two questions Boeing 747
 

I have two questions regarding the Boeing 747 and would appreciate if another forum member could answer them.

1. Why does the 747 have a lower and upper rudder and an inboard and outboard elevator respectively. As they move simultaneously, why not have only one control surface?

2. The 747 is designed to cruise around Mach 0,85. In this context, what is the Mach crit for the 747?

Thank you in advance for your help.

Control surfaces are split into separate units to give redundancy. A long range aeroplane needs more backup systems in most areas of the aeroplane. The different control surfaces (there are 4 elevators) are powered from varying hydraulic systems to provide safety in the event of multiple failures.

The ailerons are split into inners/outers (all powered from differnt combinations of the 4 hydraulic systems). At high Mach Nos., the outers would have too much of an effect and cause wing twisting, so they are locked in place whilst the flaps are up and the inner ailerons, which are behind the inboard engine, are the only ones that move. In combination with the upper wing surface spoilers for large control displacements, this is perfectly adequate.

I think the Mach crit for modern jets is about M.93. This training is done on simulator conversions only usually now because of the expense and the fact that reproduction of flight characteristics on them is excellent. The 747-400 is very happy cruising at M.86, although these days this sort of speed is frowned upon.

Rainboe,
A wee bit of a typo perhaps? Perhaps you meant M0.83, that's about right for a B747 with a nominal cruise Mach Number in the range 0.85 to 0.86.

Regards,

Old Smokey

I think he was getting mixed up between MMO and Mcrit.

I was trying to answer the question as asked, although it seems a strange thing to ask. I assume you're right- he meant MMo (which rather than just a single figure, changes, and the max indication is the computer generated red area on the speed indicator). I gave the answer of about M.93 as that is the speed where high speed buffet is significant- I got up there in a VC10 once on a maintenace check, but I didn't like doing that sort of thing. As for stalling stick push tests, I swore never again!

Mach Crit
 

Hi guys,

thank you for your reply and efforts. I actually meant the critical Mach number of the 747 airfoil. This was in the context of swept wings. I read through some notes about calculating the swept wing critical Mach number by dividing the critical Mach number of the straight airfoil by the cosine of the sweep (37,5 in case of the 747). As the given example used 0,7 Mach crit for the straight airfoil and 30 degrees sweep, the resultant critical Mach number was 0,81 Mach. Since the 747 is designed to already cruise at 0.86 Mach, I was wondering what the official critical Mach number for that airplane is.
Nevertheless, I admit that the scenario I mentioned is pretty academic and Mach crit appears to be of little operational value compared to MMO.

Rainboe,

My sincere apologies if you thought that I was taking a swipe at your post, I honestly thought that you'd made a genuine typo, particularly when the B747 Mcrit is awfully close to M0.83. Be assured that your excellent and well thought out posts are on my 'must read' list.

I too, have 'taken the pledge' to avoid fully developed stall tests forever, I survived the last hundred or so, and that was a hundred or so too many.

tornadoflyer,
That's got to be the understatement of 2005! Sure, Mmo is an extremely relevant speed, but rarely used operationally. On a Day to Day basis, Mcrit is of extreme relevance in the determination of Climb, Cruise, and Descent Mach number. The high speed drag polar comes into play only after Mcrit is passed, whereupon total drag is then the sum of the Low speed and the High speed drag polars. Best Climb (Angle, Rate or Econ), Best Cruise (Max Range, Long Range or Econ), and Optimum descent (Max Range, Long Range or Econ) ALL depend upon the relationship between the Mach Number chosen / computed and Mcrit. For an 'older technology' (but beautiful) wing like the B747, Long Range Cruise Speed will usually be in the vicinity of .02 to .04 above Mcrit.

Regards,

Old Smokey

No apologies needed! I was unsure why the question was being asked. I had no idea what the actual Mach crit for the 747 was, but I do know that noticeable separation takes place and high speed buffet sets in on these type jets at about M.93, and Rainboe doesn't like that sort of thing! I guess some people make born test pilots and some don't. I'm on the 'don't' side!

Mcrit, MMO etc.
 

Hi Old Smokey,

thank you for your additional explanation. My flying background are purely military fighter jets and subjects like Mcrit, MMO, best range, and optimum descent etc. are probably less frequently discussed in my environment. Our daily business deals more with corner velocity, turn rate, sustaining-g etc. As I will be leaving the Air Force after 20 years in near future, I am in the process of getting adequately prepared for the interviews at the airlines and consequently I really appreciate comments from experts like you or Rainboe- there are excellent technical books on the market but sometimes they lack detail.

In reference to my original question you mentioned that Mcrit for the 747 is around 0.83 and Long Range Cruise Speed is higher, up to 0.87. As a direct result, MMO must even be higher for this type of airplane. Did I understand the interrelation accurately?

Hi tornadoflyer,

Yes, you've understood the interrelation correctly, although M0.87 is a bit on the high side for the 'Grand old Lady', around about 0.85 to 0.86 would be approximating LRC (flying B777s now, can't recall B747 numbers all that accurately).

The ascending order of useable Mach Numbers is - Mcrit, Max Range, Econ (these days), Long Range, and Mmo. Then of course there's Mmd, the Mach number for minimum drag, but this only comes into play at much higher altitudes than most airliners operate, where minimum drag speed is a function of Mach number, perhaps a tornadoflyer would be familiar with these sorts of altitudes :O

Regards,

Old Smokey

Hi Old Smokey,

thanks a lot for reply. As high altitude flying in the German IDS version is concerned, it is painful. Even with variable wing sweep the engine performance and response is marginal above FL 250. It is great to fly at low level but we try to avoid IFR flying above FL 280. I flew the good old Phantom before and with its J 79 engines it behaved in FL 370 like the Tornado in FL 280. High level (Fl 250+) air-to-air refueling in the Tornado, for example, is no fun.