Which Engine to Choose?


Which engine is the best choice for the Merlin PSA aircraft? That is like asking General Chuck Yeager which was his favorite aircraft. His answer? “The one I walked away from”.

This is much more of a subjective and personal choice than it should be. Choose a car and you generally trade power for fuel economy plus you usually have limited choices anyway. The designers install enough engine to make the car viable and anything bigger than that is at the buyers’ choice and trade-off. Did you ever choose one auto engine over another because one is more likely to quit running? Probably not. And even if it does is it an inconvenience or a life-threatening situation?

So, what are the criteria for choosing an aircraft engine? The answer primarily depends on the mission of the aircraft. But for this discussion let’s assume our primary, if not only, mission is to get up in the air to enjoy the experience of flight and maybe to go somewhere for lunch. And of course, to get back.

Allow me to present several criteria for consideration:

  • Low acquisition cost. We cannot have a low-cost PSA aircraft with a high-cost engine.
  • Reliability, both perceived and actual. Example: I think the Rotax 582 is just about as reliable now as most other small engines but due to my experience flying ultralights with 2-stroke engines in the 1980s I just do not feel the comfort of reliability regardless of reality.
  • Maintenance. Some owners may wish to perform their own work. Other owners may prefer to pay someone else to do this for them. Either way the time and costs must be considered.
  • Noise. This may not be as big an issue in the USA as it is in Europe but it should be. Noise is just another form of pollution. And a quiet aircraft is most certainly less fatiguing and more comfortable for the pilot as well as being neighbor-friendly.
  • Fuel consumption. Not just for the cost per hour but less consumption means greater range and/or bigger safety margins.
  • Power. I love the 3-second take-off and the big climb!

OK so far this dribble has mostly been my subjective opinion. Now how to quantify it like most of us left-brainers like to do.



                          HKS 700E                                                                            Rotax 582

Here are my random criteria with my subjective rankings. You can insert your own data.




POWER 9 8 3
WEIGHT 6 5 1
TBO 2 8 8
RANGE 5 9 1
TOTAL 49 71 53



  • I include electric power as I know it is coming. I think the Pegasus 0-100 is coming too but likely comparable to the HKS and at a Rotax price so it could prove to be a good option someday.
  • I concede that I am perhaps not being fair about the reliability of the 582. As mentioned before.
  • Rotax has 65hp, whereas the HKS has 60hp. The HKS however has a flatter power curve and therefore the two may in fact have similar power at cruise settings or possibly the HKS has more power at lower RPM settings. Close call. The electric can have the same power as either gas motor just not for very long so its rating is low.
  • The Rotax is supposed to be the lightest. That would be true for the 503 but that engine is no longer in production. I am now convinced that the supposed ‘weight penalty’ for the HKS is next to nothing. Maybe 5-10 lbs. The Rotax has coolant, tubes, radiator, overflows, 2 oil reservoirs, etc. It all adds up. The empty weight of the 1st Merlin with the HKS is 413 lbs without paint or about 425 finished. My Merlin with the 582 is 446 lbs but includes the BRS so empty weight is basically the same. The electric motor weighs only 25 lbs. Add controller, balancer, and a truck-load of batteries and it will break the scale.


Merlin Electric Motor Engine Mount                                                                 Inside of the electric motor revealed!


  • Electric wins in maintenance as there is none. HKS has only a slight advantage here over the Rotax as all aircraft have the annual and 100-hour inspection. TBO is however very significant.
  • The other criteria are pretty mush self-explanatory. Rank them as you wish.

I am not surprised that the HKS wins under my evaluation. I am surprised that the electric and 582 are closely ranked. Of course, that would change if you want to fly for more than 1 hour or wish to go somewhere.

Here are some other data I pulled out of my hat:

OVERHAUL $2,500 $2,500 $20,000
TBO 300 1000 12000
COST/HOUR $8.33 $2.50 $1.67
$2.75 $11.00 $6.88 $0.00
ACQUISTION COST $6,500 $12,000 $20,000
LIFE HOURS 1000 3000 20000
AMORTIZATION $6.50 $4.00 $1.00
TOTAL COST/HOUR $25.83 $13.38 $2.67

The Rotax acquisition cost is 45% less than the HKS but costs almost double to operate per hour all things considered. Well, almost all things considered. I did not include the cost of oil which is sure to be more with the 582.

So, do you want to pay more now or more over time? I suppose it depends on the number of hours you plan to fly per year. If you only fly 50 hours per year, then your savings total $669 per year with the HKS which translates to a payback period of 10 years.

However, if you fly 200 hours per year the savings total $2,675/year which results in a 2-year payback period. So financially you would be better off with the HKS if you plan to fly a lot. Under 3 years is a good payback investment so 150-200 hours works.

After all this evaluation and analysis, the bottom line for most people I believe will be to choose reliability and therefore safety over cost. Moreover, in the final analysis, the perceived reliability of a 4-stroke is greater than that of a 2-stroke engine.

Unless of course you happen to have a spare 582 lying around. Then go for the most cost-effective solution.

Before you spend a lot of time searching for engines or inquiring why I am only comparing 2 engines here understand that:

  • Engine manufactures come and go with most of them going. It takes years and deep pockets to establish a brand and prove reliability. There must be a worldwide distribution and service network established. There must be hundreds of hours of hours flying in the field. Hundreds of customers being used as ‘test animals’. We would not offer an engine that has not already passed the test of time from a company that has not passed the test of longevity.
  • There should be at least ASTM certification proving compliance in the LSA class.
  • It takes a pretty big effort from the airframe manufacturer to install, document, and test-fly an engine. This includes the 3D design, cowl design, mold-making, prop choice, cooling and other systems. It can take 6 months of effort for every new engine installation package. We need to know that everything works as expected.

There are barely even a few engines in the weight and HP range suitable for the Merlin. Only recently are more 4-stroke engines coming on the market. This may be a main reason that there have not been more PSA class of single-seat aircraft on the market. I expect that will change now with the HKS, 0-100 and a couple more possible choices coming. Affordable, reliable, and comfortable engines……


  1. Thanks for the informative post Chip. I am going to share a link to it with my friends over at Pilots of America (where there seems to have been some ongoing interest in the Merlin).


  2. Hi Chip
    I am one of the first four UK pilots waiting and hoping our Merlin’s will be delivered in the next few days.
    I have read and viewed with great interest your postings on your web site.
    In this post you make some valid points regarding choice of engine for our demure aircraft.
    However in the UK the HKS is not thought of with the same regard as you have for it. Some manufactures used it but reliability and lack of power were stated as the main points for not installing it. On the other hand the Rotax 582 has a proven track record for reliability and power to weight.
    My last aircraft was a RANS S6 ES microlight powered by a Rotax 582 oil injected with a maximum takeoff weight of 450kg. Following engine inspections at the required intervals there was no discernible wear to the vital parts. What was evident following a de-coke at two hundred hours was the piston rings showed signs of sticking.
    Here in the UK we are restricted to a maximum takeoff weight of 300kg and this is increased to 315kg if we install a parachute recovery system. Therefore the Rotax becomes the engine of choice due to its weight advantage (at least 6 kg lighter). As far as I can tell a HKS 700e in the UK will also be more costly by at least 50 – 60%.
    As I fly about 50 hours per year 300 hours TBO will give me about 6 years normal servicing costs for a two stroke Rotax 582. Who knows electric power may come of age and be readily available.

  3. From what I have been able to gather, the UL260i at 97 HP has these traits going for it
    1. Designed for aircraft
    2. Low RPM
    3. Single carb
    4. Wet sump engine
    5. 40 more HP
    1. Price is $6,000.00 more than HKS

  4. The HKS engine is a fine piece of machinery but has items that I personally don’t like in an aircraft engine, to wit:
    1. Gear reduction box
    2. Dry sump engine
    3. Separate oil tank plus needed hoses
    4. Two carbs to balance
    5. Lots of noise because of high RPM
    6. Needs a muffler
    7. Needs large oil cooler

  5. Thanks Chip. Great comparison!

    The TBO for the Rotax 582 vs, the HKS 700E and the reliability of a 4 stroke and range did it for me when I ran the numbers. Just need to sock away a bit longer to afford the HKS.