Delft VTOL project uses helicopter-style main rotor for effeciency

From NewAtlas:

We have seen a few different takes on Vertical TakeOff and Landing (VTOL) drones over the last few years. The idea behind such approaches is to harness the typically longer range and greater payload capacity of fixed-wing drones and mix it with the superior agility of multicopters, allowing them to take off and land in tight spaces.

Some of these have been developed for military purposes, such as the HQ UAV and the Batwing-like AirMule, but others, like the VTOL Kestrel and SkyProwler are aimed more at hobbyists. In developing the delftAcopter, the researchers have set out to build a drone that can be used to carry medical supplies to tough-to-reach areas.

The electric drone takes the form of a miniature biplane, an aircraft design that uses two wings stacked on top of one another which became popular in the early years of aviation following its success at the hands of the Wright brothers. While some VTOL drones use tilting propellors to switch from vertical to horizontal movement, the delftAcopter itself changes orientation as it makes that transition.

Prior to takeoff, it sits upright with the propellor spinning horizontally, just like a helicopter. Then as it reaches the desired height, it shifts its position by 90 degrees so that the propellor is facing forward and it is thrust in that direction, allowing it to zip along at up to 107 km/h (66 mph). With a 10,000 mAh battery onboard, the aircraft can fly for up to 60 minutes on a single charge.

The delftAcopter is capable of entirely autonomous flight, including takeoff, forward flight transition and landing. It can travel beyond the operator's line of sight and maintain a connection through an Iridium satellite connection, which the researchers actually claim allows it to be controlled from anywhere on the planet.

It uses Parrot's S.L.A.M.dunk developer kit along with a fish-eye stereo camera to gather video, and uses an inertial measurement unit (IMU) and GPS to track its position during flight. The craft weighs 4 kg (8.8 lb) and also features obstacle avoidance and the ability to pick out safe landing zones.

The team is set to put the delftAcopter through its paces at the upcoming 2016 Outback Medical Challenge. The event takes place in Australia and tasks competitors with building an autonomous aircraft capable of retrieving a blood sample from a stranded person located at an inaccessible site around 30 km (18 mi) away.

Drones have emerged as tools with great potential when it comes to search, rescue and disaster relief situations. Various drones have been tested for these purposes in the US, the Swiss Alps and across Africa, a particularly suitable candidate due to rough terrain and the lack of paved roads and infrastructure to move cargo by land. The delftAcopter will have its chance to demonstrate its wherewithal at the Outback Medical Challenge between September 27 to 29.

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Comment by Stanley Anak Suab on September 22, 2016 at 6:05am

That is one very good idea for VTOL. Congratulations to the designer and team!
I think the center helicopter propeller can also be replace by an EDF unit to make another version of it.  

Comment by Joshua on September 22, 2016 at 6:43am

If you replace the helicopter prop with an EDF unit you lose finesse over thrust control, making hovering and safe landings harder. An EDF that can put out ~9kg of thrust is also going to be pretty large and almost certainly heavier than the helicopter system. Don't get me wrong, a VTOL EDF does have its advantages for forward cruise, but I don't think it's particularly suitable for this sort of mission.

Comment by JB on September 22, 2016 at 10:13am

Thanks Kevin.

So that's about 30A equivalent on 4S for hover (which is better if you need it), but more than 2x the current for cruise. We're doing the hover with Nylon 6x3.5 props swung by tiny 27g 2206 motors with 4g ESCs on the quad though. ;-) Complete aircraft setup cost sub $1k including avionics, comms, camera and onboard computer (lol that's probably the cheapest part with a Pi Zero! The chewy gum we used to block all the airframe holes was more expensive!). This was a intentional price target, along with as close to 2kg AUW as possible, because that is the new licence free, commercial UAV weight class in Australia.

You're also carrying 1.3kg more of course, but the mission capabilities will be similar, although we aren't using anything as fancy as SLAM or onboard imaging this time around, as there are no points for it in the competition. (With a bit of luck, theoretically you could even run it without a camera at all and could still win, given that we get the GPS coordinates within 100m to start with, from which a landing site can be predetermined by looking at a Google image of the LZ...ooops did I give something away?) ;-)

How did you go with vibration from the main motor in hover affecting imaging? Helis aren't particularly suited for stable imaging on smaller platforms, but I'm sure you found a solution to this many other challenges. The event is called that for a reason! 

Currently Wednesday looks to be the best weather, but Thursday looks stormy, and contingency Friday is windy as well. If we are lucky we'll get to fly the first day. Will be fun!


Comment by Ikaros on September 22, 2016 at 11:03am
Super cool design and good luck in the challenge! How do you handle the fact that all your antennas (GPS, telemetry, satellite) rotate 90 degrees when you switch from hover to cruise mode?
Comment by Rob_Lefebvre on September 23, 2016 at 6:17pm

Very interesting system here Kevin.  I'd like to have a look at some of the numbers. What is the rotor diameter?

So you're saying 40A to hover, at 24V or 48V?  It's not clear from the way it's worded.

Comment by Kevin on September 23, 2016 at 7:00pm

@Ikaros, thanks! We indeed have had GPS problems, so we have two of those and switch based on satellite coverage and pitch angle. The 900MHz back up datalink link has two antenna's. (one for hover, one for fwd). The Iridium antenna is placed at 45 degrees compromise.

@Rob, yep she's interesting all right...! So, I meant to say 20A at 6S x 3.7V= 22.2V (approx) in hover. If we take off with minimum weight (~2.5kg)  it drops to about 12A. Rotor diameter is exactly 1 meter. We have a 105kv motor.

What's more is that (position static) hovering becomes more efficient with wind (compared to a heli), as the wing will start to generate lift (even though it is stalled).  

Of course this a new concept, so there is still lots to be optimized. But if you have insights or comparisons we are of course very interested.  

Comment by JB on September 23, 2016 at 7:01pm

Sorry Rob I think comparing it to our quadplane system numbers here is confusing things. (bad forum etiquette I know ;-) )

Kevin said the Delftacopter uses only 20A at 6S for 4kg AUW, but our PerthUAV quadplane uses around 40A on 4S @ 2.7kg. 

Comment by Kevin on September 23, 2016 at 7:15pm

@JB, so you say 4.8A at 40 knots with 2.6KG AUW, that is extraordinary. What kind of forward motor are you using?

To answer your vibration question, that is remarkably not a problem at all. We used to do tests with a Logo600 before, and indeed vibrations all over and unusable images . However on the DELFTACOPTER vibrations are minimal, we suspect because of the symmetrical wing profile (a heli has one tail beam, we have two (or four taking the bi-wing into account).  Also we have a direct drive motor. (During forward flight there is of course also a large amount of damping from the wing, but images are already great during hover).

Another thing is the minimal sound production. DELFTACOPTER is really quite in both forward flight as well as hover, if compared to a quad rotor or heli.

Comment by Rob_Lefebvre on September 24, 2016 at 7:22am

Ok, so I crunched a few numbers.  Based on 1m rotor span, 4kg AUW, 24V (I typically use 4V/cell unless better data is available), 20A hover. And 2x 4500mAH.

Disk Area: 0.785m2

Disk Loading: 5.09kg/m2

Hover Power: 480W

Specific Hover Power: 120W/kg

Total Energy: 216Wh

Power Density: 54Wh/kg

Max hover time: 27 minutes

Figure of Merit: 36.9%

Now, I can compare that to my Procyon 500 helicopter prototype.  It has almost the same size rotor, but using standard helicopter rotor blades.  3.4kg AUW including a 100g camera.  2x 4S8000 batteries, hover power is 16A.

Disk Area: 0.86m2

Disk Loading: 3.95kg/m2

Hover Power: 256W

Specific Hover Power: 75W/kg

Total Energy: 256Wh

Power Density: 75Wh/kg

Max hover time: 60 minutes

Figure of Merit: 51.8%

So, some interesting numbers there.  Particularly the Figure of Merit, which is the theoretical efficiency of the rotor disk when compared to a perfect Momentum Theory Disk.  Your figure of merit is signficantly lower, which is why the flight time is not so long by comparison.  However, 37% is exactly in line with many other helicopters I have.  This Procyon 500 has something going on that I don't fully understand...  Wish I did, because it's good!

But, what is interesting, is despite what appears to be a superior rotor design compared to standard slab rotors from a helicopter, it's not actually hovering that much more efficiently.  I think that a big factor is the aerodynamic download on the wings compared to a helicopter.  The rotor would be creating about 4.5 m/s airspeed downward, which drags on the wings.

Also note that the Wh/kg is significantly lower.  Partially owing the weight of the wings.

Now, the wings do result in less power required in forward flight, I've got about 230W, for a flight time of around 56 minutes.  But, my 500 helicopter actually requires the same 16A in forward flight as it does in hover.  So still a flight time of about an hour.  At about the same speed.  It seems to me that the wing structure, while reducing flight power, don't really make up for the extra drag and weight they add.  I think the numbers would improve if you add more battery mass, and you probably could.

It'd be interesting to compare performance with a conventional helicopter which is about equivalent in complexity.

So I think the above does that.  The two machines are very, very similar.  I don't see any radical advantage to one over the other.  There's a few things that don't come out in the numbers however.  I think the helicopter would have far superior hover stability in strong, turbulent winds.  I think the copter-plane could probably be optimized more for forward flight.

The only major one I can think of is this could potentially be much faster as it does not suffer from RBS.

RBS is not a factor at the flight speeds we're talking about.

In comparison to a standard helicopter this obviously provides faster and longer range, and offer's better endurance in forward flight.

I don't think this is quite so obvious, as I've shown.

In terms of the swash plate design, mostly we have had trouble with control of a very lightweight rotor with a very long wing attached to it. Normal helicopter gyroscopic control is not possible, it becomes a mix between the gyroscopic effect and the aerodynamic effect. 

There is no gyroscopic effect in a helicopter rotor.  Common misconception.  The aerodynamic forces are 100 x that of the gyroscopic forces.  It's all aero.

Comment by Rob_Lefebvre on September 24, 2016 at 7:24am

Oh, I forgot a question.  The rotor blades, are they totally custom made?  Or are you doing something like taking tmotor blades and putting them in a helicopter rotor head?


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