engine requirements?

[RVbySDI]

Originally Posted by Ola

If science could make a battery that came close to gasoline in terms of watthour-to-weight, the electric motor would be perfect for aircraft engines. It's as simple as you get. Start up checklist: "Switch ON" End of list. Push lever forward to go, pull back to stop. "Switch OFF" whenever you want. The torque curve is amazingly flat, you could turn it at any practical speed without the need for gears. Silky smooth and very quiet. Shock cooling? Hah! Dive as fast as you dare and let the windmilling prop recharge your battery!

The downside is that charging time will probably never come close to the 2-3 minutes it takes to fill a tank even if discharge times were the same. Tolerable compared to the benefits of simplicity, but that's seen from the dreamworld perspective set in the first sentence...I discussed this on another forum a few days ago and did some non-scientific math. It showed that current lithium-ion batteries are 26 times heavier than gasoline per watt-hour of stored energy. Battery tech needs a breakthrough...

I would agree with Ola here. We have been talking so far only about internal combustion (IC) engines but the original question asked about the best engine for aviation. With its flat torque curve an electric motor develops its maximum torque at its initial startup and that torque does not fluctuate all the way to its max RPM. No matter what load you place on that motor it is still developing the torque clear up until it stalls and stops. That is pretty good for our needs.

Add the simplicity of basically one moving part (the rotor around the stator or, perhaps in some motors, the stator around the rotor) and you have a very simple and, might I say, bullet proof power plant. The size and weight are considerably smaller than the comparable IC engine that produces the equivalent amount of power.

The storage medium for electricity is the downfall of electric motors. At present there is no way to store the amount of electrical energy needed to feed an electric motor. I just read a few weeks ago about some research dealing with "ultra-capacitors" that sounded promising but nothing is currently available except the various types of heavy chemical batteries that are difficult to recharge and quick to discharge at high power settings.

I would love to have a very quiet, low vibration, efficient (electric motors are around 98% efficient compared to 28-35% efficiency for IC engines), powerful, simple, low maintenance motor on my plane if I could.

[rtry9a]

JCONARD,
Interesting comments made. I admire the older Lycoming engines' performance and reliability (not durability); I'm suspect of the newer offering's quality controls and too high cost; and, I believe the time as come where automotive apps are coming very close to parity with the older conventional designs. Regarding your specific comments, here are my opinions...

In #1 regarding air cooling, I agree that air cooling will generally be lighter overall than cooling fluids, pump + radiator, however, the downside is that those heads need to be heavier to act as heat sinks to transfer the heat to the cooling fins. Air cooled heads typically run much hotter than water cooled heads (reason for their high efficiency, btw), which at worst results in problems with detonation and valve problems in high-powered apps. At best, an air cooled engine needs to be closely monitored for heat buildup and typically requires operation at less than optimum power levels. A well designed water cooling system is normally trouble free, has no real operating limits, and I believe, is better for engine lubrication and overall durability).

It is easy to increase the size of the radiator to increase cooling capacity, hard to increase the air flows/fin surface area. Bottom line, I believe water cooling has an advantage with high power (performance), air cooling has an advantage at low power (cruise). At 160-180 hp, we seem to be flirting at the crossover point. To be fair, I believe the Eggs (and some early rotaries, as well) have had problems getting adequate cooling with incorrectly-sized radiators, particularly at low air speeds.

The heavy cranks/rods are more function of the displacement (and internal stresses) required in a large bore/ long stroke reciprocating piston engine than they are with air cooling.

#2 regarding weight- You are wrong regarding Mazda Rotaries; an OEM Mazda Renesis engine (w/o cooling & exhaust) weighs slightly less than 200 lbs. dry. The redrive, cooling, and exhaust systems brings it up to ~350 lbs, close to the O-320/360, however, the potential peak power output is closer to an O-540 (250+hp).

I believe you are correct with the Chevy/Subaru's- they tend to be a little heavier than equiv powered Lycs, I believe, and normally produce slightly less output than advertised (because they have a hard time producing peak hp rpm due to less-than-optimal redrive gearing, I believe). The Subs are great engines otherwise.

#3- regarding fuel efficiency, the data is not particularly comparable nor reliable. Most of the data (Ive seen) regarding the O-360s fuel burns are anecdotal and generally try to pass ideal burn rates, in a low-power leaned-out setting, as normal. At full power, the reported burn rates seem similar (13gph), as they should be when based on hp output. Normal bsfc's should fall in the .45-.5 range, anything less than .4 should be suspect.

Mazda rotaries can be safely leaned to 20:1 without detonation (a function of their large combustion chamber surface area and stratified charge effects) in cruise. Past rotary data comparisons reported by Vans did not have leaning capability, so they were richer than optimal. Real World measured 4.3 gph @ 120mph, 6.0gph at 170 mph, 8.2gph @ 202mph in an 87 octane mogas fueled R-4 and 2.85 reduction ratio. I believe these numbers are close to normal Lyc burn numbers, are they not?

Bottom line, I believe the BSFCs are close at high power settings, and the Lycs possibly have a slight advantage at low power settings. High power Lycs need high octane avgas, the Rotaries do not; the actual cost per mile is a big advantage to the rotary because of the cost differences of mogas vs avgas, where available.

#4- regarding simplicity... One of the things I like best about the rotary is its inherent simplicity- only 3 "beefy" moving parts, that see none of the stresses caused by the constant acceleration reversals seen in reciprocating engines. Rotary engines do not catastrophically break down and stop like most other designs when run as designed. Worst case, they continue to RUN at lower power, but still get you home. When broken (low compression), they will not start.

[Harvey L. Sorensen]

Originally Posted by RVbySDI

I would agree with Ola here. We have been talking so far only about internal combustion (IC) engines but the original question asked about the best engine for aviation. With its flat torque curve an electric motor develops its maximum torque at its initial startup and that torque does not fluctuate all the way to its max RPM. No matter what load you place on that motor it is still developing the torque clear up until it stalls and stops. That is pretty good for our needs.

Add the simplicity of basically one moving part (the rotor around the stator or, perhaps in some motors, the stator around the rotor) and you have a very simple and, might I say, bullet proof power plant. The size and weight are considerably smaller than the comparable IC engine that produces the equivalent amount of power.

The storage medium for electricity is the downfall of electric motors. At present there is no way to store the amount of electrical energy needed to feed an electric motor. I just read a few weeks ago about some research dealing with "ultra-capacitors" that sounded promising but nothing is currently available except the various types of heavy chemical batteries that are difficult to recharge and quick to discharge at high power settings.

I would love to have a very quiet, low vibration, efficient (electric motors are around 98% efficient compared to 28-35% efficiency for IC engines), powerful, simple, low maintenance motor on my plane if I could.

Whill I agree that an electric motor in a airplane would be ideal I will disagree on the weight of the electric vs the Lyc. I run several electric motors that are 50 and some that are 100 HP and they are heavy. I would guess that the 50 HP weighs around 400 pounds and the 100 wieghs close to 550 pounds. These are on irrigation wells so must be able to run 24-7 for several months at a time, maybe they are made heaver for that reason.
The wires coming to these motors are a big as your index finger, however they cost about 70% less to run then diesel motors of the same size.

[Ola]

Originally Posted by Harvey L. Sorensen

WThese are on irrigation wells so must be able to run 24-7 for several months at a time, maybe they are made heaver for that reason.

Yes, and I suppose for the manufacturers there is little point in spending any time/money on weight saving design or material as the hunk of iron is supposed to sit stationary after all.

Steve, I've too read about the ultra capacitors. They, and other pipeline battery tech, actually make me more skeptical to electric aviation as the "amazing new things" (often vaporware) existing only in press releases and lab experiments still aren't close to viable.

Here's Sonex' take on it:
http://www.youtube.com/watch?v=P8Pb_psj1A8

Not really a widely marketable concept, but it shows that people in the biz are dreaming and that is always a good thing.

As for marketability, I don't think the batteries need to equal gas tanks in order to sell. A personal sweet spot would be 1) at least half the range (meaning 1-1.5 at high power, perhaps 3 hrs at low cruise), 2) recharge time of 4-5 hrs and 3) easily replacable battery packs meaning you could swap to a fully charged and go again. This obviously comes with a lot of sacrifice compared to a gas burner and would rule a lot of customers out but be acceptable to a good few others. From there on it could only get better I suppose.

[rv6ejguy]

Originally Posted by rtry9a

#2 regarding weight- You are wrong regarding Mazda Rotaries; an OEM Mazda Renesis engine (w/o cooling & exhaust) weighs slightly less than 200 lbs. dry. The redrive, cooling, and exhaust systems brings it up to ~350 lbs, close to the O-320/360, however, the potential peak power output is closer to an O-540 (250+hp).

#4- regarding simplicity... One of the things I like best about the rotary is its inherent simplicity- only 3 "beefy" moving parts, that see none of the stresses caused by the constant acceleration reversals seen in reciprocating engines. Rotary engines do not catastrophically break down and stop like most other designs when run as designed. Worst case, they continue to RUN at lower power, but still get you home. When broken (low compression), they will not start.

Unfortunately the Renesis has not had great press in the real world- exhibiting very high oil consumption and premature wear, not to mention shockingly poor mileage in the RX8 sports car, despite being run at much leaner mixtures than past rotaries. Long term tests by both R&T and C&D have outlined these problem areas. Friends in the Wankel engine building business have also confirmed the Renesis is nowhere near as durable in hard street use as the earlier 13Bs. It should also be noted that the Renesis cannot achieve its rated power either with the 2.85 drive ratio and 2700 rpm prop limit. This is simply the fault of the drive ratios chosen and affect piston engines similarly. You want to make full rated power, you must choose the proper drive ratio. It is unlikely that anyone is going to twist a Renesis to 8500 rpm to match IO-540 hp levels in aviation use.

The RX8 has not sold well at all. While styling might have something to do with it, competitors such as the BMW 335, Lexus IS350 and various VG35 powered Nissan and Infiniti models outperform the Mazda in performance, fuel economy and it seems longevity as well. Modern direct injection, gasoline piston engines are at a very high level of refinement and performance. In ultimate hp turbo form, the Wankel has been entirely supplanted by piston engines such as the Toyota 2JZ and Ecotec which are capable of approximately double the hp of the best 13Bs.

In aircraft use, I'm not aware of any catastrophic internal failures of Wankels but there have been a fair number of soft failures which resulted in forced landings. Keeping these engines fresh inside is just as important as with piston engines. Typically, we rarely see catastrophic piston engine failures these days in the automotive field. In aviation, system and redrive failures lead the list of accident causes, not core engine problems.

Tracy has recently upgraded his drives with 6 planet gears to address strength and durability issues with higher power and rpm levels.

Just as with piston auto engines, Wankels converted for use in aviation seem best limited to lower rpms (6000-6500) for longevity. In this zone they have been proven by Tracy Crook and others to perform decently. I remain to be convinced that they can match the fuel economy/ speed of a Lycoming. I'd love it if Tracy would do a fly off at Van's like the Powersport RV8s did a few years back to see what a well developed example can do. That would be very interesting.

In the meantime, Wankels are an interesting alternative to piston engines but have their disadvantages and advantages the same as any other engine.

[rtry9a]

Ross,
(I believe) your Renesis comments mostly do not apply to our use. Aircraft apps with the Wankel generally do not use the OEM oil injection pumps, which create most of the automotive problems- they burn "dirty burning" crankcase oil to cool rotors which gums up the seals and affects lubrication. Aircraft apps mix 2-cycle oil (clean burning) with the fuel for cooling the rotors and use synthetic oil in the crankcase.

The high power rotaries generally are modified to a peripheral port intake arrangement. IMHO, the Wankel is a far better aircraft engine than automotive, and they love turbocharging. The exotic high-power racing mods are not needed for most RVs, though turbos might be nice for flying high or possibly in an RV10.

The Renesis is a 13B, with a big fancy intake manifold added to improve low end torque; it is best replaced with a tube intake tuned to ~6000 rpm. The Renesis intake ports use an improved (enlarged) side port design, which improves breathing over earlier designs, but still not quite as efficient as peripheral porting can be. The downside to the Renesis is that they run a bit hotter than earlier designs (more exhaust port exposure), and adequate cooling is critical. The are also noticeably quieter.

Dont know where you got your info, there is no problem running a Renesis, or later 13B's for that matter, at 8500 rpm if the intake system is up to it; most target the 5000-7500 engine rpm range as you mentioned (5000 rpm is torque peak, 6000 is most efficient speed, 7200 is the 4-port max 210 hp, 8400 rpm is the 6-port Renesis max 230 hp). The prop turns 1800-2700 rpm at those engine speeds with the 2.85 redrive. Seems to me that is as close to ideal as we can get given normal the RV propeller selection. The guys flying like the 2.85 redrive and a big 74 to 76" fixed propeller. The 2.18 ratio reportedly did not allow full power generation. FWIW, the rotors rotate at 1/3 crankshaft speed, which is loafing compared to conventional piston speeds and wear rates.

Tracy has always used the 6-planetary gear set in his high-output RD-1B and 1C PRSU models. The 4 gear went in his baseline RD-1A unit, approved up to 200 hp I believe, which is no longer made nor needed.

The rest I agree completely with you. There have been a handful of development problems as you mentioned, mostly related to Tracy's "cheapo" cooling system problems (a/c parts and poor ducting) from what Ive gathered (I could be wrong). I add only that the rotary is still in its development phase, just like the Subaru's are, and that most of the kinks are/have finally been worked out in operating aircraft.

[rv6ejguy]

Originally Posted by rtry9a

Ross,
(I believe) your Renesis comments mostly do not apply to our use. Aircraft apps with the Wankel generally do not use the OEM oil injection pumps, which create most of the automotive problems- they burn "dirty burning" crankcase oil to cool rotors which gums up the seals and affects lubrication. Aircraft apps mix 2-cycle oil (clean burning) with the fuel for cooling the rotors and use synthetic oil in the crankcase.

The high power rotaries generally are modified to a peripheral port intake arrangement. IMHO, the Wankel is a far better aircraft engine than automotive, and they love turbocharging. The exotic high-power racing mods are not needed for most RVs, though turbos might be nice for flying high or possibly in an RV10.

The Renesis is a 13B, with a big fancy intake manifold added to improve low end torque; it is best replaced with a tube intake tuned to ~6000 rpm. The Renesis intake ports use an improved (enlarged) side port design, which improves breathing over earlier designs, but still not quite as efficient as peripheral porting can be. The downside to the Renesis is that they run a bit hotter than earlier designs (more exhaust port exposure), and adequate cooling is critical. The are also noticeably quieter.

Dont know where you got your info, there is no problem running a Renesis, or later 13B's for that matter, at 8500 rpm if the intake system is up to it; most target the 5000-7500 engine rpm range as you mentioned (5000 rpm is torque peak, 6000 is most efficient speed, 7200 is the 4-port max 210 hp, 8400 rpm is the 6-port Renesis max 230 hp). The prop turns 1800-2700 rpm at those engine speeds with the 2.85 redrive. Seems to me that is as close to ideal as we can get given normal the RV propeller selection. The guys flying like the 2.85 redrive and a big 74 to 76" fixed propeller. The 2.18 ratio reportedly did not allow full power generation. FWIW, the rotors rotate at 1/3 crankshaft speed, which is loafing compared to conventional piston speeds and wear rates.

A fair comment on the pre-mix oil.

Rotaries have never loved turbocharging. While they respond very well hp wise, longevity at high outputs due to thermal concerns and turbocharger life have limited this form of power enhancement to low output street use and short duration drag use primarily. I'm not aware of any successful turbocharged road racing cars nor any turbocharged Wankels used in aircraft which have accumulated 500 hours without problems. I assisted John Slade on matching a proper Garrett turbo for his 13B Cozy some time ago but I don't think he even has 100 hours on this setup to date. Concerns with exhaust system cracking have again come to light already and turbine life may be an issue with the 1800F EGTs.

The Renesis engine makes power peak at 8400-8500 rpm hence my comment. Turning lower rpms would not allow this engine to produce competitive power with an IO-540. Longevity at near this sustained rpm is unproven in my view. Airplane engines should ideally go thousands of hours before overhaul, not hundreds. A 20B is a far more suitable choice for comparison to the big Lycoming.

My information on the RX8/ Renesis longevity issues come from 2 sources: Rob at Pineapple Racing http://www.pineappleracing.com/ and Paul Yaw at Yaw Power. http://www.yawpower.com/
Both have been heavily involved with Wankels for many years. Paul in fact received prototype engines from Japan for evaluation/ feedback before production release of the Renesis.

Rob has pulled and repaired or replaced several Renesis engines with as little as 40,000 miles on them. They were simply bagged out. His opinion was that 8500 rpm was an unrealistic redline for these engines in street use as delivered from Mazda.

I'll wait for that fly off with Tracy's new RV and engine before I'm convinced that Wankels are better than piston engines. Tracy has been the big force and innovator here so if anyone can do it, he can.

[RVbySDI]

Originally Posted by Harvey L. Sorensen

Whill I agree that an electric motor in a airplane would be ideal I will disagree on the weight of the electric vs the Lyc. I run several electric motors that are 50 and some that are 100 HP and they are heavy. I would guess that the 50 HP weighs around 400 pounds and the 100 wieghs close to 550 pounds. These are on irrigation wells so must be able to run 24-7 for several months at a time, maybe they are made heaver for that reason.
The wires coming to these motors are a big as your index finger, however they cost about 70% less to run then diesel motors of the same size.

Well, here is an area that takes some knowledgeable analysis. When comparing the output of an electric motor with the output of an Internal Combustion engine the electric motor will put out much more power/HP than will an IC engine.

Primarily because of the greater efficiencies of the electric motor it does not need to have the same HP rating as an internal combustion engine in order to produce the same output of power. I am sure there are mathematicians or engineers out there that can chime in with the specific HP numbers needed for an electric motor if it is to replace say a 100 HP IC engine. Generally speaking an IC engine rated for 100 HP can be replaced with an electric motor that may be rated for say 25 HP (someone please correct me if this number is way off).

When comparing weights of comparable IC engines and electric motors one must make sure the comparison is for engines with equal outputs. Your 50 and 100 HP electric motors would better be compared to 400-600 ci IC engines (180-300 HP) rather than the 235-360 ci IC engines (115-180) typically used in RV's. Because of this the weight savings will be due to the fact you will be using a smaller electric motor to produce the same level of power output as you would with an IC engine. Not to mention the reduction in weight for all of the other peripheral components needed for the operation of an IC engine (i.e. starter, oil cooler, radiator, carburetor, exhaust, fuel pump, fuel lines, fuel filters, magnetos, etc.)

[Jconard]

1. Rotaries will never have similar BSFC to a piston engine because they simply convert less heat into power, as a result of their design. This is why they respond disproportionately well to turbocharging. Of course retaining that heat also causes them problems.

2. You of course can always make the radiators bigger, but in an airplane the limiting factor is drag, a bigger radiator requires more air, which requires more drag. As it is, because of the multiple heat exchanges, water cooling will require more air to remove heat from the rads..as a funtcion of built up heat enrgy during the multiple transfers, and as a result of the lower delta T at the radiator.

3. Running a roatary lean of peak, even severely does not change the fuel consumption issue, because it is a function of power. When you run LOP, you also reduce total available thermal energy. Since the engine already converts less of the thermal energy to power anyway, LOP is not going to change the equation. If you need 125 ftlbs of torque for a certain flight regime, it requires a certain amount of fuel to create that power. Now, if an engine converts a smaller percentage of ingested fuel into work...less thermodynamically efficient, it requires more fuel at intake to convert enough of it for the required power. The fact that it will tolerate a leaner mixture does not change the basic thermodynamic nature of the engine. Even when done by the best rotary conversions are gas guzzlers (literally) by comparison.

4. Say what you want about weight...no installation data to support your assertion.

5. As for air cooling...totally miss the point...all aircraft engines are cooled solely by the flow of air. It can be either direct, over the engine, with a high delta T, increasing the exchange rate, or through rads with a less optimal exchange rate. Before you go warbird on me, remember that there were other packaging concerns there.

6. You talk about different performance levels, I assume you mean specific output. But why? No matter how you slice it the internal forces of an engine increase linearly as a function of weight, and increase exponentially as a function of speed. The accelerations at twice the rpm are FOUR times greater. Even if components (pistons rods, etc.) are much lighter, the stresses will always be higher at hig RPM. And Why? If a slow turning, big bore, direct drive, will power the plane throughout its flight regime, and at lower weight, what is the advantage to turning up the wick on specific output? Is it a real pleasure to have to add a new flight critical system like a redrive?

Is a rotary a cool engine? ABSOLUTELY
Have they been able to achieve some great outputs in racing trim? Absolutely.

Is it smoother?...maybe, they have their own vibration... in the early formula mazda cars, the vibration would pulvarize the welds on the steel chasis, and until you got used to it, would leave the driver feeling assaulted at the end of a race.

If you think its cool fine, if you want "modern" engine management designed for an operation regime you will NEVER use in an aircraft...great.

It is what experimenting is about. But if you want simplicity, light weight, reliablility, and durability...all with the best speed/fuel numbers money can buy, put an aircraft engine it.

One exception...I'm waiting to see how Ross's 10 does...his scoop work looks promising, and the thing is turbocharged.

[FlyJeffJ]

The perfect airplane is elusive beacuse for every good feature there is a compromise. Everyone should have a minimum of two aircraft Low and Slow and High and Fast. I only have an RV-4, so I fly Low and F A S T ! That's my compromise. It isn't as fun to watch the Minnesota leaves right now at 180 MPH, but hey, I'm going 180 MPH!

I think the RV-4 with the 0-360 and a constant speed prop is the best out there. I'm not as familiar with the latest RV's but the bigger engine recommended by vans with a variable pitch prop gives you the most bang for your buck, and a little extra. If cost is an issue, then go with the smaller engine and a wooden climb and cruise prop to give you options.