UAV testing
 

I listened to a very interesting talk from 39 Squadron on the Reaper last night. As you'd expect you leave with a million questions, etc.

One subject that I wondered if people here know much about is how are these things tested? I understand that the USAF are investigating the possibility of deploying UAV in fighter roles - but as these UAV's are flown remotely isn't there a big problem of over stressing the airframe as you simply can't feel / nor have any consequence to extreme handling?

It's been the subject of a lot of SFTE and SETP papers in recent years.

The general approach seems to be of testing primarily fitness for purpose, with a small margin above the flight envelope structurally - rather smaller reserve factors than would be normal for any manned aircraft, and flying qualities are very much only tested in the middle of the narrow operational envelope.

Not been involved myself, but the papers are easy to look up.

G

This is no problem (compared to everything else that needs to be sorted) because fly-by-wire systems can be made to observe any limits your heart desires. And, if by some amazing chance you are not using FBW, then you can have a g meter on the ground as easily as heading or airspeed or height or whatever.

JF

Is there any need to get too precious about what happen if...? We worry too much about trivia. I'm not trying to be horrible, but if you do a YouTube search on "Black Sheep FPV" you'll see some 'amateur' UAV flying. The only real difference between what these guys do and MOD sponsored events is that BAE executives don't get their wallets filled in the process. And why the pre-occupation with fighters? Get one of these things properly loaded and you'll have a pretty good weapon. Who has a fighter which can shoot one of these things down?

PM

Very true Piltdown Man, thats why the supply of technology such as the autopilot and guidance systems is controlled by the Wassenaar Agreement and is classed as Dual use technology.

It should be rememembered that costs of such technology is comparatively low with a three axis autopilot with waypoint navigation, altitude control / airspeed control with GPS intergration less than $10000 The units are also very small, in fact as little as 28gm weight.

Thank God that control on the supply and export is in place.

I believe the limits are more on components, as was stated the airframe can be built to withstand much more than a human occupant. At least one of the main nav components has been tested to 8 g's.

There are far more civilian/agency unarmed uses for UAV's than armed, and I hope that we can get past this issue....

CAA UK is much at the front lines on UAV regs with the recent CAP 722.

(nothing useful will come out from the FAA until 2020, if then)

The weak points with regards a UAV design, are in rough order - airframe limitations (quite easy to design an airframe - with modern composites, to withstand 20 - 30g. You only have to look at the range of design techniques in R/C model aeroplanes to composite manned aerobatic aircraft to see the g tolerances which can be engineered into a design) The fuel system, engine and finally avionics, which being solid state can withstand serious amounts of g in flight. 8g is a relatively low load for an unmanned aircraft; you have already removed the biggest limiting factor - a person. The avionics g limit is less of a factor than fuel system, g will starve an engine of fuel if the system is not properly designed, that is with either piston or jet engines.

In reality the flight envelope the UAV operates in, is unlikely during normal flight, to ever get towards G limits, why would you want a 20g turn as part of the normal flight profile? The worst loading would be during the recovery stage which is sometimes into a net – causing serious deceleration loads, 100kts to 0 in as little as 10 feet.

Large UAV’s such as Reaper operate a conventional flight profile – with a wheeled takeoff and landing, rather than some of the smaller UAV’s which are catapult launched and net recovered.

G stress loading is a consideration, however bigger considerations to the designer are speed, endurance and stability. UAV’s tend to rely on aerodynamic stability rather than FBW electronic stability (lower power consumption for the avionics, with the autopilot not having to constantly compensate instability, giving greater reserves / weight savings on internal batteries)

It was an IRU that I was looking at that went to a little over 8g's sustained before the compensation rate was unacceptable.

To try and establish a set of rules for UAV airworthiness is akin to writing a set of rules for everything from a paper dart to an Airbus A380.

At one extreme are the indoor ornithopters and hover platforms and at the other, Global Hawk.

There are a lot of misconceptions that UAVs only use the latest technology. One of the more successful ones in current service uses lead acid gelcells for onboard power and mechanical gyros. The flight limitations take account of the hardware used.

Generally a designer has a idea of the flight regime and designs the airframe and selects the avionics accordingly. If QinetiQ's Zephyr 7 had been built to a standard airworthiness and avionics packaging requirements it probably wouldn't have left the ground, let alone stayed aloft for two weeks.

Getting back to the original question, yes it is possible to overstress airframes if the onboard autopilot doesn't have limitations set. A decision made on the ground cannot be relied upon as it will probably be too late.

Watch the recovery of a Scan Eagle and you will appreciate that flight loads are the least of its worries.

I'd personally advocate using the appropriate manned design code (S, VLA, 22, 23, 25) and then removing or modifying what is inappropriate to a UAV. There's a whole industry that knows how to comply with these codes, so then the first major difficulty is what to remove - some bits are easy: you can drastically cut undercarriage requirements, remove most handling qualities requirements, possibly reduce Nz limits to what's permitted by the FCS. This approach is probably inevitable, although I'm not aware of much standardisation yet.

The second major difficulty, and the biggie, is then the control system - what do you require in terms of autonomy.v.direct control, reversionary logic (i.e. what does it do if the world stops talking to it), how does it sense and avoid other traffic. Tens of millions of (insert currency of your choice) are being spent on solving this one, which really is the holy grail of UAV utility.

The philosophy of all of this will remain the same however - ensuring no greater risk to people and vehicles outside the UAV/UAS than would be the case if it were certified as manned and flown by a qualified professional pilot.

G

Unfortunately applying manned aircraft design criteria to UAV design would invariably lead to huge increases in design / manufacture cost. One of the attractions of UAV’s is the reduction in cost over a conventional manned aircraft.
The question of control v autonomy is a big one; simply put, it depends upon what the planned usage is of the UAV. Some systems require a pilot, with continued data updates. Other systems require a totally autonomous flight profile, in other words the UAV takes off its self, flies a predefined 3d (including altitude and position fix) route (normally through waypoint GPS navigation) and then returns for automated recovery. There are of course numerous permutations in between, sometimes the UAV mission will have the recovery and takeoff flown by a pilot who is in visual contact of the UAV at the initial and final stages, sometimes the fully automated UAV will receive mission updates which will reset the defined course and altitude. The number of permutations is as long as a piece of string.
With regards failure mode, that depends upon various factors, even with the fully autonomous control / mission systems, a failure will trigger a pre defined event, ranging from airborne destruction of the UAV (after a predefined length of time for loss of data link, be that for GPS navigation, sensor information or loss of power), to a parachute recovery, even to a deliberate end to the flight by a crash (normally caused by motor to cut off, and a deliberate spiral dive or spin) into the ground.
Collision avoidance systems are evolving, but at this stage the most common form of avoidance is given by not operating the UAV in an area likely to encounter manned traffic.
It is difficult to compare UAV design, both practically and operationally to manned aircraft.

A bit stressfull?
 

Scan Eagle: From 52kts to 0 in half a second from catching the wire, its stressfull just watching it!

Isn't this lack of definition the issue? If I understand the answers correctly it seems to be a general view that technically one can do much more than the world is ready for, or certainly more than the world is ready to sign off as being perfectly acceptable to have around interacting with other air traffic and the general public.

So if we are saying we can test airframes, systems and they can't be broken, where are the challenges to getting UAV's signed off?

As has been said there are vastly more civil uses for UAV's than in the military but currently even our military ones can't be flown around the UK so what chance any civilian ones being used?

One could easily see a role for UAV's in place of, say, Police Helicopters but surely the role of getting them accepted is a role for testing organisations to agree how to sign these things off and convince those that need it that these are safe - or can be made safe?

As has been mentioned before, there are two fundamental issues here. Firstly 'lost link' in which the UAV loses the uplink and/or the down link to the ground control station. Not an insurmountable problem, as the UAV will be programmed to enter an autonomous 'return home' mode, whilst the ground control station will start searching for the UAV to re-establish control.

The second issue is 'See and Avoid', especially whilst the UAV is in the 'return home' mode. This is fine in a military controlled danger area, in which all aircraft have transponders and are in communication with one air traffic controller. In VFR airspace, however, it would put the onus on every aircraft owner (gliders paragliders, microlights, LAA aircraft etc) to add an expensive piece of hardware to enable the UAV to 'see' them, and to communicate with a central point, so they can be diverted out of the path of a wayward UAV. Needless to say, mandatory transponders have met with huge opposition from the light aviation community as the aircraft owners would have to pick up the tab to enable a cost saving for a potential UAV operator. There is also the issue of multiple transponders in small areas of airspace, as might happen near a gliding competition.

No doubt there are people in the UK addressing these issues, as it does seem wasteful to see a police helicopter with two 600hp engines being used simply to support a camera ball that a 120hp UAV could do just as easily and hopefully a lot more quietly.

In some ways, testing UAVs is much more difficult than testing manned aircraft. As far as the raw airframe goes, either is straightforward. Design analysis, structural load measurement, flight control system limitations, etc. are very similiar activities whether or not there's a filled seat on board. Perhaps easier without the filled seat. Once you start talking systems, a huge increase in complexity results. Manned systems have a very reliable back up system on board - the person. UAVs have to have their own backups, and have to be proven to be safe enough for the intended work.

When the work is to fly over a battlefield and gather intel, that's straightforward. The risks are easy to calculate and mitigate, airspace is controlled by the agencies fighting, and one backup is to let it crash (no immediate loss of life, risk acceptance for what is on the ground below).

Shift that into civilian airspace or over populated areas and the reliability cannot be mitigated by accepting erratic flight profiles, destructive aborts, etc.

Now consider the concerns both in the battlefield and in civilian airspace when the UAVs are armed. The risk in using short range ground attack weapons is much more easily mitigated than the risk in using medium to long range air to air weapons.

One litmus test in the perceived level of safety of UAVs is whether you would put your children in the back of one. I won't. An interesting extension of this litmus test is would you rather have your children in a UAV flying in airspace protected by manned aircraft, or in a manned aircraft flying in airspace protected by UAVs. If neither is not an option, then I'd prefer that the fighters are manned.


Cheers,
Matthew.

The UK is pushing forward with helo UAV's in preparation for the London Olympics...these platforms are unarmed surveillance only, as are most manned police platforms, (ie the platform itself is not armed)

Depending on the platform, most have multiple redundant systems, and have coded procedures on disconnect...

I would trust most of these UAS platforms to put my children in the back, rather than most GA aircraft I see in the skies...

UAV Recovery
 

The link below shows a nice picture of the chute / airbag recovery system used for the Mirach 100/5, not only can it be used as a recovery method, it can also be deployed as a failsafe in the event of systems faliure (engine faliure, data link, GPS signal loss for x time, airframe faliure -indicated by accelerometer / ASI readings)

http://www.aerosekur.com/parachutes/...erySystems.pdf

The concern is not whether a backup system can be built, that technology has been out for decades. Rather, it is whether you can assure yourself that a backup system will function and function correctly after any of the failure for which it is required. That is much more difficult to prove than whether the backup system, once activated, will do its job.

I wouldn't bother comparing UAVs to GA, two very broad fields. Perhaps comparing hobby R/C to GA is closer, but now unfair to GA. The technology we're discussing is the replacement of professional, legislated, and experienced personnel/equipment.


This isn't an overly safe, pessimistic view of certification. UAVs crash, and most have a much worse safety record than manned aircraft. A sample from different countries:

MQ-8 Fire Scout UAV resists its human oppressors, joyrides over Washington DC -- Engadget

Glitch Reporter: Predator UAV Failure

CU161009 Sperwer... | Uninhabited Aerial Vehicles | Reports - Investigation | DFS | Air Force | DND/CF

1 of Two RAF Reaper UAVs Crashes in Afghanistan - Defense News

T-Hawk UAV Fails at Fukushima | Japan Probe

ADM: Shadow tops UAV crash list