Va & Altitude

[uwochris]

Hey guys,

I'm having a bit of a brain-fart here. I understand the relationship between Va and weight (higher weight=higher Va). It seems intuitive to me that Va should also vary with altitude, however, I can't find any references to prove/disprove my assumption.

The way I look at it is that at higher altitudes, there is less aerodynamic dampening. Since there is less dampening, the a/c will be "easier" to accelerate up/down, and so it will reach its design limit load factor sooner than it would if it were at a lower altitude where the effects of dampening are greater. So, I'd imagine that Va would be lower at higher altitudes.

Any comments? Thanks in advance!

 

[sleddriver71]

Hey guys,

I'm having a bit of a brain-fart here. I understand the relationship between Va and weight (higher weight=higher Va). It seems intuitive to me that Va should also vary with altitude, however, I can't find any references to prove/disprove my assumption.

The way I look at it is that at higher altitudes, there is less aerodynamic dampening. Since there is less dampening, the a/c will be "easier" to accelerate up/down, and so it will reach its design limit load factor sooner than it would if it were at a lower altitude where the effects of dampening are greater. So, I'd imagine that Va would be lower at higher altitudes.

Any comments? Thanks in advance!

What are you talking about when you mention "aerodynamic dampening?" Manuevering speed is based off your indicated airspeed. For a given weight, it will not change regardless of altitude. Do you think that on a hotter day, hence a higher density altitude that your Va will go up too? Of course the true airspeed will be higher at higher altitudes and density altitudes as will the approach speed and stall speed, etc... Again, I'm not familier with what exactly you mean by aerodynamic dampening. Are you getting at the air being thinner and therefore producing less parasitic drag? Anyway, as you gain altitude, manuevering speed is going to be higher because your TAS for a given IAS will be higher. Maybe I am misunderstanding what your question is, I hope my comments helped.

 

[uwochris]

Thanks for the response sled.

By "dampening" I was referring to the fact that the air density is lower at higher altitudes, and so the air will be providing less resistance to the motion of the a/c. In the book "Handling the Big Jets," the author explains it as follows: Imagine a weight attached to a spring- if it's placed in a vacuum and you displace the weight, it will continue to ocillate forever (no air= no resistance/friction force to bring the weight to rest). If, however, you let air into the vacuum, then the would be slowed down by the airloads due to the motion. These airloads always oppose the motion of the object- it is this aerodynamic force which provides the "damping."

So the way I was looking at it is that if you are flying at higher altitudes (or higher density altitudes), there is less air to oppose the a/c. So if you were flying along at 150 CAS at 35,000ft and pull-up abruptly, the resisting force provided by the air will not be as great as it would at a lower altitudes. So the aircraft would be "easier" to accelerate, and would reach the design limit load factor sooner than it would at a lower altitudes.

I'm not sure if this paints a clearer picture??

 

[VNugget]

Thanks for the response sled.

By "dampening" I was referring to the fact that the air density is lower at higher altitudes, and so the air will be providing less resistance to the motion of the a/c. In the book "Handling the Big Jets," the author explains it as follows: Imagine a weight attached to a spring- if it's placed in a vacuum and you displace the weight, it will continue to ocillate forever (no air= no resistance/friction force to bring the weight to rest). If, however, you let air into the vacuum, then the would be slowed down by the airloads due to the motion. These airloads always oppose the motion of the object- it is this aerodynamic force which provides the "damping."

So the way I was looking at it is that if you are flying at higher altitudes (or higher density altitudes), there is less air to oppose the a/c. So if you were flying along at 150 CAS at 35,000ft and pull-up abruptly, the resisting force provided by the air will not be as great as it would at a lower altitudes. So the aircraft would be "easier" to accelerate, and would reach the design limit load factor sooner than it would at a lower altitudes.

I'm not sure if this paints a clearer picture??

The whole point of using CAS is that it's defined as the same dynamic pressure. At 150 KCAS at high altitude, and at 150 KCAS at sea level, the effects on the plane are the same. There's no greater or lesser "resisting force" and the plane would not be easier to accelerate.

As far as the guy's analogy, in a vacuum there'd be nothing to set the weight in motion in the first place. A plane's only interaction with the "Rest of the world" is through airloads on the surfaces, and in a vacuum the CAS is zero always.

 

[ptarmigan]

A gust at altitude has less force, due to the q factor for a given velocity being less (TAS of the gust matters less than the IAS, in simple terms). I think the reference you gave in Davies' book is actually reference the damping effect of aerodynamic surfaces wrt aircraft stability.

As for the speed, it gets impacted by mach effect more than anything else. The only possible relation I can see to damping is that the stick force/g would be less at altitude due to lower stability. That (in itself) wouldn't affect the speed itself, but could lead you to exceed a design limit or stall more easily.

 

[mar]

Hey Chris.

How's your research going?

You know the funny thing about that Davies book is that it was written for pilots in the '60s who were transitioning to jets (from prop transports) for the first time.

Since then, aircraft design has made leaps and bounds, but still I understand your question is mostly theoretical.

As the others have mentioned, IAS is really the important point when discussing Va. You already mentioned how weight plays a role but have you had a look at the V-G diagram?

I don't know if they use these in Canada, but on the written tests in the US you need to interpret a V-G diagram.

It's real basic: the flight envelope plotted against two axes: Speed ("V") and "G" force.

G limits: +3.8 to -1.52 (plus the design margins)
Speed limits: Stall to Redline

As you know, if you exceed +3.8gs you're either gonna bend the airframe or stall....depending on your speed...and your speed is just the total of dynamic plus static pressure.

So while "aerodynamic dampening" *may* have an effect on general "controllability" issues it has virtually none on the aerodynamic issues relating to Va (stress or stall).

I hope that sums it up.

 

[VNugget]

As you know, if you exceed +3.8gs you're either gonna bend the airframe or stall....depending on your speed...

I think you misphrased...

3.8G is 3.8G, and is the same margin from breaking the wing, regardless of speed. Speed is what determines whether you can exceed the 3.8G in the first place. (More speed = more available G)

and your speed is just the total of dynamic plus static pressure.

Actually your (indicated) airspeed varies with dynamic pressure only.

Anyway, a picture is worth a thousand words, so here is a V-G diagram, along with some discussion, for those who haven't seen it.

 

[mar]

Thanks for the link

Good link to the V-G diagram but I've got a few things I'd like to clear up.

3.8G is 3.8G, and is the same margin from breaking the wing, regardless of speed. Speed is what determines whether you can exceed the 3.8G in the first place. (More speed = more available G)

Airplanes certified in the "Normal" Category are approved to pull 3.8g's. If you exceed 3.8g's the wing won't *necessarily* break off. These load factors are called "limit" load factors up to which it's perfectly acceptable to operate. BEYOND the "limit" load factor, the manufacturer has built in another margin where they say some damage to the airframe or components may result if exceeded....up to the "ultimate" load factor where structural failure is likely.

Furthermore, speed does *not* determine g-force. Granted, in a level turn, with a given bank, the higher the airspeed, the higher the g's, but there are other sources of "acceleration" (e.g. the radius of an aerobatic loop or severe turbulence).

Actually your (indicated) airspeed varies with dynamic pressure only.

Well...ok. I won't split hairs over this statement but I see what you're saying. I got sloppy and should have explained that the pitot tube senses *total* pressure (dynamic + static) but static pressure is removed from the equation and so only dynamic pressure is presented as IAS.

Fair enough.

 

[VNugget]

Airplanes certified in the "Normal" Category are approved to pull 3.8g's. If you exceed 3.8g's the wing won't *necessarily* break off. These load factors are called "limit" load factors up to which it's perfectly acceptable to operate. BEYOND the "limit" load factor, the manufacturer has built in another margin where they say some damage to the airframe or components may result if exceeded....up to the "ultimate" load factor where structural failure is likely.

Yup. Ultime load = limit load * 1.5.
Notice I said "margin"; I didn't say it'll break at 3.8G.

Furthermore, speed does *not* determine g-force.

Yup. I didn't say it does. But it determines available G-force. I.E., the faster you go the more G you can pull if you want.

Granted, in a level turn, with a given bank, the higher the airspeed, the higher the g's

Actually, a given bank in level flight will give you the same G, regardless of speed. Turn rate and radius will vary, though.

 

[mar]

Sometimes it's just not worth it.

I really wish I'd left this discussion for the morning after some coffee.

Look, I think you have a point regarding bank angle, g-force and radius of turn....that's some more lazy and sloppy thinking on my part which I obviously try to avoid.

On the other hand, I think you're confusing some terms.

Limit Load Factor and Ultimate Load Factor are two completely different things. LLF is "just" a limitation. ULF is also a limitation but you'll probably die.

I really don't understand your use of the term "available g-force". I've never studied that.

To sum it up for tonight I'll say, thanks for the corrections but I really should've known better than to tackle aerodynamics on a Sun night.

Ciao.