A Rule of Thumb on Aircraft Design – You Can’t Cheat Physics

The drone industry may be on the verge of transforming air travel to some degree, but I sincerely doubt to the degree the media is telling you. There are many reasons that the innovation is moving slower than the Silicon Valley people would like. In this article, we will briefly focus on the main aspects of aircraft design and help you to understand you cannot cheat physics, when you are outside of the envelope the vehicle will cease to fly. The purpose of this article is that the people building new vehicles will use these rules of thumbs to be successful, they are not revered by designers by chance, this was figured out years ago beginning with the Wright Brothers. I am writing this because I was taught from former NASA employees, PhD’s, aircraft designers, self-taught and a couple of “fathers” of modern aircraft. It is my pleasure to pass this knowledge down because I’ve witnessed many blunders coming out of the drone and military worlds. I wonder have they lost their way? I am hoping they just forgot what our grandfathers taught us about wind passing over a wing.

Where to start? In manned aviation, it was mission based, what is the purpose of the mission? Is the mission a large passenger aircraft, or general aviation prop plane with an overhead wing configuration or maybe an innovative military fighter? It all begins with figuring out what the mission will require. Energy, Mass and Drag play a big part in the design. Energy is the propulsion system it requires engines or motors to produce movement, they require energy to keep the movement going. There are probably forgotten formulas for figuring this out so you could enter a weight, propulsion metrics and drag then figure out if the bird will fly.

Nothing New Under the Sun is a great quote from the bible: “Ecclesiastes 1:9 New International Version (NIV) 9 What has been will be again, what has been done will be done again; there is nothing new under the sun.” I believe that modern aircraft designers took this notion to the extreme and when building a new vehicle, they just take an existing airframe and slap what they need on it and hope it works. I have certainly witnessed this in the drone industry. What the aerospace industry needs to do is understand that new airfoils will not produce more lift than a conventional wing. Next we still need to know the purpose of the wing to design for loads, maneuverability and endurance. In my opinion and in many others, the most efficient manned airplane is a piper cub. If you look at the shape it is very simple yet very effective in lift, endurance, short take off and landings and is why the STOL industry uses the basic shape of these aircraft. If you need a vehicle to perform short to medium distance, carry a large load and still be able to land in a small area this design cannot be beat. This is just our first example as we move on we see that the specific mission dictates the design more and more.

Heavy lift is accomplished by having powerful propulsion systems and a very long and wide fuselage. Look at airliners and cargo planes they are designed around maximizing the space they have onboard to load up with equipment and transport it from A to B. The reason they need big propulsion systems is that it takes a lot to get it moving down the runway, once in the air it doesn’t take nearly as much power to keep it there, given the wings are shaped for the speeds that the vehicle will be operated.

Military fighter jets have a few purposes, they need to be able to travel to the fight, give them the fight and then return home. The biggest asset in a dogfight is power and maneuverability. When maneuvering an aircraft, you build up g forces along the wing called wing loading. The tighter the turn you make to throw off your adversary the smaller the wings need to be to give the least amount of wing loading. The wings also do not need to be shaped for lift as much as a heavy lift because of the overall weight of the vehicle and the massive propulsion systems. Fighter jets don’t fly like any other aircraft because the jet engine can power through gravity in most cases and the wings act more like guides than on other aircraft. When fighter jets engage enemies, it will be either a fast pass or a slow pass to start the action. To achieve this because you need to grip the air for maneuverability they have optional swept back wings that can be engaged or disengaged depending on what speed you need to maneuver.

Gliders have no engines and ride on the wind and thermals which is air rising due to convective changes in temperature between the ground and air. Gliders are built to stay afloat as long as possible and glide through the air, the wings are thin and flexible using wing loading to produce a bit of extra lift. Burt Rutan built a long-range airplane that was based off of glider principles that went around the world on one tank of gas. It is not by chance these vehicles are built and shaped this way.

I spent a lot of time looking at wind, all my life really as a kid I would get in front of a fan and experiment I was probably 5-9 years old and hadn’t picked up useful skills other than being a go-fer. While watching the wind, I begin to see patterns, movements and how drag worked in conjunction with a steady stream of air. I understood how a wing worked early because at first I thought it was magic or that the engines power was enough to get you off the ground and the wings were just what steered the vehicle. My grandfather corrected me on these assumptions taught me a lesson and sent me back to my fan. I cut pieces of paper and hung them all over the fan, then I would create airfoils cut out of paper and tape, then long streamers, then I started experimenting with the shape of the fan and next how the wing exited the fan by building covers and cones. I’m not sure where I was going with all the fan and wind time, but it has served me well later in life looking at an aircraft design and knowing if it was going to work well before most of the team knew.

Drones are mission based, but more so around the payload meaning what is the drone going to carry, and how far does it need to carry it? When I look at the Facebooks Aquila project I called failure on the first day they rolled it out because they busted many design principals, most importantly they didn’t have a working payload to beam the internet back down. The power constraints to beam wide area coverage and signals is extreme and would take a large heavy lift aircraft to achieve this, they essentially built a poor glider with a terrible airfoil. It eventually busted up at the end of the runway while enduring light and variable winds. I suspect the pilot dipped one wing too low it loaded up and snapped, a craft like that is not nimble by any degree.

Rotorcraft is similar to aircraft the rotor essentially is a moving wing and needs to produce lift for the vehicle to keep moving, instead of wind passing over an airfoil it moves over the rotors in a circle which can create some new physics that aircraft do not. The tails on rotorcraft are very vulnerable to problems with stability. The main rotors produce a lot of counter rotation that must be compensated so the vehicles compartment can stay level and facing forward. If the tail rotor stops the vehicle enters a spin in which usually a fatality is produced unless low to the ground. The rotors need to also be able to twist to (pitch) up and down to deflect the wind or to take on more or less lift. If the main rotor stops you are now coming out of the sky, this is very different than an aircraft which will continue to glide for up to several miles depending on altitude. When the rotors stop the pilot prepares for an auto rotate landing in which they time the pitch of the rotors to that of the ground then flare right before hitting the ground slowing the vehicle right before impact. Drones unlike rotorcraft do not have this ability to change the pitch of the rotors or props and this is a major concern for the industry for safety.

Flying cars are not new, the concept has been tried many times over starting with a Popular Science magazine I own dated in the 1920s. The reason they are not successful are because automobiles are not airplanes and vice versa. These modern flying cars look like Star Wars vehicles and would be great if we had laser or light drive propulsion, but we don’t. Almost every design I have seen is a failure many times over. The mere concept of making a flying car is ludicrous. Beyond the physics and safety aspects I will translate this into something everyone can understand which is dollars and costs. The vehicle will need to be certified by two different governing bodies Department of Transportation and the Federal Aviation Administration. Given the crash reduction requirements for automobiles will make for a heavy vehicle which will require greater propulsion systems and energy. None of the designs have variable-pitched propellers that can perform an auto rotate. Anyone who has had a motor failure on a drone will tell you how the vehicle bucks wildly and rotates in the air as the other props adjust and try to find level flight again.

Why do we see so many mistakes? Now this is my opinion is that drones are new and sexy and companies don’t want to be left out, however they could also be money funneling projects in the spend it or lose it cycles of budgets and are doomed to fail or will show a loss from the beginning. If we really want vehicles to work then these rules of thumb must be followed, there are more to discuss, but this should get your mind going enough for why these machines are mostly fantasies. It does take dreamers, but you can’t cheat physics. If your company is designing an aircraft and having problems, contact me [email protected].

Rob Thompson 

Co-Founder | Aviation Policy & Regulations                               

Falcon Foundation UAS L.L.C.

Maryland | District of Columbia | Virginia 

Email: [email protected]

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