Maneuvering speed vs turbulence speed

[Towner]

Probably another dumb question, but I'll ask anyway! 

Why does the airplane show a higher turbulence speed (listed as rough air speed) than maneuvering speed? I thought it was usually the opposite, or at least the same on most planes. The difference is substantial for a CTLS.

I wasn't familiar with the term "rough air speed", but the research I did always said it was the same as turbulence speed.

[FlyingMonkey]

Hmm...never heard the term "turbulence speed" or "rough air speed", at least not as something different from Va.  Maneuver speed is designed to cause the aircraft to stall before flight loads that could harm the airframe occur, which is what you'd want in heavy turbulence.  What does your manual list as maneuver speed and as "rough air speed"

There is the concept of Vh which the FAA defines as "maximum speed in level flight with maximum continuous power." -- but I'm a little hazy on that one I admit.  I think on my CTSW that number is 120kt, but it almost seems more a design goal than an operational limit.  If one cruises above Vh...let's say 125kt, what problem is that?  Doesn't Vne define the load limits on the airframe?  Sorry for the slight thread drift.

EDIT:  I think I have the answer.  Your "rough air speed" is probably Vno, which is "maximum structural cruising speed" defined as "The top end of the ASI green arc. When cruising at, and below, VNO the aircraft should not be damaged by a 35-knot vertical gust; which is at the bottom end of the severe turbulence scale. Flight above VNO should only be conducted cautiously and in smooth air."

[Tom Baker]

Rough air speed is Vb. It is not a V speed that is commonly used with FAA certification, but common with European gliders. Since the CT hails from Germany I could see them using the speed. 

[Towner]

I don't know why, but I was looking through a CTLS manual, even though I have a CTSW.

Anyway, it show rough air speed as 138 knots. I was unfamiliar with that term, so researched it everything said its the same as turbulence air speed.

From prior training, and a little research today, turbulence speed should be below maneuvering speed.

I'm guessing you are probably right and the speed of 138 knts is simply given an improper name in the manual.

Being new to CTs, I just thought there was something new here that I wasn't aware of.

[Towner]

Some of the articles I read stated Vb is also commonly used on turbine aircraft, but not normally on piston aircraft. Just seems like a strange name for the speed in the manual

[FlyingMonkey]

Hmm...I don't think I'd cruise in any CT in even light turbulence at 138kt.

But in some countries the CTs have a 163kt Vne, so what do I know?

[Garrett Gee]

In my AOI, this is a statement in the Flight Load Factor Limits section:  Up to Vra (rough air speed) (which is 132 IAS, 138 CAS), the CTLS can withstand a vertical gust of 3000 feet per minute.  Above this speed, the vertical gust is 1500 fpm.  What I think it boils down to is a structural limit.  Severe turbulence above 132 IAS (138 CAS) can lead to structural damage or even failure due to the loads that would be imparted on the structure.  Basically, if in "rough air", don't go faster than Vra, because it could lead to exceeding the flight load factor.  As for Va, my thoughts are related to full control movement, especially full elevator movement, not necessarily just rough air.

Just my 2 cents.

 

[Anticept]

See https://www.law.cornell.edu/cfr/text/14/25.335 For how Vb is mathematically defined. Vb is also know as maximum rough air penetration speed. An example is entering a jet stream.

While Va will stall the aircraft before structural damage occurs (assuming you loaded the aircraft within limits! IMPORTANT!), it does not always protect larger aircraft from being overstressed from high turbulence, which is why Vb is only required on >12500 lb aircraft.

No idea why a CTs Vb is listed as more than Va, but check out the formula, it might be a math quirk and had to be written in anyways due to some european requirement. Honestly no idea here!