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Bruce
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Windshield Wipers

Tue Jan 11, 2000 3:03 pm

I've been studying several airliner photos and had this thought. What penalty in terms of aerodynamics does the windshield wipers on a jet have? I realize pilots need to be able to see in the rain while on the ground but they stand away from the fuselage surface so that must mean the aerodynamics are reduced, right?

Even on cars, designers try to "conceal" the wipers under the cowl to smooth out the airflow from the hood over the windshield. And that's only at 60 mph!! Imagine a jet at 600 mph - the airflow up the windshield!!

Have jet designers considered some kind of concealed or retractable wiper system? Which jets are the worst in terms of wipers that disturb the airflow?
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
Buff
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RE: Windshield Wipers

Tue Jan 11, 2000 5:12 pm

In 6,000 hours on the Dash 8, I think I used the wipers while airborne twice.

On the 757, I've used them only on the ground.

However, due to their location, and their relative small profile, and the probability that they are largely protected by boundary layer, I don't believe there is any performance penalty. If there is, I would casually estimate less than 1/8 knot.

That's a very unscientific answer I realize.

Best Regards,

Buff
 
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Bruce
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Boundary Layer

Wed Jan 12, 2000 5:29 am

What is a Boundary Layer?
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
Guest

RE: Windshield Wipers

Wed Jan 12, 2000 6:15 am

The boundary layer acts as a "bubble" to the aerodynamic structure of the aircraft. It covers a small distance adjacent to the body of the aircraft, where velocity is reduced to zero(at the wall), up until free stream value (outside vortex). In other words, it is a region where a change in the airflow will not affect the airfoil and the surrounding air distribution.

I agree with Buff in that the windshield wipers would be protected by the boundary layer. Most of the air distribution in front of the aircraft will be deviated by the vortex created by the aircraft's nose, preventing a disturbance from small structures like that.

However, in the case of a take-off or a lifting force, that layer is very small and can be neglected, so if the wipers affect anything, it might be at that time. Even if it does, I reckon it is a very small alteration.

Cheers,
Leo-ERJ
 
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Bruce
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Boundary Layer

Wed Jan 12, 2000 7:34 am

So the "boundary layer" would be what allows good aerodynamics even with things like those probes that stick out of the fuselage, right? Those things then wouldn't deter the aerodynamics.
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
Fester
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RE: Boundary Layer

Wed Jan 12, 2000 2:14 pm

Hi folks !

I fly CRJ's for Atlantic Southeast Airlines (Delta Connection) and would like to chime in here with some info. First, the "boundary layer" of air exists NO MORE than a few millimeters above the surface of an aircraft so no the windshield wipers are not "protected" from drag by it. Second, yes the wipers, pitot tubes, vanes, loose rivets, etc. ALL produce drag . . . butt-loads of it. It's called parasite drag and it's effect on aircraft is exponential (i.e. to double your speed is to quadrupal your drag) so engineers try to design stuff as sleek as they can but windshield wipers still create alot of it. Proof of this is that the loudest thing that I hear up in the cockpit of the CRJ is not engine noise or airstream noise but the "whistling" of air past the wipers!

Fester
 
JETPILOT
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RE: Windshield Wipers

Wed Jan 12, 2000 2:34 pm

I agree with Fester.

Have yo uever noticed the hood of your car when it is raining and your driving down the highway at 60 miles per hour. The rain drops are hardly moving. This is because the thin layer of air (boundary layer) is hardly moving due to the friction of the air against the cars surface. The farther away from the cars surface you go the less friction there is and the faster the air is moving.

The faster you go the thicker the boundary layer. But the windshield wipers do definetly cause some serious drag.

Douglas did away with widshield wipers on the DC8 and used engine bleed air to blow the widshield clear. Look at a picture of a DC8 and just try finding wipers.

We use Rain-X on our widshields. Pilot supplied and applied.

The reason these "probes" stick out several inches above the fuselage of airplanes is to get them out of the boundary layer to get accurate measurments of airspeed.

 
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RE: Windshield Wipers

Wed Jan 12, 2000 2:35 pm

I guess that will explain why you don't get wing-mirrors on an airliner!
 
Guest

RE: Windshield Wipers

Thu Jan 13, 2000 5:45 am

The boundary layer might be a few millimeters at a given moment. Because its profile is a differential, it is not safe to say that it will be only a few millimeters long. During cruise altitude the air distribution along the boundary layer on the forward airfoil system is sufficient to eliminate drag caused by the wipers. However, you will not stay at a constant velocity, altitude, wind direction etc.... throughout the entire flight. So that is when the layer is not sufficient overcome the drag caused by it, especially when a lifting force acts upon it. Other than that it is enclosed and will not alter much the overall distribution.

Cheers,
Leo-ERJ
 
VC-10
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RE: Windshield Wipers -Boundary Layer

Fri Jan 14, 2000 12:08 pm

One of the problems suffered by the VC-10 during development was higher than anticipated drag. Research found that one of the problems was the boundary layer towards the rear of the a/c was 11 inches thick !! That is why on the Super VC-10's the engine nacelles were further outboard as compared to a Standard '10
 
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RE: Windshield Wipers

Sat Jan 15, 2000 3:55 am

Thanks VC-10. I guess it substantiates the idea of a variable boundary layer with respect to an induced external vortex. The length of the boundary layer has a differential profile and will not be constant throughout. As you said many airframes had to be altered to this fact. I would guess that an 11-inch layer towards the aft. fuselage is quite enormous. Thank goodness changes were made, as a huge drag is created when it exceeds the pressure points of the layer, and interferes in flow. The layer does not reach such a thickness in the front airfoils mainly because the velocity distribution is much higher, so certain profiles will not be sufficient to drag.

 
VC-10
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RE: Windshield Wipers- Boundary Layer

Sat Jan 15, 2000 11:30 am

Further to my earlier post I recall that during my training an instructor mentioned that when he was under training in the RAF, to demonstrate the boundary layer principle they sprinkled some chalk dust on the wing of an Avro Anson which then went flying. When it returned the chalk was still there !
 
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RE: Windshield Wipers

Sat Jan 15, 2000 12:05 pm

Bruce !

Look at theese answers,-what an enourmous kwowleadge still in this forum,-I`m learning something every day !!

(spelling ?)
 
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Bruce
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RE: Windshield Wipers

Sat Jan 15, 2000 1:02 pm

I agree, URP
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens

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