The nightmare of a B-rated 50’s horror movie has come true. A multitude of King Kong’s cousins have subdued the earth and killed off the entire human race. Since all of the fighting and bloodshed is over, a few of the Kongs have settled in Daytona Beach, Florida. With nothing else to do they head over to the airport and find all of Embry Riddle's Cessna 172s sitting on the ramp. Since they are too big to climb in, they decide to have a paper airplane throwing contest of sorts.
A few planes are picked up and tossed. To their frustration some nose dive over and crash at their feet. Others pitch up steeply and slowly waft down to a point not much farther away. A few glide well and head south for New Smyrna Beach. The Kongs are frustrated with their gliders’ consistency so they head off across the peninsula in search of other entertainment. You can probably guess why some 172s did well and others didn’t. Obviously, the Riddle students who flew the planes last didn’t follow the "After Landing" checklists and left the trim wheels in wildly different positions. Some were left full nose up, others full down, while a few were reset to the takeoff position.
In similarity to our previous articles, the Riddle 172s only had three forces acting on them. In this case the three are Lift, Weight, and Drag. We have found a scenario where one force is noticeably missing--Thrust. This situation brings up a key point in our understanding of the physics and realities of flight: every airplane, no matter how many engines it has, with a pilot in it or not, can be a glider. Some glide well and others don’t.
In the primary training world we hear so often about the “Four Forces of Flight” and “Steady Level Flight.” Yes, it is true that in steady and level flight the four forces oppose each other nicely. Thrust takes care of drag and lift takes care of weight. This, however, is just one of a multitude of situations we will find an airplane in. The Riddle planes that happened to be trimmed just right so as to glide at the toss of King Kong, are also in steady flight. In short, all of the forces on these planes have figured out a way to oppose each other. These airplanes though are in steady and descending flight.
We have two ways to look at the forces acting here, both in the “normal to us” vertical and horizontal way and also parallel and perpendicular to the flight path. Even though the second method is tilted, the two tilted directions are still perpendicular to each other so its a valid way of looking at the problem. Both cases are interesting and yield different insights.
In the vertical and horizontal case, we see that part of both lift and drag work together to oppose weight. We can see that in the middle diagram below with the large blue arrow showing the purely vertical part of lift and the tiny red arrow showing the portion of drag acting upwards. It’s interesting to think that drag is actually pointing upwards here which is a good thing; it's keeping us from speeding up and heading down towards Mother Earth faster than we may like. This is analogous to how a parachute works where the drag from the canopy points straight up. Shifting gears to the horizontal, we see that a sliver of the lift balances out the majority of the drag. This stands in contrast to steady level flight where thrust and drag balances each other out. Lift takes the place of thrust here.
Now from our other perspective we can look at the forces from our tilted viewpoint that is parallel and perpendicular to the descending glide path. We see that all of the lift (blue arrow) is opposed by most of the weight (middle diagram) and that drag is opposed by a sliver of the weight. In short, our “weight thrust” keeps the drag at bay.
Let’s shift gears now to the powerless plane itself. What about the airplane makes it go through the air at a certain speed and at a certain glide angle? As we said in the King Kong example, the position of the trim had a big effect on how well each airplane could glide. Let’s simplify the elevator trim system to a simple stabilator like Pipers have. Instead of a horizontal stabilizer with a hinged elevator and added trim tab, a stabilator is just a single wing pivoted by the yoke and trim system. In this case, we will take three Pipers and glue their stabilators in different positions. The first is full down, the second neutral, and the third full up.
If the Kongs took each of these three airplanes separately and just dropped them from a great height, each airplane would eventually stabilize at a certain Angle of Attack (AoA). Just like a weathervane is turned into the wind so will each airplane hunt for and find the angle of attack at which it can steadily pass through the air. They may waffle around for a bit finding this angle but eventually they will settle out. This AoA has nothing to do with how heavy the airplane is or how slick or rough the surface of the wings and fuselage are. In short, the AoA rules the day.
With the AoA of our three Pipers determined, let's see if we can figure out some more information about how each will glide. Think for a moment about the eventual path of every leaf that has ever changed colors in the fall and been separated from its tree. The moment that bond in the stem is broken, a slow descent for the ground begins. Since leaves are rather large for their weight, this fall through the air is usually a gentle one because each little bit of the leaf’s “wing” is only having to support a little bit of weight. A gliding airplane is very similar. The lifting surfaces of the airplane determine its AoA, as we’ve already discussed, and its weight combines with this AoA to determine how quickly the airplane will pass through the air. Of course, if the airplane weighed nothing it would have no desire to head for the earth’s surface and wouldn’t have any airspeed. If it weighed as much for its size as a leaf does, it would take a similarly gentle and slow downward journey. However, since airplanes are rather heavy for their size compared to leaves, they assume much quicker airspeeds.
We know from our intuition of slow flight here that the greater the AoA the slower the airspeed can be. This is because the airspeed and the AoA combine in symbiosis to generate lift, the opposing force to gravity. Wings have to deflect air to generate lift and the higher the airspeed the more air that gets deflected. The greater the AoA the easier it is for the wing to actually do the deflecting. This works out nicely so that the higher the AoA the less airspeed we need to do the deflecting and so we come up with a nice relationship between the two where we can trade off between airspeed and AoA and still get the same lift.
Now you may be wondering about the glide path at this point and for good reason. This is after all that oh-so-important number in the emergency section of every POH which shows how well your airplane will glide in the event of an engine-out. A crappy glide ratio like 5 to 1 and you’ll be lucky to make it farther than a five-year-old can chuck a rock. An amazing glide ratio like 40 to 1 could win you a spot at the winner’s table of a soaring event. Somewhere in between at 10 to 1 and you’ll have enough glide to pick a decent enough spot to put your Cessna down in the event of a hiccup.
At this point, the airplane can’t help but fly at a certain AoA due to the geometry of the wing and the tail. This AoA combines with weight to pick our airspeed. Drag, as you probably well know, depends on airspeed. The faster we go and the more parasite drag we get; it’s just like holding your hand out the car window. The faster you go the more the wind fights you. The slower we go the more deflecting the wing has to do through AoA to balance out gravity. That AoA works in concert with the wingtips to make wingtip vortices which create Induced Drag. So basically, our airspeed dictates how much drag we will have and the higher the drag, the worse our glide ratio gets.
So, now we have come full circle. We can finally answer the question of why one Riddle Cessna nosedived into the ground right at Kee-Kee Kong’s feet. The trim was set for a low AoA which required the airplane to accelerate to high speed to balance gravity. This high speed put the drag through the roof which gave it a glide ratio worse than a lawn dart. Another Cessna had it’s trim set at full nose-up which demanded the airplane to fly at a high AoA. It didn’t take much airspeed to help out the large AoA to make the necessary lift so it gently wafted down at low speed. However, since it was flying really slow, induced drag dominated and brought this Cessna down just as steep as the last bird we talked about. At least it didn’t hurt Ko-Ko Kong's toes when it finally made it down to earth. And finally, our last Cessna’s trim was set just right so that the AoA partnered perfectly with the best glide speed to overcome weight with lift. Since the airspeed was just right, parasite and induced drag both played nicely, with neither dominating the other. This allowed our final Cessna to glide off into the sunset and crash right into the downtown of New Smyrna Beach, ensuring King Kong would be the honest winner of the official Kong Family Embry-Riddle Sponsored Paper Metal Airplane Throwing Contest.
