questions about jets

[pplpeter]

Hello, I'm studying towards my PPL with the view to become a jet airline pilot and I had a few questions that my instructor couldn't answer. Where does pressurised air come from when used in a jet aircraft cabin? And I'm being taught to use the ailerons and rudder pedals when turning, do you use the rudders in a jet, as I have read that you don't, if not then why not? And roughly how much does a 747 weigh? This is the aircraft type I'd look to fly. Thank you in advance, Peter

[18-Wheeler]

The air that pressurises the cabin comes from the compressor section of the jet engine. If you look below ->

http://www.tsb.gc.ca/en/reports/air/1994/a94c0034/a6.gif

... the pressurised air will either come from the front compressor section or the rear compressor section, depending on how many rpm's the engine is doing. (Airflow depends on revs, and so when the engine is going slowly it's not passing much air, so the pressurised air is bled off from the high-pressure compressor)

A jet airliner typically doesn't need the rudder in a turn. A 747 has a gadget called a turn co-ordinator, and it works with the flaps extended and what it does is put a little rudder in for you to make a clean turn. About the only time you use the rudder pedals is to keep the plane straight on the runway, or for landing for the same reason.

A 747 Classic weighs about 172 tonnes empty, and 379.2 tonnes full. A 747-400 will be a bit more than that, with the heaviest being something like 404 tonnes I think .... ?

[Capt Claret]

G'day pplpeter and welcome to the forums.

I fly the Boeing 717. With an EFIS flight deck it's not fitted with the traditional skid, or balance ball. Instead it has an electronically created trapezoid (basically a triangle split horizontally in the middle), which is displayed on the PFD (Primary Flight Display) and gives similar indications of balanced flight as does the traditional ball.

If one is in a skidding or slipping turn, the bottom of the trapezoid slips into, or skids out of the turn, and to make the turn balanced, one "steps on the bottom of the trapezoid". That is, of the bottom of the trapezoid is to the left of the top, one applies sufficient left rudder to "squeeze" it back to a proper triangle shape.

Now, in the 717 if one doesn't use rudder in the turn, then one will be presented (usually) with a skidding turn indication. Some people ignore this, others, apply just enough rudder to return the turn to a balanced state.

I believe the advice not to use rudder in the turn in a jet stems from the introduction of the hydraulic powered rudder coupled with the speed of a jet. Too much rudder can be catastrophic.

[Conan The Barber]

Usually the yaw damper will provide turn coordination.

[AirRabbit]

Hey pplpeter:

First … welcome to the forum, and congratulations on working toward your PPL. You’ve received some good information from 18-Wheeler regarding aircraft pressurization and the weights of the B747 – of course, aircraft weights can vary greatly depending on the operation and the model of the aircraft being discussed.

Now, about the rudder. Every airplane needs something to keep it “coordinated” during a turn. Your instructor can explain the term “coordinated,” if you need some additional information. Most aircraft use the rudder to supply the necessary forces to keep the turn coordinated. Jets are no different in what they need – it’s just that some jets get what they need from different sources, but, actually, a yaw damper isn’t the source of this input. Some jet aircraft that use three separate autopilot axes can use the rudder axis to support this need, but that is usually insufficient for all cases. The controls that primarily allow jet pilots to make coordinated turns without using much, if any, rudder input (which is a tremendous training error in my not-so-humble opinion) are what is known as “roll control spoilers.” These are flat panels on the top surface of the wing that deploy with the input of aileron. The panels on the downside wing lift up into the airflow during a turn and “spoil” the lift produced by that wing, thereby assisting in the downward movement of that wing. This is in addition to the upward extension of the aileron on the same wing which causes the same thing. These extended spoiler panels also create some “form drag” on that wing, causing the whole wing to be moved rearward in relation to the opposite wing. This is essentially the same thing that rudder input would provide. Normally, the spoilers begin to rise with something like 5 to 6 degrees of aileron control movement and they can lift to a position of approximately 30 degrees relative to the wing surface. If you’ve flown on a commercial airline or have seen videos out the passenger window during final approach, you have probably seen those panels lifting and lowering at irregular intervals during the approach. That is directly the result of the pilot’s use of the aileron controls. Each time the panels rise, the pilot is asking for that wing to move down a bit … the higher the panel rises, the more the pilot is asking for a downward movement of the wing. The interesting point is that with each spoiler panel extension, there is a form drag component generated that has the same effect on the aircraft as rudder displacement toward that same side.

That may be all the information you need … but, if you’d like more … just ask.

[DenizD]

Now, about the rudder. Every airplane needs something to keep it “coordinated” during a turn.

http://www.servicepals.com/gallery/data/496/49641/B2-Spirit-med.jpg

I blame this guy for all the confusion about rudders

[chornedsnorkack]

Is it then correct that whenever the engine surges (and fire comes out of the front end of engine), the bleed air system and cabin should also fill with fire, smoke and carbon monoxide?

Boeing 787, if it ever were to fly, would have no bleeds, and only electrical air pumps.

[ratarsedagain]

Peter,
Re your question about the 747-some more figures for you........

The following weights are the max structural limits for British Airways Rolls Royce powered 747-400's:

Zero Fuel Wt...........246,750kg
Taxi Wt..................397,800kg
Takeoff Wt.............396,890kg
Landing Wt.............285,760kg

Hope that's of some use!
Good luck, and hope you get to fly the beast one day:ok:

[Pontius]

Of course, one very useful aspect of the rudder that, so far, has not been discussed is keeping the aircraft straight when one of the engines fails (assuming you're not flying an Airbus or 777 ;)). If, for instance, your right engine fails, then you find yourself in a position with the same thrust coming from the left side of the aircraft and distinctly less from the right, causing the aircraft to yaw to the right. This will then require left rudder to yaw the nose back to the left and keep you on your merry and straight way. The good thing is, just like elevators on light aircraft, airliners have trim for all of the flight controls (ailerons, elevators/horizontal stabiliser, rudder), so you don't have to sit there all day with a bootful of rudder in to keep the thing straight. Once you're level and the thrust of the live engine(s) on the opposite side to the 'dead' engine reduces, then so does the need for as much rudder input and, therefore, rudder trim. Finally, the further from the fuselage the engines, the more of a yawing moment in produced. For example, a 717 with it's engines mounted on the body of the aircraft will produce far less yaw than a 767 with the engines out on the wings, just as the failure of an inboard engine on a 747 will have less effect than if an outboard engine gives up the will to live.

Apart from that, jet pilots are very lazy because they don't use the rudder pedals, apart from a bit of steering during takeoff and landing and somewhere to rest your feet during the hours of cruise.....until it's time to go into the bunk. :ok:

[AirRabbit]

Pontius –

I don’t necessarily disagree with your comments – in fact, the one about jet pilots becoming “lazy” because they aren’t regularly required to use the rudder is dead-on correct. That is why I say most jet pilot training programs are very incomplete without including a series of training tasks where the rudder is used. However, the primary point I was making was about coordinated flight. Most of the time this “coordination” becomes an issue during normal operations during a turn – of course, that coordination becomes an issue with or without turning when experiencing engine malfunctions and asymmetrical flight control drag – all of which are corrected with proper use of rudder to regain and maintain coordinated flight.

Also, while the pure understanding of engines mounted on the aft fuselage vs. mounted under-wing gives one the impression of requiring more rudder for asymmetrical thrust situations with wing-mounted engines than with fuselage-mounted engines is understandable, that actually isn’t exactly true. The reason for this anomaly in understanding is that with the engines mounted on the fuselage, the aerodynamicists that designed the airplane got away with using less vertical fin area and a correspondingly less rudder surface area – in the interest of saving weight. Therefore, when an engine malfunction occurs on the aircraft with aft fuselage-mounted engines – the amount of rudder pedal control required is actually at least as much as required on airplanes where the engines are mounted under-wing. Ask any DC-9/MD-88 pilot.

And, a note for DenizD … while the B2 certainly looks like it hasn’t any horizontal control (rudder), by using the computer to deftly apply those split ailerons appropriately, the B2 yaw control is maintained quite nicely. As said, EVERY airplane needs something to keep it coordinated.

[smo-kin-hole]

Other fun things about jets:
-They are far more difficult to deal with when you have to abort a takeoff.

-They produce most of their useable engine power at the upper third of the
throttle.
-They don't slow down immediately at the flare when throttles are reduced
like a propeller plane.
-If the engines are close together like a DC-9, single-engine flight is much
easier than if the engines are on the wing. Of course, a yaw-dampener
helps alot.

You can buy jet manuals on Ebay and some flight simulators are dirt-cheap and have really good information in them. I suspect by the time you retire, most airliners will be entirely autoflight, possibly flown by teenagers on sim consoles only when there is an emergency. Just wait and see. Good luck!

[Lookleft]

Any multi-engine aircraft needs rudder input if an engine fails which includes airbus. The main difference with airbus is that you don't need to arm wrestle the side stick to control the roll when you put the rudder in.

[AirRabbit]

-If the engines are close together like a DC-9, single-engine flight is much easier than if the engines are on the wing.

Please ... talk to someone who has flown these "easier-to-handle" jets before you make such statements ... you just might be surprised to find out that they won't agree with you!

[18-Wheeler]

Any multi-engine aircraft needs rudder input if an engine fails which includes airbus. The main difference with airbus is that you don't need to arm wrestle the side stick to control the roll when you put the rudder in.

You don't in any Boeing either - If you are doing it right, all you have to do is a little bit of aileron and apply enough rudder to keep it all straight. The fine tuning is simply enough rudder to keep the controls (aileron input) level.
It's very easy.

[SNS3Guppy]

Other fun things about jets:
-They are far more difficult to deal with when you have to abort a takeoff.



Really? They are?