the load factor. However, if I run the factory numbers in that, I see get: 63 x √(3.8) = 123, and not the published 148kts. Are you seeing a different formula?

After some thought, I think the difference between CAS and IAS may be more likely the answer you are looking for. The number that needs to go in the equation is CAS. The difference can become large near the stall speeds. So, if 63 is your Vsl IAS, convert it to CAS, multiply by sqrt(3.8) and convert it back to IAS.

 

[Henning]

You aren't aounting for the additional static load from the increase in GW, due to that the calculations may get a new baseline to work off. What did they do to the plane to get the weight increase?

 

[HighFlyingA380]

So as this STC paperwork shows, the other published VA weights don't change, but they replaced the max weight speed with the aforementioned 143KIAS @ 9,062:

Looking on, this is clearly accounted for by reducing the standard +3.8g load limit down to +3.36g:

So, the APE II changes the slow-speed/stall characteristics to remain within parameters at higher weights, thus not changing the stall speeds. Now for weights above the original max gross of 8,750, they needed to reduce the load factor limit to +3.36g in order to not exceed the structural strength of the airframe, which thus reduces VA for the increased weight.

Man, I just love when everything finally comes together and just 'clicks'.

In addition, something I never thought of but makes perfect sense is these new limitations (shown below). When above the original max ramp weight of 8,785, you can't jack or tow the aircraft, because that may also impose excessive loads on the wing spars and nose strut, thus exceeding what it was designed for...

 

[James331]

Off subject, will a 675SHP redline in level flight?

 

[Henning]

Ha, interesting.

 

[Rigged4Flight]

I just finished reading 'The Thinking Pilot's Flight Manual' by Rick Durden. One of the chapters was on aviation myths, and this was covered. The bare bones of it is that maneuvering speed is a function of stall speed. Increase stall speed and maneuvering speed increases as well - and vice/versa.

Or put another way: "at maneuvering speed the airplane will stall before it suffers structural damage" during an extreme/abrupt control input.

Side note: I highly recommend the book, especially for student pilots.

 

[HighFlyingA380]

Off subject, will a 675SHP redline in level flight?

Yup, but it's not the HP rating that makes a difference, it's that turbines are operated differently from recip engines. You're always watching torque (the most critical with a P&WC), prop RPM, inter-turbine temperature (ITT), and gas-generator speed (Ng). Any and all of those can easily be exceeded during dormal operation if you don't closely monitor during all power and altitude changes. With a recip, you simply firewall it and go; Not so with a turbine. Do that and and the engine will be destroyed in a short time...

 

[HighFlyingA380]

I just finished reading 'The Thinking Pilot's Flight Manual' by Rick Durden. One of the chapters was on aviation myths, and this was covered. The bare bones of it is that maneuvering speed is a function of stall speed. Increase stall speed and maneuvering speed increases as well - and vice/versa.

Or put another way: "at maneuvering speed the airplane will stall before it suffers structural damage" during an extreme/abrupt control input.

Side note: I highly recommend the book, especially for student pilots.

Thanks for the tip, I'll look into it. Always looking for ways to pass the time in cruise, yet not waste it playing Candy Crush...

 

[Henning]

I just finished reading 'The Thinking Pilot's Flight Manual' by Rick Durden. One of the chapters was on aviation myths, and this was covered. The bare bones of it is that maneuvering speed is a function of stall speed. Increase stall speed and maneuvering speed increases as well - and vice/versa.

Or put another way: "at maneuvering speed the airplane will stall before it suffers structural damage" during an extreme/abrupt control input.

Side note: I highly recommend the book, especially for student pilots.

Right, what happened in this case was the Load Factor Limit required to do structural damage.

 

[James331]

Yup, but it's not the HP rating that makes a difference, it's that turbines are operated differently from recip engines. You're always watching torque (the most critical with a P&WC), prop RPM, inter-turbine temperature (ITT), and gas-generator speed (Ng). Any and all of those can easily be exceeded during dormal operation if you don't closely monitor during all power and altitude changes. With a recip, you simply firewall it and go; Not so with a turbine. Do that and and the engine will be destroyed in a short time...

I was talking airspeed, I flew a TPE331 powered 208B, we could hit VNE in straight and level on a light plane, just didn't know if a 600ish HP van could do the same

 

[HighFlyingA380]

I was talking airspeed, I flew a TPE331 powered 208B, we could hit VNE in straight and level on a light plane, just didn't know if a 600ish HP van could do the same

Oh, gotcha. No, lightly loaded (for us, light is about 1,000lbs of gas and another 1k in payload...) and torqued out I've hit 160KIAS. Pull it back to about 1,600lbs and you will be right on a standard 3* glidslope at VNE. I would love to give it a try solo with minimum fuel; Probably could tickle 170KIAS...