Vertical Takeoff And Landing
While the idea of a vertical-takeoff-and-landing aircraft sounds interesting to just about everyone, few people are acquainted with the long and interesting history of the diverse designs that attempt to achieve this. A large fraction of the population of the western world has first-hand experience being flown inside conventional (non-VTOL) airplanes, but few have ever been inside a helicopter. And while airplanes dominate the aviation world, helicopters only fill small often-unseen niches, and VTOL airplanes and other VTOL machines are even less visible.
Many people today realize that VTOL airplanes exist only because they have seen True Lies (an action movie with a terrific Harrier scene) or heard of the troubles with the V-22 program. Many of the people who are aware of the JSF program (probably because of its escalating costs) do not realize it is a VTOL airplane (although this is probably changing since Die Hard 4 came out). Most people do not at all realize that this kind of airplane exists. And even those who are familiar with the designs of the Harrier, JSF, and/or V-22 do not realize that these airplanes are only the tip of an iceberg spanning half a century of extremely unusual, clever, interesting, unique, but ultimately unpopular airplane designs.
So VTOL technology has had extremely little impact in the world of aviation. VTOL aircraft (including helicopters) are extremely inefficient and heavy, having smaller payloads and greater fuel needs than regular airplanes. Despite this, the several varieties of VTOL technologies are so interesting, in my opinion, that I want to write a little text about them. I guarantee that, in reading it, you will learn about some amazingly clever designs, and about aircraft that do things you did not think were possible. Perhaps most importantly, you will learn about the reasons why good ideas sometimes just don’t catch on.
Let’s get to it.
Thomas A. Edison once said;
The airplane won't amount to a damn until they get a machine that will act like a hummingbird-go straight up, go forward, go backward, come straight down and alight like a hummingbird.
As we know, he was quite wrong. Airplanes that require long runways have revolutionized long-distance transportation, have allowed millions of people to cross continents and oceans quickly, safely, and relatively cheaply, have changed the meaning of warfare, and were an important stepping stone into space. They are also fun to fly and to watch, giving many people a kind of freedom that is impossible to enjoy while bound by the surly bonds of earth.
From even before the start of aviation, designing an aircraft that could hover was naturally one of the goals of aeronautics. Leonardo Da Vinci envisioned a platform that could be lifted by a ship-screw-like device spun above it by human-muscle power. Balloons came along in the centuries that followed, and dirigibles were all the rage in the late 1800s.
Dirigibles are great aircraft – they can go up, down, left, right, they can even hover in place without using any fuel… Sometimes I wonder if it is possible that aviation pioneers in the late 1800s and early 1900s could have been satisfied with dirigibles and never bothered to design gliders or airplanes at all. Luckily for us, aviation pioneers did realize that a winged heavier-than-air vehicle could move much faster than a dirigible (even – or maybe especially – with the primitive engines those folks had at the time), and that a small airplane could carry a similar payload to that of a much larger dirigible. So when it came to transportation (getting people and/or stuff from point A to point B), airplanes were just better. Airplanes were also much more agile, which made them easier and more fun to fly, and better-suited for combat, than dirigibles.
Still, airplanes were restricted to operating from long flat spaces such as fields or calm bodies of water. What if the same force that pulls an airplane forward could be used to pull it straight up off the ground?
Here we hit THE major reason why VTOL aircraft are not as practical as they sound. As we learned in the “Thrust To Weight Ratio & Lift To Drag Ratio” part of this course, the Thrust and Drag forces are usually quite a bit smaller than the Lift and Weight forces. An efficient airplane’s thrust might only need to be 1/20 to 1/60 of its weight in order for the airplane to be pulled forward fast enough to stay in the air. (Typically it’s more like 1/5). This means an airplane that weighs, say, 100 tons might only need 10-20 tons of thrust to stay in the air. But a VTOL airplane that weighs 100 tons obviously needs 100 tons of thrust to get off the ground. Since modern airplanes’ lift-to-drag ratio is about 5, 20, or up to 60, this means a VTOL airplane needs 5, 20, or 60 times the thrust of an equivalent non-VTOL airplane.
“So what?”, you may ask. “It only needs that much thrust to get off the ground. Once it transitions to normal forward winged flight, it can use as little thrust as a normal plane”. Yes, that is correct, except that the engines carried by the airplane weigh 5, 20, or 60 times as much as the engines carried by an equivalent airplane that did not take off vertically. (Remember that the engines are usually the heaviest parts of the airplane). This means that the useful payload is greatly reduced, and the added weight means more drag (and therefore less range and/or speed).
Despite these shortcomings, an aircraft that could use thrust for lift (therefore not requiring motion in order to get lift) seemed like a worthwhile goal. Helicopters were first successfully flown in the early 30s. Much like today, their roles were limited to situations where the ability to land anywhere and/or hover is key: Search and rescue, medical evacuation and transport, military troop transport, construction, and filming/journalism. But the helicopter’s short ranges, slow speeds, exceptional mechanical complexity, and extreme fuel demands, though, meant that a regular winged airplane should be used whenever possible.
Now here comes the interesting part.
While an aircraft that can use thrust to lift itself off the ground will necessarily be a little heavier and more complex than a regular airplane (since it needs big engines), it does not have to be as bad as a helicopter. Helicopters hover all the time. If you could hover only during takeoff and landing, and then direct your thrust forwards for regular winged flight, you could fly as fast, as far, and for as long as an airplane, and not need much more fuel. If this kind of dual-mode VTOL technology could be developed, most of the disadvantages of the helicopter would go away.
But how to build a vehicle with this kind of dual-mode ability? That is the million dollar question, and its answers have been almost as diverse as the rest of the history of airplane design.
When turboprops were first being introduced right after World War 2, some people noticed that these engines could provide more thrust than the weight of the airplane. Some airplanes could climb vertically for extended periods of time, their weight being completely cancelled out by the thrust of the engines. Theoretically, such an airplane could be sat on its tail like a rocket, and take off straight up like a rocket!
That sounds simpler than it is. For one, how do you control such an airplane? In order to control (i.e. turn/rotate) an airplane, you need air flowing over the control surfaces on the wing (ailerons) and tail (rudder and elevators). This problem is one all VTOL airplanes have to face, and there are now a handful of “standard” solutions: They range from placing control surfaces in the downwash from the prop, to ejecting high-pressire air from little holes in the wingtips and nose and tail, to changing the angles of the blades on the prop so that it pulled the airplane in different ways (much as a helicopter rotor does).
Most importantly when it comes to “tail-sitters”, when a pilot lands, he is landing the airplane onto its tail while looking towards the nose. In other words, he is landing the airplane “in reverse” while laying on his back and trying to look over his shoulder! As if a conventional landing weren’t tricky enough! In the end, this was the reason why the tail-sitter concept was abandoned: Any commericial application is obviously hopeless (how would passengers get in or out of a vertical tube in which they are laying on their backs?), and military pilots did not want to land on their backs in an aircraft carrier. (Some modern UAVs do use this technology, however).
Well, you don’t need to turn the whole AIRPLANE, just turn the ENGINES! Ok, but while a tail-sitter can fly straight up and then just turn nose-down into normal horizontal flight, in this tilt-engine design you need to tilt the engines forward while in flight. This was not possible at first, and only became possible when engines were powerful enough to (when set at a diagonal angle) keep the plane in the air AND accelerate it forwards at the same time. The tilt-engine approach was tried in many variations (including some aircraft where the wings turn upwards along with the engines), and eventually (after many accidents and many failures over many decades) found itself operational in the V-22 osprey.
Well, you don’t need to turn the whole ENGINE, just the part that moves the air back! This was tried in propeller airplanes where the propellers (but not the whole engine) was tilted up into a helicopter configuration so as to push air down instead of backwards, and in jets where the engine nozzles could similarly be turned downwards instead of backwards. This thrust-vectoring approach became operational in the Harrier jump-jet.
Or, instead of turning anything at all, just deflect the air downwards AFTER it is pushed back by the engine, and then retract (stow away) the deflecting mechanism for normal forward flight. This involves placing “bucket flaps” behind the engines. While some experimental VTOL aircraft managed to fly under this system, its main application is in STOL (short takeoff and landing) aircraft, primarily on the C-17.
Here’s another approach: A fan mounted inside a duct can be made much more efficient than a prop, since there are fewer blade-tip losses, and since the duct itself can act like a diffuser and nozzle, sucking air in the front and acceleratig it out the back. Many VTOL airplanes use ducted fans, the only “successful” one being the Lockheed JSF (the Boeing JSF was rejected since its Harrier-like thrust-vectoring setup required more engine power for vertical operations than the Lockheed JSF’s fan did). One more interesting application of the ducted fan is the flying car: an engineer named Paul Moller is trying to introduce a line of “Skycars” that use ducted fans along with the bucket-flaps idea. You have probably seen his designs on TV news shows or in science magazines. But like most other flying car ideas, it’s not really taking off (although he has a very cool working prototype, powered by Wankel-style engines).
One last approach involves having dedicated lift engines that turn off while flying forwards. Instead of having one huge engine and some mechanisms to rotate it (or its nozzle or prop or whatever), maybe have one big engine (or a few small ones) mounted vertically and a small engine mounted forwards. This may sound like a good idea (it’s heavy, but not that much heavier than previous ideas since you don’t need a turning mechanism), but it is actually VERY unsafe.
Why? Because you need ALL the engines to work in order to be able to fly safely. “Well, you need ALL the engines to work in order to be able to fly any single-engine airplane safely!”. Yes, ok, but if the chance of engine failure in a single-engine airplane is, say, 10% (which is too high, but bear with me, I’m making a point), then the chance that no engines fail is 90%, and the chance that no engine fails in a plane with two engines is 81% (that’s 90% squared), the chance that no engines fail in a plane with three engines is 73% (that’s 90% cubed), the chance that no engines fail in a plane with four engines is 66%... you get the idea. Whatever the probability that your engine will NOT fail, the chance that multiple engines will ALL work properly is that probability raised to some power (that power being the number of engines). Regular multi-engined airplanes can fly with one fewer engine than normal, so despite the fact that it is more likely that one engine will fail, you need a DOUBLE engine failure before you really get into trouble, and this is ridiculously unlikely (it’s the probability of an engine failure squared – so if the probability of an engine failure is 10%, then the probability of a double engine failure is 1%). Despite this risk, the Russians actually developed a naval VTOL airplane, the Yak-38, that used two lift engines. Most Yak-38s crashed.
One variation on this idea is the helicopter-airplane hybrid: A helicopter with wings and with a forward-propulsion system (not just a tilted rotor). The rotor can be used for takeoff and landing, and can provide lift throughout the flight as necessary (possibly as an autogyro, possibly powered), but during forward flight all of the thrust comes from the horizontal-pushing engine and most of the lift from the wings. Much experimentation has been done trying to combine the airplane and the helicopter in this way – an airplane with a helicopter rotor on it, or a helicopter with wings and with horizontal engines, depending on how you look at it – but none has been too successful. Modern autogyros do use this approach, but as you probably know, you don’t really see a lot of them around.
While discussing VTOL technology, one could also include the personal platforms: Rocket-belts, jet-packs, tiny personal helicopters, and many ducted-fan contraptions. Yes, those are real, they actually work. A couple of the ones seen in movies are even the real thing! While amazingly cool and very easy to fly, these are extremely expensive, terribly wasteful, surprisingly loud, and only have a range of 20 seconds to 5 minutes, not really long enough to get anywhere or even to really have much fun.
So now that you have a basic idea of the different kinds of VTOL aircraft (tail-sitters, tilt-engines, thrust-vectoring, bucket flaps, ducted fans, lift-engines, airplane-helicopter hybrids, and personal platforms), let’s talk about them. Yes, all of them. Well, all the ones that have ever actually flown, anyways. I won’t spend too much time on each one (especially as some have very little information on them available unless you’re willing to travel to the research group that made them and flew them, if it still exists), but I want to give you an idea of how many of these there have been, of how diverse they are, and of how they’re just so cool.
- ducted fans
- thrust-vectoring and bucket flaps
- airplane-helicopter hybrids
- personal platforms