Concorde Vzrc

[DuncanF]

I wonder if you guys can elighten me. I've been reading a few books on Concorde recently and the concept of Vzrc is introduced.

In Chris Orlebar's book he goes further to show a table of Vzrc for an aircraft carrying maximum weight with gear up/down and for 2/3 engines. This is used to illustrate the deep trouble the Air France plane was in once engine 1 went offline for the second and final time. That's where I get confused.

My question is: why does Vzrc vary with the number of engines? Surely the minimum thrust required to keep the aircraft in level flight is a fixed number, and if you can keep X knots on the clock why does it matter how many engines are used to achieve this?

Sorry for being at home to Mr. Thicky!

Thanks,

Duncan

[NW1]

If you are below min drag speed, then reducing speed increases drag. You have a fixed maximum amount of thrust available, so there will be a maximum amount of drag you can have beyond which level flight will no longer be possible. This drag figure will correspond to an airspeed - reduce further, drag will go up and you will go down. That is the Vzrc.

More engines=higher max thrust available, so more drag (slower flight) is possible in level flight. so Vzrc4<Vzrc3<Vzrc2, put the gear down and the Vzrc speeds go up.

Not a concept unique to Concorde, but the higher min drag speeds of the slender delta wing and its lack of a conventional stall speed (increase AoA and lift just keeps increasing) put the focus onto Vzrc as a reference for low speed flight.

Hope this helps.....

[DuncanF]

Thanks. I think the penny has dropped.

Level flight can be maintained but with increasing drag as we slow down which is offset with more thrust. So there must be a Vzrc with four engines at full chat, and gear up, below which speed the aircraft will start to resemble a (beautiful) brick I guess?

So on a hypothetical delta winged aircraft with engines of infinite power, is there an angle of attack where the vertical component of the thrust is actually supplying the lift and the forward component has dropped to a level which is not enough to overcome the drag by itself?

Sorry for the nerdy questions, but it intrigues me!

Duncan

[niallcooney]

Duncan,

The Shuttle would be a good example of your question. T/W ratio would be (in theory) about 8, first part of flight is vertical, Shuttle is delta-winged. Think about it, although lift doesn't come into the equation for the Shuttle, control is effected by gimballing.

Niall

[MK940]

NW1, great explanation makes perfect sense. What I would like to know is what does Vzrc actually stand for.
Is it Zero Rate Climb or something similar?
It's the first time I have heard of this speed.
Thanks in advance.

[Keith.Williams.]

DUNCAN

If an aircraft has a thrust:weight ratio of 1 it can hover (provided ther is a means of maintaining control with no airspeed). If it has a thrust:weight ratio of greater than 1 it can climb vertically, with all of the weight being carried by the engine thrust. The type of wing has no relevance here. But if the aircraft is to climb vertically above a fixed point on the ground it will need to be flown at its zero lift angle of attack.

[Bellerophon]

MK940

Yes, correctly worked out, Vzrc is the Zero Rate of Climb speed.

It is the speed at which the total available thrust exactly balances the total drag, and two assumptions are made. Firstly, that the (remaining) engines are all producing full power, and secondly, that the aircraft is clean with no unexpected drag.

So, when quoting a Vzrc speed, we can qualify it further by stating the number of engines that are operating, as Vzrc will increase with a reduction in the number of operating engines.

This is what NW1 meant by the terms Vzrc4 (all engines operating), Vzrc3 (three engines operating) and so on.

We can further qualify Vzrc by talking about extra drag, and for Concorde principally this is about whether the landing gear is up or down. We end up with terms such as Vzrc2 Gear Down.

This is the minimum speed at which Concorde could maintain level flight, on only two engines, with the gear still down, and is calculated for every Concorde departure.

If the gear could be raised, then total drag would reduce, there would then be an excess of thrust over drag, and the aircraft would begin to climb.

Regards

Bellerophon