Always kind of wondered.
You can see that modern commercial airlines have windshield wipers. I know that they are not used in normal flight, but still, they must be able to withstand 600 mph wind when not in use.
Do the retract under a cowling to protect them? Just made of unobtanium? Little of both?
Don’t you mean Adamantium? (The metal named after the British musician 
)
Answered by an airline captain:
The speeds at which they can use them are limited (explained in detail in the video), and you can see that the wipers are below the wind stream when not in use (see around 2:58 in the video). Different aircraft obviously use slightly different designs.ETA: I also found this on another message board:
On preview I see I’m beaten by engineer_comp_geek’s good ref and post. Oh well. I typed it so you can darn well read it. 
Nope. They just sit out there in the wind all day long pressed against the glass along the bottom edge of the windshield. They’re stouter than the stuff on your car. But not hugely so. The spring that holds them down against the glass is a monster though. If you lift the blade, stick your finger in there and let go you’re gonna get cut bad. Ask me how I know. On second thought, don’t.
On narrow body Boeings (e.g. 737) and similar older aircraft there’s a pretty good kink where the relatively vertical windshield meets the relatively flat top of the nose. The wiper blade sits there in the kink. The airflow doesn’t make that sharp corner so well. The result is there’s a small permanent eddy there and not too much wind force on the parked wipers. Later designs from all manufacturers have a smoother transition from nose to windshield and the wipers are more exposed to the wind.
Most wipers have 3 speeds. Slow is about like medium on your car. Fast is like 3+ complete back-and-forth cycles = six-plus wipes per second. It’s insanely fast. And stupid noisy. You have to shout to be heard when they’re in high. Even thinking is challenging during that rapid cyclic noise.
When landing in heavy rain with high wipers the blade leaves about a 3" clear slot behind it. The entire rest of the windshield is opaque with water. It’s like somebody has a fire hose aimed at it. So you’re trying to spot the runway through this strobing rectangle of clarity that’s swiping back and forth across your field of view a few times per second. At least it’s usually dark and the wind is blowing; that’s a help. :eek:
As engineer_comp_geek quoted, in lesser rain they don’t work all that great versus just letting the airflow clear the windshield. So they don’t get that much use. Commonly in ordinary rain the pilot not landing will turn them on for a couple of swipes a few seconds before touchdown then cut them off.
I saved the best part of the wipers for last. They’re also our best icing detector.
Like in a car, the blade arm is held on to the motor shaft by a ~1/2" nut and cotter pin or safety wire. For various aerodynamic reasons if the airplane is going to start growing ice this small shaft and nut is generally where it’ll happen first.
So when we’re driving through the clouds low enough that ice is an issue every couple minutes somebody will lean forward, peer out there (using a flashlight at night) and loudly announce “No ice on the Captain’s nuts or shaft!”
That’s always reassuring to hear.
LSLGuy, thank you once again for the first-hand insight you bring to these discussions – fascinating stuff!
Though I must say, from a car owner’s perspective, if I was paying between $80.6 million to $400 million for a vehicle, I’d expect the windshield wipers to be both quieter and more effective! :eek:
Rainx™ my friend, Rainx.™
or
Pledge™ which is just as good but easier to apply. Do not exceed .92 Mach.
Yep thanks for the answers all. Specially LSL.
Simply Not Done. Years ago Boeing had a rain repellent product and a spray system to apply it. Long since removed from all aircraft and no longer offered on new ones.
@ enipla: Going back to the OP … The max airspeed of airliners, bizjets, etc. is about 400 mph and cruise speeds tend to be 275-350 mph. The airplane can go faster across the *ground *at higher altitude where the air is thinner.
IOW, if you think of “airspeed” as “how hard is the wind outside blowing?” you’ll find that it never blows harder than the force equivalent of 400 mph at sea level. With 300mph-worth being a more realistic long term stress applied by the wind. So that’s the target the blades need to be designed to survive against. Not 600mph or more.
Thanks. Yeah, I do know the difference between airspeed and ground speed. I should have thought of that. Still, 300-400mph airspeed must require some pretty damn robust wipers. I thought that maybe I was missing something.
Follow-up question: Does an airliner have a horn?
(I’m sure this question has been answered before, but this opportunity to ask again seemed sufficiently germane).
Yes they do. See http://boards.straightdope.com/sdmb/showthread.php?p=19670207#post19670207 from earlier this very month.
I assume that by “cruise speeds tend to be 275-350 mph” you’re referring to airspeed at sea level or low altitude. Otherwise I’m not sure what you’re saying as jet airspeeds tend to be much higher than that, but presumably are only achieved at cruising altitudes.
For instance from I could find, the cruising speed of a 737 is about 485 mph (for all models), and the 747-8 is much faster, rated at a cruising speed of Mach 0.855 and a maximum advised speed of Mach 0.90. I realized when doing the conversion to MPH that you have to adjust for altitude because the speed of sound drops in thinner air, but it seems that a 747-8 at 40,000 ft would thus be able to achieve a cruise airspeed of 570 mph and a maximum speed of 594 mph.
If you do the Mach conversion for sea level, you get something silly like a cruising speed of 656 mph for a 747-8, and I assume in reality at low altitudes it could only achieve a fraction of that, the kinds of speeds you mentioned above. I presume this is what you meant, but it wasn’t initially clear to me.
You are referring to ground speeds. LSLGuy was talking about air speed which is what matters in this case. An airliner travelling 500+ mph relative to the ground will still only show less than 300mph on the Air Speed Indicator at 35,000 feet because the air, and the force it exerts, is much thinner. The airplane (and its wipers) couldn’t care less about how fast it is moving relative to the ground. All that matters is airspeed and that is much lower at cruising altitude than ground speed.
No, I’m not talking about ground speed. A Mach number is certainly not ground speed.
I think perhaps the confusion is between indicated air speed (IAS) and true air speed (TAS). From what I can tell, if you only have an IAS indicator, it will underestimate your TAS by (very roughly) about 2% per 1000 ft of altitude. So when LSLguy says “cruise speeds tend to be 275-350 mph”, if one interprets that as saying that a jetliner’s IAS will (correctly) show a maximum cruising air speed of, say 350 mph at sea level, then applying the above rule of thumb, at 32000 ft that same jetliner will have a true air speed of somewhere around 570 mph, very roughly. Which seems about right. I think this was my confusion.
But this is not the same as ground speed, of course. If I’m understanding this right, if you’re flying into a 100 mph headwind in this example, at 32,000 ft, you’ll have an IAS of 350 mph, a TAS of around 570, and a ground speed of 470 mph.
It seems to me that the point is that indicated airspeed is all the wipers, wings and stuff care about.
He he he
Yeah I know but on a DC-3 if you would go out and hand polish it like it needs, it might help. Bawahahahah
…which makes sense since an airspeed indicator works in precisely that way - measuring the dynamic pressure of the air “hitting” it.
Bolding mine.
Yup. I didn’t want to dig into the details of airspeeds: indicated -> calibrated -> equivalent -> true -> ground. Much less mixing in Mach or converting between knots vs MPH. So I explained it as ground speed in mph with a reduction in indicated speed in mph with altitude due to thinner air having less impact force. The wipers care about impact force and are designed accordingly.
Ballpark we cruise at 250 to 300 knots indicated. 300 at lower altitudes reducing to 250 at very high altitude. Regardless of altitude, that converts to about 475 knots true. Which in no-wind conditions converts to 475 knots ground speed. Converting all those ballparks to mph by multiplying by 1.15 you get cruise indicated = wind blast of 275-350 and ground speed of 550ish. Which relates to the OP’s original claim about 600mph wipers.
Clear as mud yet? 
Right. Indicated airspeed is really a misnomer because it isn’t a speed at all. It is the pressure acting on the airplane and it takes a much higher velocity to produce the same pressure at high altitudes than at sea level because the air is much thinner at altitude.
However, indicated airspeed is one of the most important numbers in flight because it is all the aircraft surfaces care about. They couldn’t care less if you have a 100mph tailwind, headwind or how high you are as long as you keep those air molecules hitting the aircraft surfaces at a sufficient rate within a fairly narrow range. For airliners, that means you can you can fly low and fairly slow or high and fast but you can’t mix and match those parameters. The Airspeed Indicator needs to stay within the same narrow range either way.
If the Airspeed Indicator actually read much over 300 knots on a typical jet airliner at any point in flight, it would be in great danger of an over-speed condition followed by a possible air-frame failure. That includes the wipers too.
Max allowable indicated airspeed varies a lot by type.
Some RJs are as low as 320. DC-9 / MD-80 variants and 777 are 330. 757 & 767 top out at 350. 747 and 727 is (IIRC) 380. We typically stay 10+ knots below the limit. Not because the airplane turns into a pumpkin if you brush the limit. But because the tattletale systems mean a 3-second 5-knot excursion over the limit due to bumpy air buys you a couple hours of talking to the boss and filling out reports. The limits are always to be respected, but they aren’t points of brittle failure.
There’s a separate max Mach limit on each type. Workign together the combined effect of both is that the indicated limit is the controlling limit at low altitude and the max Mach is the controlling limit at high altitude. The crossover point is usually around 30,000 ft.