What would be a good answer??

[silentbob]

What we be a good answer to the following question.

Why are aircraft considered to be less manoeuverable at high altitude?

Only a short 'bullet point' answer is required and there seem to be so many factors that influence the answer (Thin air being the obvious one).

Thanks for any assistance.

[noisy]

The silentbob school of getting other people to do your homework for you

As you've pointed out it's largely down to reduced density and viscosity of the air at higher altitudes.

Try typing 'coffin corner' into a search engine. I think this a condition where the Vne coincides with the stall speed of the airframe although I don't have a good definition to hand. (This is of course an extreme case)

Good luck with your assignment.

PS this thread would probably have been better off in the flight test forum

[ft]

Two suggestions off the top of my head.

Since the speed range between mach buffet and stall is smaller?

Since there is typically less power available (and power is what we convert into all kinds of peculiar accelerations)?

Cheers,
Fred

[silentbob]

Only seeking more profesisonal answers than i can find in my study guides

This coffin corner may be referred to as 'buffet boundary' perhaps? May be slightly different. Buffet boundary is a position on the corner of the V-n (flight envelope) diagram where positive and negative load factors are limited due to turbulent airflow on the tailplane exacerbating the airframe loading.

The thin air answer just seemed too easy. Perhaps That is all i need. I also wonder whether less manoeuverable means that is is 'physically' less manoueverable (ie cant control) or less manouverable in the sense that it can't be safely manoeuvered without encountering high speed buffet regions. Hmm. More thought required.

[oxford blue]

Because in order to have the right conditions for safe manouevre you need sufficient RAS (to avoid stall, to have a safe margin in turns, etc). Therefore your RAS is pretty well fixed for a given manoeuvre. However, at altitude, TAS is higher for a given RAS. As TAS gets higher, rate of turn for a given bank angle becomes less and radius of turn for a given bank angle gets greater. Result - less manoeuverability.

Simple, really.

[noisy]

The equation for the coefficient of lift contains the term:

CL= k x 0.5 x r x V˛

At altitude, the density of the air is lower and the indicated airspeed is low. Therefore the control surfaces will generate less force as will the mainplanes and empennage. This will result in reduced manoeuvrability and reduced stability.

The simpler the issue the longer the debate….

[Dantruck]

As it's a bullet point you want, how about:

'Generally speaking, ground speeds tend to be greater at higher altitudes.'

[noisy]

Sorry Dantruck, am I a*** about face here? I mean that there are fewer molecules hitting the back of the pitot tube the higher up you go, so there are also fewer molecules to support that big heavy wing and do work on the control surfaces.

I'll have to go home and stick my head in the avionics folder

[PAXboy]

This is not an answer but a supplementary question!

I see the reasons for the a/c to be more difficult to handle at high altitude, if you are able to increase your airspeed - do you regain some ease of handling?

Only asking as my boy hood dream is due to come true in something over 48 hours. On Friday 8th, I shall be on BA 0001, ex LHR.

[noisy]

Hi Paxboy

one of the limiting factors on a subsonic aircraft flying very high e.g. 777 is running up against compressibility effects. This can cause flutter and ultimately structural failure.
However, Concorde is obviously designed to completely ignore the high subsonic region and just kinda plough straight on into supersonic flow.

Don't know about stability and manouevrability in supersonic flight.

Enjoy!

(envy)

[av8boy]

Why are aircraft considered to be less manoeuverable at high altitude?

Some potential reasons:

1. Inverse square law. Every time an aircraft doubles its distance from a ground observer, it gets half as big. Pretty soon you can't even see it from the ground. Although its weight also halves every time the distance doubles, the weight-related distance is calculated from the center of the earth, not the point of view of an observer standing on the surface of the earth. At about 18,000 feet above the surface, the area of the control surfaces on the aircraft is no longer appropriate for the aircraft weight, and it becomes more difficult to maneuver. Although pilots have, for years, attempted to counter this effect by setting their altimeters to 29.92 as they pass through this critical altitude, the effects of the change are minimal and performance degrades, none-the-less.

2. As you know, an aircraft in flight is supported on a column (a cushion, if you will) of highly compressed air. Actually, "column" is probably the wrong word here, because the cushion of air supporting the aircraft is actually pyramid-shaped. From the edges of the aircraft wings the cushion extends outward and downward at a 45 degree angle until it is stopped by the surface of the earth. (This is why the areas near the ends of runways are clear of everything except for the highly reinforced structures supporting lights and the localizer. Anything in that area which is NOT built from incredibly strong materials will be crushed by the weight of aircraft passing directly overhead). At low altitudes, the atmosphere below the aircraft is quite compressed, because the weight of the aircraft is concentrated in a lower volume of air. This compression allows the aircraft to operate in an incredibly stable environment where control surfaces need only turn, bank, climb, and descend the aircraft. In contrast, at high altitude the ratio describing the relationship between the weight of the aircraft and the volume of compressed air supporting it is such that there is very little support provided. Therefore, aircraft at high altitude must devote the majority of their control surfaces to maintaining the stability of the aircraft. This leaves very little surface area to be used in maneuvering. Aircraft at altitude always run the risk of departing from controlled flight whenever a pilot touches the controls, because the aircraft is "concentrating" on just staying in the air. The "coffin corner" refers to the actual corner of the control surface that is left for use by the pilot in maneuvering. As the aircraft flies higher, the "coffin corner" gets smaller. At some point (depending upon complexities which I will not go into here) the ability to maneuver the aircraft at all disappears, and ANY control inputs are met with certain disaster. Because the aircraft cannot turn, climb, bank, descend, slow down, or speed up without being destroyed, it is a "coffin" waiting to run out of "gas."

3. It is not the aircraft, per se, which is less maneuverable. Rather, it has to do with the fact that there is less oxygen at high altitude and the ability of a pilot to function in this environment is impeded. Every few years we hear of an adventurous soul who tries to take his "bird" above 12,000 feet and does not live to tell the tale. Oxygen deprivation is a serious issue in aviation, and the extreme cold at altitude just makes things worse. Perhaps someone will find an answer for this scourge some day, but for now, we would all be safer if pilots stuck to the rules.

There are other reasons as well, but let's just start with these.

Dave

[ft]

noisy,
your comment on the lift equation about IAS could be understood to indicate that the V is the IAS. I'd like to point out that it is indeed TAS to avoid misunderstandings.

As the dynamic pressure will be essentially the same at identical indicated speeds close to the lower end of the usable speed region (i e same equivalent airspeed), the aero forces will remain largely the same.

OTOH... at high altitude you'll need to consider the difference between IAS/CAS and EAS as compressibility will come into play all the way down to Vs...

I do know that dutch rolls can occur at altitude when they do not down low... but I'll have to think (or even *gasp* read!) to come up with an explanation for that one. There's not enough coffee here to make that possible right now! Someone, point out the flaws in my reasoning please!

What are your air molecules doing hitting the back side of the pitot tube? Tell the daft ******s to get up to the front end where they belong!

As for GS, I wouldn't be concerned about turn radius way up there. The turn rates are proportional to TAS of course...

PAXboy,
if you are wobbling around at Vs, on the verge of stall, yes, you do regain control by increasing airspeed. If, and this is the crux of the matter, this does not put you above Vmo/Mmo which is the velocity/mach speed at which you run into compression related difficulties instead. When these two converge, the only way back into the flight envelope is down where there's a usable speed range again.

Cheers,
Fred

av8boy,
could you explain the implications of Passenger Fear Induced Lift on all that? Especially in cargo ops?

Ah, yet another reason to use hPa on the altimeters rather than inHg! Of course a small change such as 29.92 to 29.87 (change of 0.05 units) will not have any effect! If you instead change from 1013 to 1011 (change of 3 units, or a whopping 60 times as much!) things will really start happening!



/Fred