Why such loud flow noise on some Canadair Regional Jets?

Question

I am a frequent flyer from KMCN to KATL via Comair and ASA; the usual equipment is a CRJ, either the -200 or -700 variant. On the northbound flight (usually AM), the flight is typically very quiet - conversation is easy at a normal tone of voice. On the southbound flight, however (usually PM), once the plane exits the departure pattern, there's a very definite - and very LOUD - flow noise that starts up and stays with us until the pilots throttle back and extend flaps for approach. The noise is so loud as to inhibit conversation anywhere on the craft. These flights are short - 10-15 minutes, and if I had to guess, I'd say they operate in the neighborhood of FL90. I haven't been able to correlate presence or absence of the noise with variant type, operator, age, or any factor other than northbound vs. southbound. Any ideas? Thanks!

Answer 1

I think you are asking about the sound of airflow in the cabin. This is most likely the air conditioning system. On the CRJ 200, we usually taxi and takeoff with the air conditioning running off the APU. The airflow is constant when running off the APU. After takeoff, we switch the airflow to run off the engines. The amount of airflow is dependent on the power setting in use. So, after takeoff the engines are operating at a high power, thus more airflow (and it is a little louder). On the CRJ 700, the air is better regulated, so there probably isn't a drastic change in sound. I almost always shut down the APU after engine start because it has a much better air conditioning system. That's all I can think of regarding airflow.

Most of the other anwers are not correct. Winds and crosswind component don't have any noticeable affect on the sound you are talking about. The "ram air turbine", or in the case of the CRJ 200/700, the air driven generator, is NOT deployed unless you are having a really bad day. Nobody tests them on passenger flights. The engines on both types of aircraft are tubofan. And the "drag" answer, the drag would be constant on both aircraft. You wouldn't know the difference in sound.

PS I'm not trying to make fun of everybody's answers. I'm just trying to share my experience having flown thousands of hours in both types of aircraft.

Answer 2

The only thing that could make sense would be inlet distortion due to the cross wind component; but that should not extend all the way to the front of the cabin, and during normal flight, the aircraft heading is to compensate for the coss wind anyway(crabbing).

Note that flying North will have to be done at a different altitude than flying South, as per airplane navigational rules.

Do you notice a change as a function of the environmental condition (strong winds versus days with very little wind)?

The final thing I can think of, is that the pilots are deploying the RAT (ram air turbine) for a test. When those come out, they do make a hell of a racket. But to have that tested all the time seems a bit excessive.

Have you considered talking to the crew? I mean the pilots must know there is a difference in the noise level as a function of the direction.

Answer 3

The CRJ is based on the Challenger Series Business jets that were a lot shorter. The fuselage of these jets would have been optimized for minimum drag to save fuel and go faster.

When they stretched the jets, they kept the nose, tail, and fuselage cross section the same to keep costs down, but form drag would increase even though frontal area is the same on the CRJ as the Challenger. This could be the souce of the flow noise: the air not flowing smoothly abound the fuselage. A longer nose and tail section with a wider fuselage would have produced less drag.

Answer 4

The difference may be in the type of engines used.
Most quite turbine engines are turbo fan, meaning a fan produces 80% of the thrust needed for takeoff and a turbojet engine uses raw thrust to attain takeoff.

Problem is, if you restrict the flow of the hot gases on a turbojet, you kill the thrust. Do this, take a paper towel roll, scream through it with you mouth as if you are a jet engine. Note the sound. Then scream through it while blowing. Note the difference.

Turbofans are far quieter and more efficient than the turbojet.

Answer 5

The nosie you hear is most likely the "packs" that heat and cool the airplane. in the morning it is cooler and there needs to be less air coming in to the plane. As it gets hotter, the air conditioning works harder and makes more noise.

Answer 6

Newer aircraft have better acoustical damping and noise suppression/insulation than older aircraft. Maybe you are on an older aircraft when it is noisier.

Answer 7

Keep wind in mind. they might be using more power on the way back to overcome stronger headwinds.