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Vzrc Vs Vso: Attn. Bellerophon  
User currently offlineRedcordes From United States of America, joined Jan 2006, 245 posts, RR: 0
Posted (7 years 9 months 3 weeks 4 days 4 hours ago) and read 2465 times:

How does Vzrc (zero rate climb) and Vso compare in std. atmospheric conditions at sea level for some different aircraft? I understand that the Concorde has published Vzrc speeds. Is Vzrc only relevant with an engine or two out? Why is it that even though the wing isn't stalled and the Vzrc speed is well above stall-speed that the aircraft can't climb? This seems to contradict information I've read in other threads. Please help.


"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
6 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (7 years 9 months 3 weeks 4 days 3 hours ago) and read 2458 times:

When you slow down, induced drag increases. This is especially so for delta wings. With enough drag, the power required will be equal to the power available, meaning the specific excess power is zero and thus your ability to add energy (climb or accelerate) is zero. The only way out of the situation is to descend.

Another way to describe it is to be on the backside of the power curve, when it takes more power to remain in level flight the more you slow down.

For most aircraft, the stall will happen at an airspeed high enough for this to be a non-issue. For other aircraft, this is not so and Concorde is one of them. Delta wings generate vortex lift, which is lift generated by vortices formed on top of the wing by separated airflow around the leading edges, which gives them a tremendous ability to generate lift at low speed and high angles of attack. However, this comes with a very severe drag penalty.

Rgds,
/Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 2, posted (7 years 9 months 3 weeks 4 days 1 hour ago) and read 2437 times:

Concorde doesn't 'stall' in the conventional sense as the flow on the upper wing surface seperates (by design) at quite a low angle of attack/ high speed. However, at slower speeds/higher angles of attack it's wing is designed to optimise the seperated flow into 'vortex lift' as mentioned above. So clearly Vs has no meaning in the traditional 'lowest flying speed possible' sense and Vzrc is relevant.
As alluded to above, delta's producing high lift at high angles of attack are extemely 'draggy' and have a large 'back side of the drag curve'.
This was tragically demostrated by the Concorde lost at Paris - unable to maintain height & speed on two engines with a gear that wouldn't retract. I read somewhere that the Vzrc in that condition was over 300 knots !

[Edited 2006-12-10 21:23:03]


...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (7 years 9 months 3 weeks 3 days 23 hours ago) and read 2421 times:

Quoting DH106 (Reply 2):
Concorde doesn't 'stall' in the conventional sense as the flow on the upper wing surface seperates (by design) at quite a low angle of attack/ high speed.

While this is not news to you I would like to clarify, as the above has the potential to mislead, that while delta wings don't stall in the conventional sense they do stall. This happens when the vortices collaps (burst). At that point, lift drops off rapidly and drag increases even more, just as with any wing.

Just to preempt the discussion from spinning off on a tangent.  Smile

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineRedcordes From United States of America, joined Jan 2006, 245 posts, RR: 0
Reply 4, posted (7 years 9 months 3 weeks 3 days 23 hours ago) and read 2416 times:

Do airliners with typical wings have Vzrc speeds or would these be below stall speeds?


"The only source of knowledge is experience." A. Einstein "Science w/o religion is lame. Religion w/o science is blind."
User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 5, posted (7 years 9 months 3 weeks 3 days 22 hours ago) and read 2414 times:

Quoting FredT (Reply 3):
While this is not news to you I would like to clarify, as the above has the potential to mislead, that while delta wings don't stall in the conventional sense they do stall. This happens when the vortices collaps (burst). At that point, lift drops off rapidly and drag increases even more, just as with any wing.

Yes fair point Fred.
Also - while re-reading I think I've confused the 300+knots speed I quoted as 'Vzrc' for a 2 engined gear down Concorde. This speed is I believe in fact the minimum speed at which the available power (i.e. 2 engines) will be enough to overcome/exceed the drag and allow continued flight. Not the same as Vzrc.



...I watched c-beams glitter in the dark near the Tanhauser Gate....
User currently offlineBellerophon From United Kingdom, joined May 2002, 583 posts, RR: 59
Reply 6, posted (7 years 9 months 3 weeks 3 days 15 hours ago) and read 2390 times:

Redcordes

…How does Vzrc (zero rate climb) and Vso compare in std. atmospheric conditions at sea level for some different aircraft?...

On many aircraft types, Vzrc will be below Vso, and the aircraft will stall before it runs out of excess thrust. I suspect that this is probably true for most aircraft types.

There are some aircraft types, particularly delta winged ones, will reach Vzrc before Vso.

However, unless we talk about a specific aircraft, it is hard to comment further.


…I understand that the Concorde has published Vzrc speeds…

Correct.


…Is Vzrc only relevant with an engine or two out?...

Vzrc is always relevant, in the same way that a stall speed is relevant, in determining the minimum safe operational speed of an aircraft; however, there is a significant difference. Broadly speaking, Vso doesn’t increase following an engine failure or two, but Vzrc certainly does.

So Vzrc becomes a lot more relevant when the total thrust available is reduced following an engine failure.

When we talk about Vzrc, we are talking about a rate of climb (even though it is zero), and an aircraft climbs because of an excess of thrust (not lift). The number of operating engines, and hence the total amount of available thrust, is critical to determining that speed.


…Why is it that even though the wing isn't stalled and the Vzrc speed is well above stall-speed that the aircraft can't climb?...

There is a very good answer from FredT posted above.

I would just add that he is referring to slowing down below Vmd, the minimum drag speed, when total drag also increases as the induced drag increases.


…This seems to contradict information I've read in other threads….

I’m afraid that’s the story of aviation!

You will have to make your own mind up about which threads, sources and posters you should believe.  Wink

Regards

Bellerophon.


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