The Housai HS-6 engine

History and background

The AI-14 engine was given to the Chinese, together with the design of the Yak 18 A military training aircraft – of course the predecessor to the Yak 52. The Chinese decided to completely redesign the aircraft to produce the Nanchang CJ-6, but did very little to change the engine for their use.

Chinese overhauls of these engines

The Chinese government decided to sell large numbers of Nanchang aircraft into Western civilian markets where they have been a great success, and indeed are an excellent aircraft, and better in many respects than the Yak 52. At the same time, the Chinese offered overhauled engines to Western owners operating Nanchang aircraft. From what we have seen these overhauls were not particularly high quality – seemingly disassembly; a good clean; new piston rings; grinding valves; possibly changing crankshaft bushings; painting and some test running. But certainly good enough for a good period of further use.

However, approximately three years ago, for reasons that are somewhat inexplicable, the Chinese government decided that they would not overhaul any engines unless for their own air force. We have not seen any explanation for this decision, and it does seem surprising that they would not even overhaul engines for friendly air forces using Nanchang aircraft – and indeed we have been approached by the air forces in both Sri Lanka and Bangladesh to see if we could overhaul their HS-6 engines.

Technical differences between AI-14 and HS-6 engines

Four years ago, we obtained two HS-6 engines and carefully dismantled them, comparing every component both in terms of material used; dimensions and so forth with the AI 14 engine with which we are very familiar. As we expected, the differences were relatively small, and all the differences are given in the table below. As can be seen, the differences are typically not significant, and probably the only one of significance is the change in the supercharger drive ratio – and clearly this is where the additional 25 hp increase was obtained with the Chinese engine. However, again as can be seen from the table, we have worked out the solution to all these small differences in terms of overhauling the Chinese engine.

Our program to overhaul the Housai engine

We see there being two somewhat different markets for these overhauls. Firstly, for the military market, and indeed we are in discussions with the air forces in both Bangladesh and Sri Lanka, we can see that their requirement is for an engine that is absolutely standard, so our proposals have been to keep the engine as original as possible, but to use the significantly superior SPC pistons which, although using the same piston “blank” have three totally modern piston rings, so significantly reducing oil consumption and related issues. And also the technical specialists in these air forces seem to accept the idea of our excellent automotive plug conversion. But otherwise the engines would remain as standard.

However, for the civilian market, we believe an important issue is the need to produce an engine of significantly more power than standard, which would hopefully overcome the natural tendency of some owners to go to the considerable extra expense of converting their aircraft to the M 14 engine, which involves important airframe modifications and of course the requirement for a new propeller.

The modifications required for increased power

Our program includes:

  • Using higher compression pistons. The standard compression ratio of both HS-6 and M 14 P is 6.2:1. With our most powerful M 14 PF engines we use pistons giving a compression ratio of 8.2:1, but for the Housai, we are having pistons manufactured by Carillo in the USA with a compression ratio of 7.4:1 which we feel is an appropriate compromise between power and long life.
  • An important part of our development of the M 14 family of engines is to have recognised that they were originally manufactured on a production-line basis with semi-skilled workforce and so they were allowed quite significant tolerances in dimensions. In particular within the supercharger, the closer the rotating impeller can be placed to the fixed diffuser, the greater the boost pressure will be and so the greater the power. So with very careful assembly, we have demonstrated that our supercharger assembly will give an additional 25 hp with the M 14 engine.
  • The use of our latest automotive plug conversion kit, as standard, together with our new AYC magneto coils, both of which have been extremely successful and have been fully authorised by EASA.
  • Using standard engine tuning technology, we have made sure that all induction passages are smooth with minimum resistance to airflow.
  • We have increased maximum rpm.

Our overhauls of HS-6 engines

These include painting the engine in the colour of the customer’s choice, and of course we can use different colours for separate parts like magnetos; valve covers et cetera; nickel plating of all exposed metal parts; incorporating our new automotive plug conversion kit and our AYC coils; the high-compression pistons with modern piston rings; assembly to much tighter tolerances; balancing to a factor that is three times as good as originally allowed; every part of the engine totally overhauled to a condition that we believe is better than factory new since we are using skilled specialists rather than production-line workers; testing the engine for a minimum of three hours on a proper dynamometer and all results given to the customer.

We completed our first HS-6 overhaul for a Norwegian Nanchang owner and it was tested in January. This was done on the Aerometal calibrated dynamometer, and we were very pleased that our conservative anticipation of 330 hp was significantly exceeded – we obtained 354 hp at only 2500 rpm, and are absolutely sure that at 2600 rpm the power would be in excess of 370 hp. However today there is no propeller that will fit the Housai propeller shaft that can transmit powers of much more than 320 hp. This is not a structural issue, but one of aerodynamic efficiency.

But we are talking to MT propeller in Germany; GT Propeller in Italy and Whirlwind propeller in the USA, and are confident of having a solution for an improved three-blade propeller which could deliver 400 hp in the medium term.

We are currently working on orders for a further six Housai overhauls, all to the same specification as this engine.