For (modern) gliders with highly optimized laminar flow airfoils , bugs (or dirt) are a non-negligible factor, especially in competition gliding.
There are some older laminar airfoils that are notoriously allergic to bugs (and rain), but all are to a certain degree.
This is also the reason why any glider pilot worth his or her salt will painstakingly clean their ship each afternoon and before each competition flight.
Sufficient bugs will degrade your performance by a noticeable amount - most gliding computers even have a special "bug" factor to include that into glide path calculation (even though the measuring and correllation of that is quite subjective).
I´ve had a few flights in somewhat biblically bug-infested conditions (summer, hot, low thermals) that more or less painted the leading edge black (like in this picture from an article on the DG Flugzeugbau website ).
Bug wipers are quite common on high-end and competition gliders and can be found e.g. at http://www.storka.at/ .
Or try to google for "Mückenputzer".
Of course bug wipers are not without drawbacks (concerning cost, installation, ease of use, drag etc.).
A friend of mine once stuck a banana peel on his wing and tried to remove that with the wiper, resulting in banana and wiper being stuck in the middle of the wing.
It took some creative maneuvers to clear that mess, by which time probably a lifetime aerodynamic benefit of cleaning had been spent.
As for the comparison with cars, their aerodynamics differ in a lot of ways.
In comparison to a wing there is no laminar flow to talk of at higher speeds (most of it is turbulent), which drastically reduces the influence of the surface smoothness on the drag produced.
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One of the main reasons why it's not a good thing to have dead bugs on the windshield is that it becomes very difficult to spot other traffic.
Other planes are just little specks at a distance and when your windshield is covered with specks from dead bugs it becomes very difficult to spot another aircraft in the distance.
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Bugs or other dirt can have several effects on a (light) aircraft: It will have some effect on airflow, however slight It may affect the paint or surface by corroding or leaving stains It can block pitot tubes or other openings
It can build up on windows and obstruct the view A heavy buildup of dirt could make the brakes less effective The effect of dead bugs on airflow is probably negligible (frost or ice would be a different story), but a blocked pitot tube can be very dangerous because the airspeed indicator will read incorrectly.
Dead bugs on the windscreen may seem like a minor issue, but even a small blind spot could hide another aircraft and prevent the pilot seeing it.
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A lot of bugs on the leading edge of a wing does significantly degrade performance.
Years ago I gave a lot of float plane instruction in a J3 with an 85 hp engine.
If the student was heavy and there were a lot of bugs, it wouldn't fly.
Clean off the bugs and it would.
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Mud dauber wasps are credited for at least two total losses: Birgenair Flight 301 and Florida Commuter Airlines Flight 65 .
Airfoil soiling can make a large difference to performance.
I don't operate aircraft, but I have operated wind turbines.
Older stall-regulated wind turbines can take a performance hit of 20% if they're not kept clean.
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The first thought is possibly that a build-up of dirt/bugs on the surfaces of the aircraft would affect the smooth flow of air and thus affect the amount of drag produced.
This could, albeit quite slight, affect the flight characteristics of the aircraft.
There is a secondary thought, certain substances - especially bird poo - are quite corrosive and could therefore lead to more structural damage which is extremely hard to spot.
You should keep your car clean for much the same reason - however on a car
the worst you're going to get is a little rust spot on your bodywork.
In the wrong place on a aircraft that could be a corroded rivet or bolt.
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A rough surface on the leading edge of the wing has a surprisingly large effect.
I tried to answer a similar/related question on the Skeptics site: Can a sandpaper-thick layer of ice reduce lift by 30 percent and increase drag up to 40 percent?
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The worst thing is a perfect flat surface, as you get a bond of static air to the surface.
The flowing air has to pull away from this.
The best I have seen is when the wing surface has millions of small features which make the air turbulent
- this is all on a macro scale at the surface.
The above flowing air rides over the turbulent air more freely. Like a stack of steel plate on thousands of ball bearings.
Imagine taking the bearings away and trying to slide the steel across the floor!
Sharks do this, their skin has details which stop the flowing water sticking.
Another way is the repel the air molecules at atomic level from the wing surface - reduces sticking.
You use a high voltage electrode on leading edge to charge the airSo
the wing and the air now share like charges and repel each other - less sticking.
Thx for the opportunity to share odd memories of random facts from decades ago.
I have ran out of skill now and will probably not bother you again,, Steve
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I know that the Supermarine company painstakingly blind riveted the leading edge of the spitfire wing as it proved faster.
They did a series of experiments some involving gluing split peas in rows to mimic domed rivets!
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It's noted in many POHs to keep your aircraft clean all the time, but is there a reason to this?  Dirt hardly add any sort of weight to the aircraft, bugs are extremely small and other things such as cars easily move even when dirty as a mud pig.    Does having dirt or bugs on your aircraft seriously affect the structure or performance signficantly? If so, why?
Why are dirt and bugs on an aircraft so bad?
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Sufficient bugs will degrade your performance by a noticeable amount.  It will have some effect on airflow and will corrode the paint. In light planes and gliders it is indeed a factor that will affect performance, especially on the leading edge.
Commenters agree that it is a factor