In my last post I mentioned that there have been several attempts at making functional vertical take-off and landing (VTOL) aircraft. In this blog I’ll have a look at some of the ideas which almost worked, and what Passerine is doing differently.
“VTOL, the place good engineers go to die”
I can’t remember who said this to me, but the words themselves have stuck with me. It is true that many good engineers have tried to solve the problem I outlined in the last post: basically, efficient flight without needing a runway to take-off. Very few aircraft, if not none at all, have been developed which achieve this goal. With the many failures, there is a lot to be learned from previous designs. One of the first things which becomes apparent is…
People are creative, and stubborn
The diagram below gives a great summary of the different methods which have been used to create a VTOL aircraft. It is often referred to as The V/STOL Wheel of Misfortune; not only because so few designs worked, but also due to the high fatality rate in the development of VTOL aircraft. The diagram is from 1997 — over 20 years back — and yet, to my knowledge, the only ‘new’ VTOL aircraft to become operational since then is the F-35. It was in development as the x-35 at that point.
I find all these aircraft fascinating because of the a huge amount of knowledge the failed designs impart. The aircraft in the wheel are divided up according to their propulsion systems. More specifically, by how the systems are used in hovering and/or conventional flight. Most interesting are the concepts which use the same system for both modes, as this seems to be a more efficient use of resources. Intriguingly, the newest kid on the block, the F-35, doesn’t do this. It shows that there’s no definitive solution to the VTOL problem, which brings up the question: “Why have so few of them been successful?” In my opinion, most of these aircraft were too complicated to fly and too heavy to be truly efficient. This also links to what I’ve said before about many of these aircraft having too much thrust.
The few that worked
Very few VTOL aircraft have gone into production, and even less have actually been successful. Probably the most famous VTOL aircraft is the Harrier jump jet (below). It was developed in the ’60s and is still in use by air forces today. Of the VTOL design successes, nearly all of them are for defence force use. And it makes sense, because military jets are the prime candidate for VTOL aircraft, as generally, they are designed with excessive thrust (greater than their weight). This means that if the thrust can be directed downwards, they can work as VTOL aircraft.
The only VTOL aircraft which is not a military jet is the V-22 (below). This aircraft is able to tilt its rotors forward to act like propellers during normal flight. There are a few issues which make this solution undesirable for use outside of the military. The main one (subjectively) is noise. The V-22 is excessively noisy, even by helicopter standards, due to the rotors being a compromise between an effective propeller and rotor.
Great for STOL (short take-off and landing)
Most of the aircraft which were designed for vertical take-off actually performed better as short take-off platforms. Even the Harrier and V-22 mainly perform short take-off runs, with vertical or short landings. Another example of this is the F-35B (the newest plane on the list). At maximum flight weight all of these aircraft can’t actually perform a true vertical take-off, but rather do a short ground run, and sometimes ramp off a “ski jump” like ramp (as seen in the picture below).
Many of the stranger concepts, like the Ryan Vertiplane (below), also performed very well with short take-off runs, but were near uncontrollable when hovering (vertical flight). This is largely linked to the different requirements for positioning of thrust for hovering and forward flight, as well as structural concerns. Something very difficult to get right with a conventionally (fuel) powered aircraft.
A new era for VTOL aircraft
The onset of small electrically powered aircraft has solved a lot of issues previously faced by designers. Distributed thrust (mounting motors all around the airframe) is relatively easy to do, which helps with the control issues; and (at a small scale at least) very high thrust to weight is achievable with a very low cost in efficiency. As a result, a great number of electric VTOL designs are being created. Linked to this new propulsion method is the fact that many of these aircraft are unmanned, removing the other major difficulty: flying the aircraft. It’s a very exciting time to be in the aerospace industry.
Once these designs are scaled up, however, they run into many of the same issues as the ‘old’ VTOL aircraft. Power densities of electric motors drop as they get larger, and the batteries to power them get larger and heavier to compensate. This is a major problem, as unlike fuel, you still have all your batteries onboard at the end of a flight, making the aircraft heavier for landing. Too much weight, makes hovering impossible, hence a lot of aircraft just end up just doing short take-offs instead. I actually think it’s the best scalable solution to the V/STOL problem. Basically, try and avoid the part which generally causes the most issues — hovering. I would call this type of design a Standing Take-off and Landing Aircraft.
Standing Take-off and Landing
What I mean by this is that the aircraft can take-off from a spot, without a runway, and can land in a similar way. With the limitation that the aircraft can only perform forward flight. This design has the advantage of not being compromised by the requirement to hover. Many larger drones/UAVs fit this criteria. For example: Insitu’s ScanEagle is launched by a small ground based catapult (slingshot), and is recovered by a net or similar ground based device. This is not a bad solution as the space required to launch a large aircraft is massively reduced and the airframe can be designed purely for efficient flight. The draw back is that you need to have ground based infrastructure for both launch and recovery, which limits the roles for which the aircraft can be used.
Passerine Aircraft is taking this principle a step further; allowing this type of performance without the need for any ground infrastructure. We are using a light weight, bird like leg (pictures below) to provide the aircraft with its launch assist, and a unique end of flight manoeuvre to land.
The goal is to provide all the efficiency of fixed-wing flight, without the downside of needing an airfield for operation. We are using the lessons from failures in the past to make the perfect drone aircraft for the future.