Vb (Turbulence penetration speed)

[Snapper_head]

How is Vb ie Turbulence penetration speed caculated.

We realise that it the design speed for max gust intensity but how do the designers come up with 156kts for a baron.

Is it a percentage buffer for Vs or a structual limitation or OTHER

Cheers

Honestly this is an argument over a few beers G.F.

[compressor stall]

I cannot answer the question, but I know I operate a turbine type that has no published turbulence penetration speed....so it would be Vne...

Would have thought it would be structural... :confused:

[czechmate]

I would have thought it to be similar to Vno, the speed only to be passed in smooth conditions. Makes sense to take that if Vb isn't given in the flight manual or POH.

[piontyendforward]

Va is the close approx for Vb if it is not published in the aircraft flight manual. Most light GA aircraft dont have a published Vb, at least I can't think of any offhand that do have a Vb published.

Vb = design speed for maximum gust intensity(best speed to give protection of both stall and structural failure) i.e. might be at a higher certification assumption e.g. 45fps

while

Vno = maximum structural cruising speed (where maximum turbs by "definition" is 30 fps vertical)

Vno (green segment upper limit) the aircraft must be able to with stand a vertical gust of 30 feet per second without STALLING or STRUCTURAL FAILURE at Max weight. You would have to make sure that it did not exceed the Vno speed or the 30fps gust while at Vno to avoid stall or damage, hence the Va recomendation by some engineers for light aircraft.

30 fps gusts can be found in the vicinity of thunderstorms, while 45 fps can be found near thunderstorms, and in them can be well over those values

Large aircraft are in a different certification rule.

As an aside the C172 model has had two reported inflight structural failure accidents last year. The first in its long history (one a C172S and the other a C172P)

Take care out there.

[wingnutt]

Turbulence penetration speeds in my aircraft are a range, which provide an adequate margin between low and high speed buffet in severe turbulence. ie between max structural and stall.

[OZBUSDRIVER]

By coincidence this subject was in the June issue of Flying.

Peter Garrison put the formula for Va as-

Vs x sq root of limit load factor.

for normal catagory a/c limit is 3.8g so for an aircraft with a 60kt stall speed the problem would look like-

60 x 1.95=117kts

Hope this helps.

Regards

Mark

[Wing Root]

I'm interested in this too.
First off Va is different to Vb
Aerodynamics for Naval Aviatiors defines Va as
"the minimum airspeed at which the limit load could be developed aerodynamically. Any airspeed greater than Va provides a positive lift capability sufficient to damage the airplane."
And gives the same formula as Ozbusdriver for its calculation... I can find no mention of turbulence penetration speed in that book.
So, it was on to the famous "Handling the Big Jets" by Mr. Davies. Now I got really confused.... the definition for Vb here is "Design speed for maximum gust intensity; one of the parameters used in establishing Vra." And Vra is "The rough-air speed; the recomended speed for flight in turbulence." so... it would seem that Vb isn't strictly Turbulence penetration speed or as Davies puts it "Rough-Air Speed."
So I think what we are interested in is "Rough-Air Speed?" If so Davies has a section titled "The history and choice of rough-air speeds." (Page 220) This is a long section with diagrams so I don't know how I can explain it, especially since I am trying to come to terms with it myself, but here are a few interesting points from it:

"In piston-engined days rough-air speeds were close to Vb."
Davies seems to say regarding Vb that it's the "Stall speed in a 66ft. per sec. gust." is this correct? He says that there was concern in the beginning of the jet age that using Vb as rough-air speed presented the hazard of a stall in severe turbulence so the rough-air speed was increased to a speed half way between Vb and the "Maximum strength speed for compliance with the 66 ft. per sec. gust." Does that speed have a name?
Phew... this is a Techlog thread now! :D
I really don't know how all this is fitting together so far. I think I need it explained to me by someone who knows what they are talking about. :confused:

[Snapper_head]

Excellent post Wing Root. As you correctly stated Aerodynamics for Naval Aviators only discusses Va which is of course different to Vb and D. P. Davies explanation is close to answering the question but not quite there.

Yes pointyendforward a Baron flight manual gives only the Va speed but states to use this speed in the absence of the Vb speed. There is apparently a few light GA machines which have a published Vb speed possibly a Chieftain / Nav.

With reference to D.P Davies in Handling The Big Jets it is my interpretation that he has used Vra as the best means to demonstrate the parameters used in calculating a turbulence penetration speed although this is not in fact Vb. As stated above he defines Vb as "one of the parameters used in establishing Vra" which is "the recommended speed for flight in turbulence" This is where I become unstuck as he does not explain how Vb is calculated although Vra is related to Vb.

Once again I pose the question how is Vb calculated.

It would seem that the pilot orientated texts that define or discuss this topic fall short of explaining the parameters for its development. (Anyone feel free to correct me):8

[4dogs]

SH,

Try FAR 25.335(d)...

Stay Alive

[djpil]

4dogs SH,Try FAR 25.335(d)...

Stay Alive
That's right 4dogs - same goes for smaller airplanes with FAR 23 (i.e. no Vb there) although older aeroplanes of UK origin would have a flight envelope per the BCAR's. The basic problem is that people use that book for regulatory aspects of flight envelopes - it is fine for the physics only.
Same goes for Va - there is scope for the designer to choose something different than Vs x sqrt (n1)