How much does wind resistance slow a runner? How hard is it to drag a sled up a muddy hill? How much gas does a rocket burn going through the atmosphere on its way to the moon? What do all of these questions have in common? They all involve a struggle against friction. You may think friction and aerodynamic drag are a total nuisance and life would be better without them but let's take a look at how the elimination of drag would change flight as we know it. Through its absence, you might just catch a glimpse of how influential it is both in good and bad ways. Furthermore, you should also get a glimpse into the world of energy management and how airplanes trade energy back and forth between potential and kinetic energy.
In high school physics I remember the teacher saying things like, “let’s ignore friction here” or “assume the angle is small” or “neglect wind resistance in this case.” I couldn’t understand how you could just disregard things like friction and wind resistance when solving a problem. It seemed that doing so would mess everything up and change the problem. Early in college, I learned that this approach made problems understandable and solvable at our novice levels. We just accepted that we weren’t getting the whole picture yet. In later years, we added these factors back and although we could get more accurate answers, wow! it really complicated things! After solving problems the simple ways and the hard ways, I learned that cancelling factors was an art that required careful intuition. Cancel the wrong factor and you get an unsolvable problem or make-believe scenario. Cancel the right factor and the math would get easier and the result would be close enough to reality to be useful.
Today though, we do want to wildly change the problem from reality. We want to get a glimpse into a world where normal aerodynamic drag doesn’t exist. Let’s be clear though, we are only getting rid of drag. If we got rid of every other kind of friction, our problem would turn into a never-ending slip-and-slide of uselessness. Basically, what I mean is we aren’t going to get rid of the friction keeping our pilot’s shorts on, or the friction that slows the tires down after takeoff, or the friction in the engine. Just to be clear, lift, thrust, and weight are still in play here. So let’s follow along with Cheapskate Chester as he boards his Cessna 150 bugsmasher in the middle of a hypothetical hot summer Drag Drought.
Chester finally has a free Saturday to fly so he wakes up early and checks the Forcecast and sees the Drag Drought is still going strong. No storms with torrential drag drops are on the horizon. Nothing seems out of the ordinary as Chester climbs in and cranks up. His engine idles a little faster than normal since it doesn’t have to overcome any induced or parasitic drag like it used to. He just pulls the throttle back to full idle and doesn’t have any problem. Taxiing out for takeoff is no different than normal. Runup is routine. After making a half-hearted radio call on his cruddy communicator (don’t forget he’s a cheapskate), Chester takes the runway and pushes the RPM up to redline. Expecting a violent neck-snapping acceleration experience, he is disappointed at the usual slow lumbering of the 150 on takeoff. You might think that without drag, the takeoff run would be greatly reduced. However, since parasite drag only builds with speed and Chester is still relatively slow, the total lack of drag isn’t much different than having small amount of drag. Normally, Chester rotates and is in the air by the 1,500 ft marks. Today, he is airborne a hundred feet or so short of them. Not much of a boost.
Despite the abnormally normal takeoff, Chester is really surprised by how high he must lift the nose to maintain his normal climb speed. The vertical speed jumps up to a healthy 1,800 ft/min versus the usual 400 ft/min. Now this is more like it! Since the engine doesn’t have to constantly overcome drag it can devote 100% of its effort to climb.
Even though there is no drag, there is still a limit to how quickly this little plane will climb. Imagine an elevator with two motors that normally pull it upward. Lose one motor and your trip to the top floor will take longer. The usual drop in climb capability still slows his ascent since the engine has less air to breath for making power the higher he goes.
Reaching 6,000 ft in a hurry, Chester decides it’s time to level off and see how fast he can go. He pushes the nose down and keeps the rpm up on the redline. Up at this altitude on a summer day, our friend is usually happy to get 100 mph. The airspeed steadily builds. He blows past 100 mph and into the yellow arc. As the needle makes the last sprint for the redline, Chester reaches over and yanks the mixture back to cutoff and the engine quits. The airspeed settles at 160 mph, just under Never Exceed Speed. With the engine off, we are now just down to lift and weight. Thrust and drag have departed the picture.
Now Chester has never liked being up near redline. His instinct tells him to pull back on the yoke and slow down. He can’t think of any other way to try slowing down so he gives it a careful tug. Everything seems normal as he pulls back; the nose comes up, the altitude starts winding up, and the airspeed drops off. Also being afraid of stall, Chester decides to level off at 60 mph, six mph over the Cessna’s 54 mph stall speed. He checks the altimeter and finds he is at 6,730 ft. He pushes the nose down, the speed builds, and the altitude unwinds. Out of a sense of curiosity he decides to get cozy with redline again and is amazed to find himself back at exactly 6,000 ft with an airspeed of 160 mph. He repeats his climb and descent pattern ending back up at exactly the same ending altitudes and airspeeds as before. He is amazed! This is the first time Chester has ever witnessed the true one-for-one trade of airspeed for altitude!
Despite his amazement, Chester slowly realizes a horrifying fact. He can’t think of a way to get below 6,000 ft without going over redline. If he were to push the nose down, the airspeed would immediately go out of limits. From our vantage point, we can do the math to find out what his airspeed would be back down at sea level and it comes out at a whopping 452 mph. Chester knows his airplane could definitely not take that kind of airspeed structurally, and a landing at that speed would be foolhardy. Thankfully, since he isn’t burning gas, his trusty fuel gauges aren’t imposing a time limit on his flight.
He tries lowering his flaps, slipping, and even cracks his door but finds that none of these tricks help bleed the speed. The distant words of his flight instructor slowly start to creep back into his cranium. “Confess Climb Conserve Communicate Comply” The 5 Cs from lost procedures come back to his memory and so he starts to act them out. He has already admitted to himself that he has a problem. He climbs to slow down a bit and thankfully doesn’t have to worry about conserving fuel. He flips on his cruddy communicator, dials it to 121.5, and lets out a wailing cry for help. Thankfully, nearby air traffic control hears his plea and asks for his position and altitude. They inform him to circle and to wait for help.
After about an hour, Chester notices a large white vehicle off in the distance headed his way. A gargantuan rigid airship similar to those of days gone by climbs to meet him. The radio operator onboard this monstrosity asks him to climb until he reaches minimum airspeed and take up a northerly heading. He passes along his final airspeed and altitude and the airship maneuvers out in front of him. To Chester’s amazement, the airship carefully starts slowing down and the radio operator tells Chester to aim for the large hangar door open on the back of the airship. His 150 is slowly eaten by the airship and wing walkers gently guide his airplane onto the deck.
Overcome with curiosity, Chester immediately asks how the airship is able to slow down and finds that the engines onboard have reversible thrust. This lets the airship speed up, slow down, and also go down without overspeeding. Before long, the airship gently settles to earth at Chester’s home airport, and his Cessna is rolled off the back onto the tarmac. As the behemoth gasbag sails away, Chester’s friend stops by and in conversation asks if he has complied with the latest Airworthiness Directive for the Cessna 150 fleet: the installation of a reversible pitch propeller. Chester shakes his head in dismay as he recounts his recent saga of being stuck at 6,000 ft without a way to get down. If Chester ever gets around to prying his padlocked pocketbook open, he will be able to fly his 150 again without needing a rescue.
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