Turbine (Turboprop) Seawinds

There are at least three Seawinds flying with Allison turbo-prop engines.

In spite of the problems encountered in adapting engine to the airframe, and visa versa, they are magnificent machines. There are at least two other turbine Seawinds currently being built, both by ISPA members. Not everyone is using the Allison turbines. Others being adapted include the Pratt & Whitney PT6A-20. What follows is an extremely brief and limited discussion about the turbine Seawinds.

(Photo above courtesy of Leon Pesche)

John Hare's Seawind. Powered by the Allison 250-B17 turbine engine. Built with assistance from Turbine Design Inc. (The ISPA is a small informal organization of Seawind enthusiasts. We have no formal affiliation with either SNA Inc. or Turbine Design Inc.)

Because of the reliability and high power available from turbine engines in such a small and light package, turbines are a likely power plant for the Seawind. Some turbine power plants produce upwards of 550 HP. Almost as soon as the Seawind kit arrived on the scene, those who were familiar with turbine engine applications started designing and adapting these engines to the kit.

The early leader in turbine adaptation was Doug Karlsen of Turbine Design Inc. Doug completed three turbine Seawinds using the Allison 250-B17 engines. Turbine Design’s first completion was featured in Popular Science, and plowed a lot of new ground for the turbine Seawind enthusiasts and pilots (also see the "Seawind Piloting" page on this site).

Allison turbine engine installed on a Seawind (photo courtesy of Doug Karlsen)

Another view of the Allison turbine engine installed on a Seawind (photo courtesy of Doug Karlsen)

Of course, early on, to the newly founded SNA company trying to bring a newly designed kit plane to market, the turbine applications added complication to the already monumental task at hand. Understandably, in an effort to limit their own liability (among other things I'm sure) they denounced the whole turbine engine venture. Also understandably, since SNA has not undertaken the hugely expensive engineering effort necessary to redesign the Seawind kit for a turbine application, they still do not support the application of a turbine engine to their airframe.

Nevertheless, there are builders who can see the excitement wrapped up in such an undertaking, and continue to push the envelope. This is what experimental aircraft are all about. The ISPA seeks to support and promote these builders in any way we can (however limited our support may be).

Jorge Ortiz's Turbine Powered Seawind

Following the ISPA posting of this page, our ISPA members sent the additional information that follows. The ISPA wishes to thank Doug Karlsen (Turbine Design, Inc.) for adding the clarification shown in green.

Turbine Design had several bolt-on Allison kits available. Engines and cowls were from NOMADS. [Following some turbulent times over the adaptation of the turbine engines to the Seawind,] Turbine Design became un-interested.

The Allison weighs quite a bit less than the Pratt PT6A-20, but has a forward facing intake which houses a compressor wheel that turns 55,000 RPM and is more vulnerable than the PT6. The Walter is configured essentially the same as the PT6. Turbine Design is putting the Walter engines in many a/c (check their web site).

The Allison weighs 195 lbs. The Lycoming weighs about 469 lbs. The Pratt weights about 375 lbs fully dressed. Weights are reported differently depending on the accessories attached to the engine. The Walter weighs 440 dressed. This statement about he compressor wheel spinning at 55,000 rpm is not true. The highest RPM of the Allison is about half that, not that it really matters. I would say that the Allison weighs about half the Lycoming. And also about half of the Pratt. A little less than half of the Walter. The Walter is not a candidate for the Seawind, and we really never considered it an option.

I would not think that there is a great advantage in trying to install a Walter in a Seawind, except they are cheaper than the PT6's. The Walter would have tremendous excess power, which in the Seawind airframe would be unusable. Turbines are a little "peaky," meaning that they run better at the RPM for which they are designed. You can run at something less than design RPM, but they are less efficient.

[Turbine] fuel efficiencies are such that they may compare generally well against the reciprocating engines. They will do this at altitude and being realistic about the whole thing...I rather like the idea of flying over a lot of the weather. I would flight plan for 22,000 to 24,000 FT. The service ceiling of the Seawind with the PT6A-20 is right at 37,000 FT. This is, of course, a good reason to put 2-60 cubic ft. Oxygen bottles in the nose compartment and get rid of the ballast. All the rationale may prove to be interesting!

We had one of the Turbine Seawinds up to 15,000 ft, but I expect that that would be the limit. The little Allison C model starts to run out of air at that point. I would say that your Lycoming powered Seawinds, would probably not make it much past 10,000 ft. All things being equal, the Seawind would handle about the same at 10k with the piston and at 15k with the turbine. I would say that the service ceiling with PT6-20 will be about 22,000 ft. and it won't handle very well above that. This opinion is based on my experience with PT6 on King Airs, and replacing those engines with Walters.

We installed the Allison on the Seawind only because of the weight. That was the most important thing to us. In those days, we only knew what we ourselves had tested. Paul Array's plane was about 2500 lbs. In those days, the max gross was 3200 lbs. Paul had 110 gallons of fuel storage. That is 660 lbs, and if he filled the plane up with fuel, he couldn't get in it. Never mind that the plane needed ballast if he was solo. [Also see the page about "Gross Weight" in the builder tips section of the ISPA site. ed]

My customer, Gerard Pesty, didn't need a plane that he couldn't fly if he filled it with fuel. That was the reason we started looking at a lighter engine. At some point, much later, we felt that we could lighten and stiffen the plane if we built it out of carbon. At least that was the test, to build a fuselage only from Carbon. (That of course, as we all know, turned into a 5 year lawsuit, that continues to this day.)

One of the problems that you mentioned is the fact the the prop hit the mid deck. We could take care of that if we cut the prop down to 76 inches, as the Seawind piston takes (I think) but we would rather use a 78 inch prop, and instead, teach pilots not to get into the violent porpoising in the first place. [Please note that problems with porpoising are not only encountered in turbine Seawinds, but all Seawinds and seaplanes. Porpoising is a piloting issue. Also see the page about Seawind "Piloting" on the ISPA site. ed]

That is where we [Turbine Design, Inc.] are at at this time [early 2003]. One of the benefits of using Carbon fiber, would be a much stiffer plane. Have you ever grabbed hold of your Seawind on the trailing tip of the float, and given it a good shake. The fiberglass / vinyl-ester combination, although a good material, is very elastic. Carbon would stiffen the plane, and if it didn't flex as much, we might find that the prop would be held away from the mid-deck better during very violent nosedives. [The obvious trick here is to improve the stiffness while retaining sufficient elasticity to withstand the beating that an amphibious aircraft is subjected to during water operations. We won't know unless (or until) more research and design development is done on the Seawind, and under the current circumstances, this seems highly unlikely. ed]

The following is provided in an effort to address or clarify some of the observations listed on the Turbine Design Inc. web site.

Since completing the three Allison powered Seawinds, Turbine Design Inc. has stopped their turbine adaptations to Seawinds. Several reasons are sighted on their website. In spite of the criticisms found on their site, there are still many of us who feel the Seawind is an exciting and worthwhile endeavor. Right or wrong, what follows is some of our reasoning. We provide the information so you may draw your own conclusions.

As with any experimental kit plane design, there are limitations that ISPA builders continue to find innovative and elegant ways to deal with. In the Builder tips section of this site, there is a page entitled "Seawind Gross Weight" that addresses existing weight limitations. In spite of it's huge comfortable cockpit, the Seawind is still and airplane, and requires careful and prudent planning with regard to weight and balance.

Furthermore, when equipped with auxiliary fuel tanks, the Seawind has a huge fuel capacity. It cannot be flown with full fuel tanks except in some very limited circumstances, but flying with fuel tanks at full capacity is not often required. As most pilot experience bears out, the fuel tanks almost always outrange the human bladder anyway. Of course, the following quote also applies: "The only time an aircraft has too much fuel on board is when it is on fire." (Sir Charles Kingsford Smith, sometime before his death in the 1920's.) As with any airplane, the gross weight limitation is largely a factor of landing weight. If takeoff gross weight is pushed to 3400 or 3600 lb, and flight planning allows for flying off fuel to reach the 3200 lb landing specification, the Seawind does indeed become practical for many of us.

As for freeboard capability (also sighted as a limitation on the Turbine Design Inc. site), the following photo (of a piston powered Seawind) gives a pretty good illustration. Of course, in rough water, prudence would dictate that the canopy be kept closed. Note also that there are large wing sponsons that add flotation at the wing tips and prevent tipping.

Terry Goodyear's beautiful Seawind, C-GTSG. (This is NOT a turbine powered Seawind. This picture appears here to illustrate the amount of freeboard on a Seawind. See text above.)