One of the most repeated questions from our customers is: Why did you choose to build a helicopter instead of a multirotor UAV? 

It is a great and valid question, given the popularity of both amateur and professional multirotor UAVs in the market. However, at Alpha Security and Defense we are experts in UAVs and aeronautical engineering, and we concluded that the helicopter form factor has many advantages when compared to multirotors, and we will explore those in this article. 

Gasoline powered UAVs vs Battery-powered Electric Drones

The first and most evident advantage of our helicopter is that it is gasoline powered. This allows for longer flight times, as the energy density of gasoline is still much greater than the one offered by even the most innovative batteries.

Some quick calculations with approximated realistic values reveal the following:

In aeronautical applications, on the one hand more weight means a higher energy requirement (and thus more batteries required on board), and on the other hand a lower payload capacity available, as Maximum Take-off Weight is fixed for each aircraft and fuel or batteries are included in that weight.

This comparison applies not only to multirotor UAVs, but also to other fully electric helicopter UAVs.

Alpha vs Hybrid Drones

Other existing solution for increasing the autonomy of multirotor UAVs is to create hybrid drones. In this design, a small generator set is added on-board to produce the electrical energy needed by the drone.

This type of drone achieves significantly better performance that battery drones, but our gasoline powered UAVs are still more efficient, as we’ll show in the following sections:

 

Energy generation and transmission efficiency.

Both designs extract the power of gasoline via a combustion engine, so we will evaluate the efficiency of the different transmission methods below. We’ll consider the output power of the combustion engine 100%. 

 

Alpha’s UAVs Helicopters: 

Our helicopter UAVs’ transmission is composed of just shafts, gears or belts from the engine to the two rotors (main and tail) to transmit the generated power. This means that the power transmission is very efficient. The overall efficiency from the output of the engine to the rotors on a conventional helicopter can easily reach up to 95% depending on the number of stages.  Alpha’s UAVs use a two stage transmission. 

For reference; normal efficiency values for different transmissions are: 

  • Direct coupling of shafts: 100%
  • Timing belts: 98%
  • Gear set: 98-99%

The following diagram represents the transmission on an Alpha UAV helicopter:

Alpha’s UAVs reach an efficiency of 96%

 

Hybrid multirotor: 

In hybrid multirotor UAVs, a gasoline motor is attached to an alternator which generates AC current. At a later stage, this AC current is converted into DC current, in order to charge the batteries and to provide DC current to the Electronic Speed Controllers (ESC) of the motors. ESC generates multiple AC current waves, that, when passing through the coils in the stator phase of the motor, generate magnetic fields that cause the rotor to turn.

The battery is only used to supply the system when high peaks of demand occur so is constantly being charged and discharged, which affects its durability. Having those conversion steps and knowing the efficiency of each step we can calculate the overall efficiency of the complete system:

  • Motor – generator transmission: 100% (normally shaft)
  • AC  Generator: 90% (heat generated in the generator coils)
  • Rectifier (AC to DC converter): 
    • 95% (ideal, used for calculations
    • 85% (diode-based converter, most common)
  • Battery Charging*: 90%
  • Electronic Speed Controllers: 95%
  • E lectric motor: 90% (heat generated in the engine coils)

*Here we consider only the amount of energy that is consumed due to the high peaks of energy, and thus multiplied by the efficiency of the step. We will assume only 10% of the energy goes through the battery but this may vary significantly depending on the platform and its weight.

So, the overall energy efficiency (best case scenario) would be: 

0,9·0,95·(0,9+0,1·0,9)·0,95·0,9= 0,7237  72%

This means that, for similar conditions, a hybridmultirotor UAV has 24% less energy transmission efficiency than an Alpha UAV helicopter. 

 

Weight penalties and flight time calculations.

In order to see how this would affect a real operational scenario, in this paragraph we’ll compare our Alpha 800 UAV to one of the most common hybrid configurations in the market: a DJI Matrice 600, turned into a hybrid powered UVA using a Nova 2400 generator.

The A800 empty weight with no payload is around 9,8 kg.

The DJI Matrice 600 with 6 TB48S batteries weighs in at 10 kg. If we remove the 6 batteries at 680 g each, we save 4080 g.  The Nova 2400 generator used to convert an electric multirotor into a hybrid multirotor weighs 4,2 kg. Therefore, the final empty weight of our newly built hybrid Matrice 600 is 10,2 kg. 

In the following tables, we calculate fuel capacity for a 1,8 kg payload, the maximum payload that the A800 can carry with full fueltanks.

*The density of the gasoline is 0,755 kg per liter.

 

Following DJI’s specifications, a regular Matrice can fly a mission of 40 minutes with no payload and 18 minutes at MTOW. With this data and the available specifications of the batteries, we can calculate its power consumption:

Total power available is: 6*(5.7 Ah*22.8 V)= 6*130Wh = 780 Wh

Power consumption at 9,6 kg= 780 Wh/0,66 h= 1181 W

Power consumption at 15,5 kg= 780 Wh/0,3 h= 2600 W

For the hybrid Matrice of our example we have a 1.8 kg payload plus the fuel. Then we will need an estimated average of 2300 W during the flight for a total weight that varies from 15,5 Kg to 12 kg, depending on the amount of remaining fuel. 

Let’s calculate how much flight time we would have under that circumstances: the NOVA Hybrid generator’s specifications indicate a fuel consumption of 830 g / kwh. This means that we need 2300 W of average power delivered to the ESC. To calculate how much input power we would need, first we need to estimate the upstream efficiency losses in our hybrid configuration until the ESC input:

AC Generator Efficiency · Rectifier Efficiency · (%PWR Batt · Batt Charging Efficiency + %PWR ESC) =
0,9 · 0,95 · (0,1·0,9 + 0,9) = 0,846 = 84,6%

Then, we calculate how much fuel is needed per hour:

2300 W / 0,846 = 2718,7 W that equals to 

2,7187 kW · 0,83 kg/ kWh= 2,2565 kg/h.

As we have 3,5 kg of fuel that means that the endurance is 

3,5 kg / 2,2565 kg/h = 1,55 hours.

While the ALPHA 800, that consumes 1400 W for constant hover (the most fuel-consuming flight mode) at 1000 m ASL, would stay in the air for 2,5 hours, so the ALPHA 800 can fly more than 1 hour longer using less fuel, which is also beneficial for the environment.

 

Aerodynamic efficiency.

We have seen already that a multirotor with similar take-off weight as a helicopter consumes more energy due to the energy conversion steps. Additionally, a part of the reduced efficiency is due to the fact that a multirotor creates lift with 4, 6 or 8 small propellers instead of one big main rotor. 

It would be like comparing a fighter plane to a glider. Which one do you think is more efficient? Yes! It is the glider, obviously, and that is mainly due to the larger surface of the wings.

Basically, Alpha helicopter UAVs are able to create more lift, as their large main rotor (the equivalent of a wing in a Fixed Wing Aircraft) has a bigger surface than the smaller rotors of a multirotor, and lift is directly proportional to that.

Other considerations: Safety.

Finally, we have seen how a multirotor needs up to 2600W of average power to fly with 15.5 kg of total weight. However, the NOVA generator only provides 2400W of output power, and the average required power during the complete mission would be around 2300W.

That means that, most likely, the backup battery that is there to provide extra power during peaks of demand, would be almost discharged at the end of the flight. It also means that the generator’s engine would be at maximum throttle most of the time (also affecting its durability) and it would not be able to even provide all the necessary power for the flight. 

Finally, and most importantly, in case of engine failure the pilots would only have the little time the batteries can provide to reach the ground safely. Let’s calculate how much time that would be.

With the two 6S batteries of 3300 mAh each that NOVA provides with the kit, we would have: 

22.8V · 3.3Ah · 2 = 150 Wh

Assuming we lose the generator engine right at the start of the flight when the batteries are still fully charged:

150 Wh / 2600 W= 0,057 h = 3,46 minutes of flight

This leaves the pilot with very little time for executing a safe landing, even right after take-off.

Additionally, Alpha UAV helicopters have a unique safety feature that makes them much safer than multirotor UAVs: autorotation, which is performed autonomously by the autopilot in case of an engine failure situation. Simplifying a lot, as a result of having bigger propellers and the possibility of inverting their angle of attack, helicopters can use their blades to glide and execute a controlled emergency landing with minimum damage. As Alpha UAVs are equipped with back-up batteries, the communications would be still up and running and you can command the helicopter to a landing location of your choice. 

On the other hand, as multirotor UAVs can not execute an autorotation manoeuvre, they need to incorporate parachutes if they want to increase safety. Typical parachutes for a DJI Matrice 600 weigh more than1,1 kg, which consequently reduces payload capacity by that much. Additionally, you can not direct the fall of the multirotor UAV, which significantly increases the probability of losing it or causing damages to people, buildings, goods or to the UAV itself.

 

Conclusions

As we have demonstrated both quantitatively and qualitatively, the choice of buying a gasoline powered Alpha UAV is the smarter one, specially for demanding missions in terms of endurance, payload needs and/or safety. 

Alpha UAVs are more efficient than hybrid drones and fly for significantly longer than purely electric drones. 

Alpha UAVs are also safer than multirotor UAVs, as, in an emergency, they have the possibility of directing their landing. They also avoid the weight penalties that a parachute causes to payload capacity.