Speed vs Turbulence
 

Reduce indicated airspeed by 25% = Reduce effect of turbulence by 50%

True or not true?

The g loads from vertical gusts are linear with speed IIRC, so reduce speed by 25% - reduce effect of turbulence by 25%.

However, you can't reduce your speed by 25% in a jet transport In cruise...

I tend to lean toward a very strong FALSE. At economical cruise a 25% reduction in IAS would put a whole lot of aircraft at just about the pre-stall buffet margin or worse. The best option for is to avoid the turbulence. If unable, maintain turbulent penetration speed as published for the airframe at given altitude. Disengage altitude hold and allow some variation in altitude. Frequently a strong updraft is followed by a strong down draft. Remember to let the ATC know what is happening.

Of course you can reduce airspeed by 25% in cruise if you are cruising at FL250 due to ATC restriction.

Anyway, the topic is not how to avoid turbulence or what to do if you are experiencing turbulence.

Let me make it easier:

Reduce indicated speed by 0.1% = Reduce effect of turbulence by 0.2%

True or False?

@Sidestick: Do you have any reference? what about horizontal gusts?

Depends on what kind of airplane you are talking about. In a jet transport in cruise altitude reducing speed by 25% would get you into the low speed buffet/stall region.

Horizontal gusts don't really produce g forces, the main problem is airspeed excursions up, or down, in extreme cases outside of the flight envelope. That's why turbulence penetration speed is somewhere in the middle of the operating speed range, to keep maximum margin to both the low and high speed limits.

As for the vertical gusts - I don't have any reference, but it is basic aerodynamics and some geometry. G-load in a gust is proportional to the square of the speed and to the change of the AoA.

The change in the AoA in turn is proportional to the magnitude of the gust and inversely proportional to the airspeed.

If you combine the two together, the net effect of airspeed will be linear - i.e. twice the speed, twice the g'load for the same gust.

you might want to give this a read:
http://www.nar-associates.com/techni...ide_screen.pdf

the available lift increases with speed squared.

if we assume that the turbulence increases the angle of attack a non-speed-dependent amount additional lift due to turbulence is reduced proportional to speed squared.

now at a higher speed the absolute angle of attack increase due to a change in vertical windspeed (updraft) is less.
so our previous assumption is not very helpful and i'd say the updraft induced additional lift is proportional to speed.

but and this is the big thing: the maximum available lift at the stall angle of the wing is proportional to the speed squared.
therefore if your wing temporarily stalls due to an updraft the total lift available increases with the speed squared.
the additional lift due to the updraft decreases even more dramatically approaching stall speed.

but as the linked pdf describes you normally fly a comfortable speed at which the wing at maximum AOA can't rip your wing apart and you are still fast enough for good controllability.


EDIT:
reading the other pdf linked ..... what i linked seems like it would not apply to jet transports :)
stalling might be good if it's the only way to keep your wing from falling apart but the accompanying loss of control is really bad.

Well that discussion speared off on a tangent pretty quickly.

I reckon it's 25% gives 50% more bump ie the V2 thingy. But I think I said that in an old thread here on Prune and somebody shot me down. All I know is that if I slow down in the bumps, the bumps get less.

https://www.google.com/patents/US4796192

Haven't read it so I'll store it here for later

Mustangsally.
My reading of limitations is that Turb Penetration Speed is for severe turb. Check the definition of severe. In my book it requires things to be lifting of the floor. Too often seen speed reduced unnecessarily, reducing the low speed margin to uncomfortable levels.
maui

maui, severe turbulence only.
Maybe, but how will you know if the turbulence yet to be encountered is severe, or for that already encountered, then it's probably too late to change speed.

The AFM turbulence speed is just that; and why all this concern about low speed margin?

Test pilot at one of the two largest airliner manufacturers - "we laugh at you airline guys who slow down for every bump. It's for turbulence penetration and not the stuff you experience every day."

Severe turbulence encounter is very, very, rare. I can recall on in over 20,000 hrs.


We've prepared for severe turbulence more often but actual encounters is very rare.

You're not slowing an airliner's KIAS by 25% at cruise.


Speed range ('window') between low speed 1.2 margin and redline at cruise altitudes is frequently 30-50 kts. Mach changes of .01 is a small amount of indicated airspeed (3-5 KIAS). KIAS is 250-300 KIAS. That means the 'window' speed range is approx. 15-20% of KIAS.


Actual cruise speed is typically about the mid point, or slightly faster, of the 'window'. So typical cruise KIAS is about 15-25 KIAS above the 1.2 yellow band ('hook'). So the speed available to reduce by, while staying above the hook, is maybe 9-12% of KIAS.


KIAS speed reduction from typical cruise to 'turbulence penetration speed' is more like 1-2% of KIAS.

Because pilots appear to be illiterate. The vast majority cant understand that the topic of this discussion is not how to avoid turbulence or how to operate transport or not transport cat airplanes, or about the stall or slow flight etc.

ITS A THEORETICAL AERODYNAMICS QUESTION! :ugh:

I have to admit, that it is a very complex topic and only few people here will have a good grasp of this matter and majority will just guess (including myself), but I am still hoping that something good will come out of it.

One more of this kind.

People, please don't assume that it is all about your operation, or your type etc. It is not.

For example, in business aviation its not about the structure of the airplane, but instead it is about keeping your job. Many owners are very nervous flyers and fire people for bumps.

Edit:
Please more of that.

Don't know where you should fit this in, but stall speed is proportional to square root of G load. Such that you can't stall at zero G ( when everything is floating in the cabin.)
Conversely if you encounter a +2G event, your stall speed will be 1.414x more.

and here I was thinking it was a force-deflection issue.

I had thought that the idea was to penetrate it with the primary vector being along the intended line of flight.

So to minimize excursions up or down or sideways, don't slow down in it.

Of course this presumes that the aircraft has been designed and certified for the level of turbulence forces.

Dear Test Pilot, we slow down to make the ride nicer for the FA and pax. :rolleyes:

That's rubbish. Slowing down doesn't improve the ride, just prolongs your time in the turbulence.... the test pilots have it right. Turbulence penetration speed is for a category of turbulence that occurs very very rarely.

As far as I have read, nearly all the major severe turbulence occurrences have occurred in clear air With little, or no, prior turbulence to slow down for. But nearly everyone slows down at first bump, for no other reason than they see everyone else do it. Sheep operating procedure!