This is the helicopter we need for the most dangerous threat we face

A Helicopter the World Needs

Dr Ron Smith joined the British helicopter company Westland in 1975, working in Research Aerodynamics, remotely piloted helicopters, before becoming Head of Future Projects. He had a strong influence on the design of the NH90, and was involved in the assessment of the Apache for Britain. We asked him what to consider what helicopter the world most needs.

The Problem

Having been asked the question “what helicopter does the world need?”, I am thrown back to a query raised with me a couple of years ago. “Could you develop a fire-fighting or crane helicopter with a water / fire suppression load of 20 tonnes or more?”

With climate change, there is an increasing wildfire risk worldwide. There is a very good discussion of this at www.carbonbrief.org/explainer-how-climate-change-is-affecting-wildfires-around-the-world

In recent years there have been significant fires affecting Australia, the Amazon region, the US West coast from Washington State down to California, Indonesia, southern Europe and Central and Southern Africa. With climate change in the far north, significant fires have also broken out in both Alaska and Siberia.

Given this wide geographic spread, fire seasons are now occurring pretty much throughout the year, and indeed seasons are beginning to overlap in geographically dispersed locations such as those listed above.

The US Forest Service’s Aerial Firefighting Helicopters

Notable on the above figure is the markedly increased risk of severe fire risk weather conditions in Southern Europe and Southern Africa in addition to significant percentage increases in many of the current high-risk locations.

Solutions?

So, what do we need and how do we get it?

I see this as having some parallel with the difficulties currently being experienced in dealing with the present COVID-19 pandemic.

One’s first reaction is – well, fix the emissions and it will go away. However, the world’s scientists and governments have known about climate change for decades and (despite some encouraging changes in direction in the US Administration) there is a lack of effective global coordinated action. Nowhere is this more evident than in Australia, where the Government seems more focussed on maintaining its coal exports to China than on reducing emissions.

In the case of the pandemic, the World Health Organisation has said that effective vaccination is needed world-wide, but many wealthy nations are taking the ‘after we’ve looked after ourselves’ approach. This despite the acknowledged rationale that any pool of unvaccinated populations will provide a source of mutated variants that could stretch the crisis out for years.

So past and current behaviours tell us that there isn’t going to be a magic reduction in world-wide emissions (and even if there was, the path is already set for significant climate change).

There does not seem to be a military need for large crane helicopters and the largest helicopter available today originates from Russia, which is currently something of a problem child in terms of global diplomacy, and this type is not available in a fire-fighting variant.

The largest lifting capability currently in servie is the Erickson CH-54B / S-64K Skycrane fleet operating at up to 47,000 lb max weight and the Billings and Columbia CH-47 adaptations at around 50,000 lb max weight. Some 31 Erickson Skycrane have been built.

Mil Mi-26 helicopter - development history, photos, technical data

A high-capacity fire-fighting crane could potentially be generated using a Mil Mi-26 dynamic system (rather like the Mil Mi-10K derived from the Mil Mi6) but this note examines what a new-build aircraft for the role might look like.

A helicopter makes a night water drop on the leading edge of the Lake fire which has burned more than 10,000 acres near Lake Hughes north of
Los Angeles. (Photo by David Crane)

How would it be Funded?

With no military need, could funding be raised from concerned nations? Europe, the United States and Australia might head that list, but past and present behaviours suggest that those, with existing helicopter design and manufacturing capability, would lobby to have any such work placed with their own domestic industries.

The future wildfire risk forecast suggests European nations, particularly Italy and France, which have significant land areas near the Mediterranean, and strong helicopter industries; and the US, which has been having significant fire problems, particularly in the West, might be best to develop this capability.

While noting these countries as having the greatest need and the requisite capabilities, in the absence of any current project activity, I am forced to leave the question of funding and acquisition management on one side for the moment.

What is the operational vision? There probably needs to be around 60 – 80 aircraft allocated to this task, with greater numbers concentrated where there is the greatest risk to human life and economic impact. The fleet would be dispersed with perhaps 10 in southern Europe, 20 on the west coast of the United States, 15 in Australia, 10 in South America and a reserve fleet to respond to emergencies elsewhere and to provide a surge capability when required.

The climate data indicates that additional capability might be required in Southern Africa and in Russia, should funding become available. In view of the global nature of the problem, consideration should be given as to whether there might be support available from the UN, as well as an assessment of the level of interest in countries such as Russia and China, both of which are likely to have future need for such a helicopter.

The Air Vehicle and Equipment

Drawing on my thirty-year old experience of helicopter preliminary design, I will outline some very basic rule-of-thumb thoughts on what a new fire-fighting crane might look like.

The discussion will necessarily be highly simplified at this stage but will give some idea of how to get into the ‘right ballpark’. From there, we can evaluate what other areas would need to be investigated to harden up the design.

The Mil Mi26 will give some idea of the size of helicopter required. The quoted figures for this type include an empty weight of around 62,000 lb, a ‘gross weight’ of 109,000 lb and a maximum take-off weight of 123,000 lb. The aircraft has an eight-bladed rotor of 105 ft diameter and is powered by two ZMKB Progress D-136 engines, each rated at 11,400 shp.

Where to Start?

We start with some ground rules for the new design:

  • The helicopter must use engines that are already certified and in production in the west. The parallel development of a new powerplant would result in excessive risk to the project.
  • The target payload is 20 tonnes (roughly 45,000 lb)
  • Design ambient conditions should reflect those in typical fire risk regions – perhaps 2,000 ft and ISA +30C (although this aspect can be subject to confirmation).
  • The aircraft needs to be capable of regional self-ferry operations. Possibly three hours endurance at 125 kt+. In actual use, most drop operations are likely to be between hover and minimum power speed. Modular arrangements to allow long range tanks to be fitted should be investigated.
  • The rotor should be optimised for hover and low speed operation, thereby maximising payload and endurance. This implies composite blades with modern aerofoils and a non-linear blade twist of perhaps -14 degrees, probably with an anhedral tip. Between six and eight blades would be used and, for hover efficiency, the rotor tip speed would probably be around 660 ft/sec (similar to that of the Sea King and AW101).

Where do these assumptions lead us?

The above constraints allow us to make the following decisions:

(i) The aircraft is likely to be in the same weight class as the Mil 26 and therefore we are looking for an in-service powerplant in the 11,000 shp class if two engines are to be used. The only candidate currently available is the Europrop TP400-D6 used on the Airbus A400M aircraft. Basic information has been drawn from the EASA Type Certificate Data Sheet (TCDS) for this engine. The 5 minute take off rating of this engine is 8,251 kw (11,000 hp).

(ii) The likely payload fraction of the helicopter is estimated at 45%, although 50% may be achievable for a crane configuration. This implies a maximum weight of around 100,000 lb. Taking a hint from the Mil Mi-26, we will assume a rotor diameter of 100 ft.

(iii) Will the power be sufficient? Our data suggests that two of the TP400-D6 engines used on the A400M, coupled with an appropriate main rotor gearbox, would be likely to be sufficient.

Clearly, a proper design with detailed weight estimation and specific attention to both engine and gearbox rating structures would be required to firm up the figures suggested in the text box. A key question in respect of the Europrop engine would concern the implications of providing a short duration contingency rating to be used to fly-away following an engine failure.

Gearbox Requirements

Mi-26 helicopter firefighters

The existing TP400 engine comes with a propeller reduction gearbox that reduces the take-off prop rpm to 864, from an output shaft speed of 8580 rpm. For the helicopter, the proposed tip speed of 660 ft/sec on a 100 ft diameter rotor implies a nominal rotor rotational speed of 126 rpm.

The helicopter gearbox is likely to be lighter if the A400M propeller reduction gearboxes are not used and the overall reduction is accomplished within the main gearbox. This implies an overall ratio of around 68:1 between the engine output shaft and the main rotor drive.

The TP400 engine would need to be certificated for helicopter applications, whether or not the existing reduction gearbox was retained. One specific consideration would be the vibration environment to which the engine would be exposed in any helicopter application.

Notionally each engine would be spaced outboard of the helicopter main gearbox. A bevel gear would redirect the drives toward the gearbox. A further bevel gear would turn the drives vertically into two planetary (or epicyclic) stages to drive the main rotor shaft. A tail rotor drive would be provided to the rear, with an accessory gearbox mounted forward.

This arrangement provides four reduction stages (two bevel stages and two planetary stages). The overall 68:1 ratio would be provided using an average reduction of around 2.8:1 per stage.

Other aspects of the airframe design would broadly be similar to an enlarged version of the Sikorsky S-64.

Sikorsky S-64 Sky Crane | Aircraft |

Fire-fighting equipment

Delivery of water, fire suppressant chemicals, or a mix of the two is anticipated to be through the use of a fire-fighting turret, as this provides an opportunity for greater precision in application than a simple water drop system, The directional fire-fighting turret could be mounted on a suspended water tank arrangement. Arrangements to stabilise the position of the turret with respect to the helicopter are likely to be required. (This would be needed to control the cg position of the heavy load and should also lower operator workload and ease the design of the helicopter Flight Control System.)

The operator could use sensors to define the jet aim point(s) and an active control system could then adjust the fire suppression jet onto the target, or along a defined line. An increased payload (longer delivery time) and targeted delivery should significantly increase efficiency and reduce operator workload.

Provision will also be required to allow the helicopter to take on water through a suction pump arrangement similar to that used by many other fire-fighting helicopters. This will allow flexible operation, particularly where lake, dam or oceanic water is available close to the location of fires.

Unanswered Questions and Risks

Identification of funding and commercial principles.

Market analysis and solicitation of government, national park agencies, fire services and end-user opinions.

Selection of helicopter manufacturer based on facilities, experience, capacity, etc. Almost certainly an existing helicopter manufacturer.

Allocation of sub-contract and supplier elements. This to include selection criteria and sub-contractor qualification and management.

Powerplant development and certification for helicopter applications.

Understanding of powerplant constraints, including physical, electrical, electronic / digital interfaces, engine vibration and other environmental clearances.

General engine performance characteristics – power / fuel flows vs altitude & temperature; intake and exhaust constraints; particle separation.

Definition of engine and gearbox rating structures (including contingency rating(s) and one engine inoperative operation). Possibly linked to dynamic simulation of post-engine failure fly-away manoeuvres.

Mass estimation and loads modelling including crashworthiness

Flight performance modelling

Manufacturing tooling of 50 ft composite blades (tape laying, autoclaves, etc). There are likely to be significant non-recurring costs for such items, to be amortised over a relatively short production run.

Main gearbox test facility compatible with engine power available, another significant non-recurring cost to be amortised.

Structural static and fatigue test rigs

Rotor head and blade design. Control power when operating high inertia system in turbulence?

Fatigue life of critical components (and their validation / verification)

Digital architecture and flight control system design (hardware & software)

FADEC responsiveness (taking into account the fluctuating power requirements likely to be found in the turbulent conditions encountered in the vicinity of large fires).

Failure modes and effects analysis, Health & Usage Monitoring Systems

Cockpit design / human factors – for both pilot and fire suppression system operation

Design and development (hardware and software) of fire suppression stabilised turret

Vibration control and structural dynamics

Flight test and certification

In service support

The overall task would be managed with a defined work breakdown structure (WBS) such as Mil Std 881D:

This would typically include

a. Integration, Assembly, Test, and Checkout

b. Systems Engineering

c. Program Management

d. System Test and Evaluation

e. Training

f. Data

g. Peculiar Support Equipment

h. Common Support Equipment

i. Operational/Site Activation

j. Industrial Facilities

k. Initial Spares and Repair Parts

Included within item (a) above is the design, integration, assembly, test and certification of all Air Vehicle elements and systems / sub-systems.

Planning (including taking account of long lead items) for all the above activities will be required to generate an overall development programme. This plan, with suitably realistic contingency allowances, will be required to establish programme costs and the associated spend (and investment) profile.

Overall Conclusion

A new large crane helicopter could feasibly be developed based on the use of a pair of Europrop TP400-D6 engines adapted for helicopter use.

There is a clear need for a helicopter of this type based on current experience and projected increases in wildfire events worldwide.

Bringing such a project to fruition requires a significant effort on a number of fronts. Not the least of the challenges is the raising of investment funds (possibly on an international or global basis) to see the project through to completion.

Without the availability of a significantly increased fire-fighting capability, there is likely to be a severe penalty in terms of the loss of property, livelihoods and lives, in a number of the widespread regions that are at risk. The potential economic damage of future wildfires is such that investment in the development of a modern, capable, helicopter system to fight these fires appears not merely prudent, but essential.

RV Smith

May 2021

8 comments

    • Ron Smith

      Co-Axial designs eliminate the tail rotor (which saves some weight and power). Another advantage is that the overall design is quite compact (useful for hangarage on board ship). Against that is the drag from two separate rotor heads – not necessarily a big issue where crane operations, logging and firefighting are the main roles. The other issue is that efficient hover power favours a low disc loading (weight over rotor disc area). Therefore a large radius single rotor is a straightforward approach. The saving from eliminating the tail rotor on the co-axial design would help, but one would still need a high rotor diameter to minimise hover power. My instinct points toward the single rotor solution, but one would need to do a design exercise on both configurations looking at both mass breakdown and performance (hover and forward flight) to make an objective comparison.

      • edin

        Firstly, thanks for the reply and I have to say this was a great article with sketches and planning and all. I’d love to see more like this.

        I am an electronics engineer so I have no aerospace knowledge except I’m into it so forgive me if I’m being an idiot. The point about the disc loading is very enlightening, but it’s calculated by both of the rotors’ disc areas, right? So instead of a 6 or 8 blade rotor, the same blades but a 3 or 4 blade coaxial configuration (with the same radius) should have the same disc loading except some interference between the rotors. Or is there something (like blades bending and hitting each other maybe?) that limits the radius in coaxial configurations?

  1. Gray Stanback

    So if the Mi-26 “is not available in a firefighting variant”, what’s the one in the photo doing?

  2. DBA

    I am from Canada and have worked a few significant forest fires. We have a saying: aerial firefighting is just using dollar bills to hold the fire until rains can put it out. This is not to say that that water bombers and helicopters do not play an important role in forest firefighting (they do), but one must be realistic that, utlimately fires are put by ground ground crews or a change in weather.Nonethless, this was fun exercise in helicopter design .

  3. JB

    I enjoyed the technical discussion, but can’t agree that forest fires or even global warming itself are the biggest threat we face. I would much rather face 50 years of global warming than deal with a serious global pandemic or even a limited nuclear exchange.
    Is there a reason that 50000lb class helicopters have been so successful but 100000lb class helicopters have not? Perhaps two Chinooks or three Merlins would be more effective than one mega fire bomber, all things considered.

    • Duker

      ‘reason that 50000lb class helicopters have been so successful but 100000lb class helicopters have not”
      The really successful ones are under 10 tons, ie the low 20,000 lbs. Once the very large helicopters come along the costs skyrocket and you get into tactical airlifter territory which is better for running cost

  4. Andrew Wagner

    I really enjoyed the blog and thorough thoughts about designing a super helicopter for fire fighting operations. I have been on numerous fires in a UH60 with a 660 gallon bambi bucket and most of the are remote with small dip sites. Countless times it has been difficult to fill a bucket in shallow ponds without pumps to fill the bucket. I think a significant challenge would be finding dip site close enough to the fire to fill an aircraft that large and then return to drop it, in that scenario the heavy fix wings with retardant may be more effective. In my experience a dip site close to the fire with a chain of helicopters is the most effective use versus just one large drop occasionally. Thanks for the blog!

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