Greener Aircraft, Innovative Design Topic

The more you have to lift, the larger the craft you need and the more fuel you will burn. If you want environmentally friendly, you need "light weight".

 

I'm only 37 and have been flying since 2007.

There have been a dozen new designs in the last 10 years but all those companies failed. Not because the design was bad but because the government makes is sooooooo cost prohibitive to launch a new design.

 

That's perfect. That's why I want this engine. I like flying mid teens, VFR, NORDO, Direct. There's no faster way to get somewhere.

 

Unelss you like to buy a lot of gas you really don't. That engine has a specific fuel consumption above .6 lbs/hp hr. At 250 hp you are burning over 150 pounds per hour (more than 23 gph). A good recip will be burning 18, so that's over $25/hr in DOC...

 

A gas turbine can be made to run on most any fuel. The Model 250 is actually allowed to run for a short time (I think it was like 30 minutes, but don't quote me on it, I'm going from memor here) on gasoline. Diesel fuel is a very close cousin to JetA, but turbine fuel nozzles are susceptible to coking up, and of course, diesel begins to solidify and have problems with wax coming out when it gets cold, do it's really a bad aircraft fuel. Gasoline doesn't have the lubricity of JetA or diesel, so that's why you have to rebuild the fuel control if you run it on gasoline. If the fuel nozzle is designed so that it doesn't coke up, you can burn most any liquid fuel in a turbine. JetA costs a lot at the airport, but it's really not that much more expensive to make than diesel, just the cost of insuring it is what it is and the ability to follow it from cradle to your tank is what makes it cost more. Airlines don't pay all that much for it, but anything bought at an airport does..

 

That's true for big engines, but small engines have a lot of room for improvement, and the technology to do that has been demonstrated, but it isn't in production yet.

Let's start by defining big and small. As it is now the dividing line is at about 1300 hp. Above that size engine you can cool the blades and the compressor is big enough that higher cycle pressure ratios (above 13 to one) become common. Those engines typically get SFC's of about .45 lbs/hp-hr. To put that into perspective that's actually a bit better than a good normally aspriated gasoline recip (.48) and not as good as a diesel (.36-.38). So if you look at engines like the T700 (2100 hp), T800 (1500 hp) or Series IV PT6 (2100hp), you are looking at cooled blades a bit higher PR's and very respectible SFC. Some people are thinking that the PC12 is thirsty, but when you consider the amount of power you are using, it really has a pretty efficient engine. Not cheap because of the cooled blades, but they are efficient.

The problem is that small turbines, below 1,000 hp have lower pressure ratios (around 8 or 9 to one), and the blades aren't cooled, and consequently they burn a lot more fuel than recip's. That's all true, but the real reason is that there hasn't been as much money invested and development carried on in small engines. The leading small engines (PT6 series II, RR Model 250 C20, and TPE331) are all really old engines. They were first laid out over 50 years ago. Think about that for a minute. How much better are car engines than they were in 1960? Those legacy engines have an SFC of .6 or higher, while even the latest engines in that size class have an SFC of about .52 (think Honeywell HS900), which is better, but still not quite as good as a recip.

Recently there has been some development and testing of smaller engines (in the 750 hp class), that have much better SFC than the current production engines. Honeywell has run a 750 hp engine under the Army's Small Heavy Fuel Engine program. The Army put up $25mil and Honeywell matched that and they have demonstrated the program goal of an improvement in SFC of 20% from the baseline (old tech engines). There is another company working under a government contract that will run a core early next year using advanced materials that will have an SFC of less than .4 lbs/hp-hr. That's within 5% of a diesel, and the engine actualy weighs less than the legacy turbines. One of the reasons the airframers aren't bothering to put an RR500 into a 4 place airplane is that about the time they get it certified, they are going to be facing some new technology and they will have paid a big bill to cert their airplane only for it to be obsolete in a couple of years. To put that into perspective, think about an engine that weighs about 200 lbs (a lot less than a PT6), makes 700 hp at takeoff, and at altitude, making 360 hp, pushing a Lancair propjet at almost 400 kts and burning about 20 gph....

Bottom line is that the technology to improve small turbines is out there and is being developed, but the market for turboprops is relatively small. Unless you have other places to sell the engine, you can't make a business case for the cost of development and certification on the turboprop market by itself.

Pratt is in no hurry to replace the PT6. It is the ultimate cash cow, paid for a long time ago and making money every day. The issue is, as it almost always is, more a matter of economics than it is technology. When somebody makes a move in the market, the others will have to jump in, but until that happens it's still a standoff. It's happening slowly, but it's going to happen.

 

It makes more than 250 in the mid teens if it makes 500 at sea level.

But yes, recipes are much more efficient. Just not as sexy.

 

Well, it would if it was capable of running at takeoff power at cruise. Unfortunatley that isn't the case. About all it will do at 16,000 ft at takeoff power is about 270 hp, (take the pressure at that altitude, add ram pressure, divide by 14.7 and multiply that factor (.54) by the sea level static horsepower) but that's assuming you can cruise at 100% power, which in that engine you can't. As I said, it depends on how it was rated and what percentage of the rating you can use for cruise. If you ran it at 80% power at 16,000 ft, you are only getting 215 hp, which is, like I said ealier, pretty anemic.

Sexy, yes and it makes a nice turbine whine, but don't plan on going very far or very fast with one. At altitude with a turbo recip (and yes they aren't sexy), you will be able to get 260-300 hp and that translates into cruise speed. With 215 hp in a four place retractable at 16k, you are looking at something like maybe 230kts cruise. You gotta want a turbine really bad to pay $150k more to go slower and burn more fuel. I'm sure there's a market for it, but it's pretty limited.

For a four place single turbine, you pretty much need an engine rated at about 750 thermodynamic horsepower as a minimum, 500 isn't going to cut it. You can put a 500 hp gearbox on it to keep the weight down and flat rate it at 500 hp, but for that market you need a hot rod and be able to go much closer to 280-300 kts at altitude to justify the higher cost. A 750 hp turbine is going to cost almost the same as a 500 hp one. The parts aren't that much bigger, and a lot of parts (like the expensive fuel control) are the same pieces. While the actual cutting time on the machine tool is a bit longer (like 7%), setup time, inspection time and assembly time are the same.

This is why, unfortunately, that engine isn't likey to find a home in an OE application. Mooney learned a hard lesson with the PFM series of engines. Customers want something for their money. Although the PFM was sexy and smoother and much more quiet than the Conti, when you pushed the throttle to the wall, it didn't go any faster and it wasn't any more reliable either. So in response to that they voted with their wallet and bought the the Conti.

 

Got it.

Didn't Mooney recently advertise development using the RR500?

What engine does the Lancair Evolution use?

 

Mooney and RR jointly announced that they entered into a "cooperative agreement" to evaluate the RR500 in future products. My spies tell me that this is going nowhere for pretty much the above reasons.

The Evolution uses a PT6-34 rated (not concindentally) at 750 thermodynamic horsepower for takeoff. It has a 560 hp gearbox on it, so it is flat rated at 560 up to about 13,000 ft. That's a pretty thirsty engine with an SFC in the .595 lb/hp-hr at takeoff power, and a bit higher at cruise. If you are crusing at 300 hp, you are looking at 180 lbs/hr, or about 25.5 gallons per hour. Cha ching....

 

A Baron burns at least that. That's less than half what a PC12 or TBM burns. It's not bad.

 

Thanks, it's a tough biz in that you really need to have the vision thing down and have a lot of guts. As they say aviation is the best and fastest way to turn a large fortune into a small one..... The amount that it takes to cert any engine is so large that if you can’t make a business case it isn’t going to happen, and the market has gotten more fractured, that it is difficult to figure out where you can sell enough to make it pay off.

I’m really not familiar with the LSA market. To me it’s kinda like a bugeye Sprite. It’s fun to run around the neighborhood in, but if you want to go a long distance it isn’t much fun. Good for training and bumping around the patch, but it isn’t what I’m interested in. I think it’s great for the hobbyist, but the limitations on speed don’t do much for me so I lose interest.

If you look at what Cirrus and Columbia did it was pretty amazing. In a very short time they totally changed the look of GA airplanes. That said, a lot of low hanging fruit in airframes went away with the change to composites, and that hasn't slashed the cost of airframes. If somebody would figure out how to stamp (or superplastic form) and then inexpensively JOIN aluminum, you might have a way to cut the cost of airframes down a lot and that would be a game changer.

Not to be too parochial, but propulsion will have a lot to do with what comes next in the four and six place single engine market. Airframes using the existing engines are pretty well optimized. You could put retractable gear on a Corvalis and make some more improvement, but it won't be huge. If you look at the price point for these airplanes the incremental cost of a new generation of turbines will add about $120-150k to the selling price and that is probably a price point that would sell. Cessna and Cirrus have sold a lot of airplanes and those customers would have something nice to move up into. With good SFC the useful load and range would actually improve relative to the existing turbo recips. Think of an Evolution burning 18 gph going more than 300 kts in the mid teens with a slightly pressurized cabin. I think something like that is doable with the emerging technology and it would probably sell over 200 units a year for a number of years.

I was always in love with the Epic aircraft, they really nailed it with that airplane. If somebody could get something like that certified, it would be a strong seller. If Klapmeier can get his version certified it will sell. Would like a potty in the plane for that price, but if it’s fast enough maybe you aren’t up there long enough for it to be an issue.

If there were a 250 hp diesel that didn't weigh a ton (ok, well 1/4 of a ton) I think that would have an impact on the lower end of the market.

Direct injection gasoline technology could also make a diesel less attractive. There are already spark ignition outboards that can run on JetA and that would allow a much lighter engine than a diesel if you wanted to run jet fuel.

What happens with unleaded avgas will also shape what happens. If Swiftfuel is adopted you will have 100 octane and a direct injection smaller bore engine could run a compression ratio of 11 or 12 and have nearly the sfc of a not too hot diesel. Think of a 360 cu in aluminum small block turning at 2600 rpm making 250 hp (I'm no fan of geared engines). It will be little lighter, could live a long time and the cost might be 60% of a lycosarus if it was based on auto technology. Lower cost motors would go a long way to making aviation a lot more affordable.

Sorry for the long post, kinda rambling here.. The biggest impact will come if costs come down a lot compared to what they are now. Green is nice, but I don't think it will drive the market. Initial cost improvements will have a bigger impact, and lower operating costs are nice too, but will take second place to initial cost.

 

Yeah, the Epic is great but not certified. The certified version will be more and if that's the case, I'd rather have a TBM850 made of metal. I used to have an SR22 and I'm no fan of composite. Too feeble (at least what Cirrus uses is). What Cirrus calls composite, I call fiberglass.

I've seen a couple Epic's but I wouldn't say they've sold too many. Over a $1M for a non certified 6 place, pressurized home built is not a big market.

 

All very true. The Epic isn't certified and at $1.9M with a lot of assemby required you aren't going to see very many around. I've been through the Cirrus factory and it's not built as tough as a metal airplane. That's why I'd like to see a real metal airplane that didn't cost an arm and a leg.

I've seen friction stir welding and it sounds like a great thing, but it's a lot harder to do in practice than with two flat pieces clamped to a table. When the Eclipse started using FSW, it was supposed to be the holy grail and was going to cut the cost of the airplane to where they could make them for what they had sold them for... Well, we all know that didn't happen and the biggest reason that that Eclipse failed was that they couldn't sell what they were building for what it cost to build it. For what ever reason, schedule, tooling development, strength, I don't know, but if you look inside the hull of the Eclipse all of the stuff below the floor was put together with a bizillion rivets and looked pretty conventional.

Aluminum is a bear to bond together, it forms an oxide very quickly and that makes bonding difficult. I know that everything from the GA Tiger to the Cessan Encore and all the way to 777 have bonded stuff, but it really isn't cheap. If you want to make a lot of money in aviation, figure out how to connect thin aluminum to thin aluminum without expensive bonding, causing no distortion and do it for pennies per length of joint... People a lot smarter than me have been racking their brains on that one for a long time and still haven't found anything better than rivets.

Composites are cheap in boats, but somehow in airplanes they got a lot more expensive. And Cessna learned an expensive lesson in quality control of composites when they sent the wings of the Corvalis to Mexico and got back some junk parts.

I used to have a T shirt that said: I know there's a lot of money in aviation, I put it there... True then, true now.

 

You mentioned an extruded wing profile. This was done by Lockheed on the Electra. They extruded the skin and integral stiffeners as a rolled up tube. When cured, they sawed the tube's outer skin lengthwise and flattened the curled up item to become a wing skin with integrally formed stiffeners. The problem with that kind of thing is that you cannot taper the thickness of all materials as you go out to the tip so you're carrying around stuff that you don't need....weight.

 

So do they have to do the bonding in a non-oxidizing environment... maybe a nitrogen environment?

 

They typically don't because you would have to protect the part from the air from when it was last cleaned until it was coated with the bonding agent. I understand they do it with coatings or abrade it after the bonding agent is applied. Either way unless the process is controlled carefully you can end up with bad bonds and that gets expensive if you have to redo it.

If you are a big company with expensive equipment you can do it successfully (Boeing and Cessna do it all the time), but it just isn't cheap. Once you get the two parts together, they have to finally cured in an autoclave at temperature. I've been in the Cessna facility where they were doing parts for the Encore and they had huge autoclaves for doing fuselage parts.