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03-21-2012, 04:28 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by WingsOnWheels
Very true, but you can increase the HP output of most engines by increasing operating rpm (to a piont). HP doesn't tell the whole story for an engine that must drive an rpm-limited propeller. Most 1 liter motorcyles these days are putting out close to 200hp, that doesn't mean it will work on your RV-6/7/8. The 10,000 RPM peak hp output would be a problem. Aircraft engines run at low RPM because they have to and the design of the engine reflects that requirement. The point I was trying to make by noting the high torque way that just because the hp numbers are low does not me the engine is a poor design. Try to find another engine with the same output at that RPM (even with a reduction) for the same installed weight. You won't.
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HP under the curve within the usable rpm band is the whole story actually whether you are driving a prop on an airplane or boat, a bus, car or locomotive. That is why all engines are rated in hp first (sometimes only) because this denotes the amount of work they can perform. Torque is meaningless in that regard.
I'm not comparing different types or sizes of engines here, only trying to put people on track here about motive capability.
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03-21-2012, 04:43 PM
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Join Date: Jul 2006
Location: Corvallis Oregon
Posts: 3,547
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Quote:
Originally Posted by flywithme623
See, with a liquid cooled diesel you would not even need to talk about that. You can build much more aerodynamic cowlings.
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You can build as an aerodynamic cowling as you want, but the problem is you still need to cool the engine.
The fact is that liquid cooled engines need more cooling air flowrate which equals more drag (yes I know the famed Mustang overcame this at 400mph, but no one has ever done it for a 150mph homebuilt)
This means: liquid cooling = more drag= less fuel efficency.
Basically the reciprocating engine is not THAT much more efficient n matter how you slice it, diesels are better but the fact you have to discharge heat to a cooling airstream at 200F (ish) in a water cooled motor, vs 350F in an aircooled one..Well the only way to dump more cooling horse power is more flow.
I.e BTU/Hr = 1.09 * flowrate in CFM * Delta T.
For an outside air temp of 100F
delta T is 200-100F = 100F, in an aircooled engine 350 -100 = 250F
Thus you can see for the same BTU/Hr flowrate goes up by 2.5 times for the same HP in a lquid cooled motor.
if you want to find out how much.. 1kW= 3413 BTU/Hr and 1Hp = 0.746kW.
Frank
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03-21-2012, 04:49 PM
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Join Date: Jul 2006
Location: Corvallis Oregon
Posts: 3,547
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Back to detonation, Dynamic compression is one thing that limits us, the other is the cylinder head temperature. Detonation is function of temperature.
Now we don't know what the actual temp of the inner wall of the cyl head is, but most likely it is higher than on a water cooled motor as water is exceptionally good at removing heat, thus lowering temperature.
As to what can be run.. I run premium car gas on an 8.5:1 FI'd Lyc with auto advancing EI with no problems.
Rocket Bob runs regular mogas with the same result..I haven't been brave enough to try that ..yet.
Both of us run corner store ethanol blends.
The other thing you really should have is a proper fuel pumping system to avoid vapour lock..But I'm not going there..
Frank
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03-21-2012, 06:34 PM
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Join Date: Jun 2011
Location: GREENBANK
Posts: 106
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It is all about mind set. More so in the US than anywhere, Lycoming and Continental piston engines have almost a cult following. Yes they are an adequate engine (and I love my XP-360), however, I always laugh when I see people refer to them as RELIABLE.
If a car or truck manufacturer brought out a vehicle with an engine that needed cylinder replacement due to cylinder heads cracking, worn valve guides, broken valves, piston pin wear leaving a little metal in the filter, broken rings, sticking exhaust valves etc etc etc, they would go broke. What if the engine spalled lifters and cams etc all the time?
Ok, there may not be a heap of catastrophic engine failures but there are way more than in the automotive industry, so I don't agree that they are hugely reliable.
In Australia and Europe, the Rotax 912 is a very prominent engine in the LSA class and have been flown there for many years. They are high revving, water cooled heads, electronic ignition and now electronic fuel injection etc. They are such a beautiful engine which except in very rear circumstances, always make it to TBO without much more than an oil change every 50 hours. And guess what, you don't have to fly them for an hour once a week to make them RELIABLE. Lycoming is trying to match their success with the IO233 but they still can't match the installed weight per HP.
On the other hand, the US seems to love the Australia Jabiru engine. In Australia, the fields are littered with them. Most Australians will not fly behind one and prefer the Rotax 912. I used to own a Jabiru and refused to fly it over anything except open paddocks. I sold that aircraft as the WORST ever aircraft I have owned or flown.
So what is GOOD about Lycomings?
1. Easy to top overhaul (which is handy because it always has to be done).
2. Simplicity (the thing is a tractor engine but things still go wrong frequently) It is reasonably easy to fix in the field.
What is bad?
1. Lifter and cam spalling. I roll my eyes every time I hear the old wives tale (most likely spread by Lycoming) that the reason for spalling is due to the position of the cam. I am a Diesel Mechanic by trade and have built numerous diesel and gas engines with overhead cams. You don't get much higher than that and yet I have NEVER seen a spalled cam.
2. They are a heavy, hugely inefficient, slow revving, noisy, dirty engine (that is part of the romance of the engine for me).
Mark.
__________________
Mark
RV-7, Superior XP-360.
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03-21-2012, 06:47 PM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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Nice formulas Frank, but don't forget the surface area of an aluminum radiator, right now we are using cylinder cooling fins AND an oil cooler cause the fins just don't cut it. Did any one see Mikes post # 36 about ADEPT engines? go to their site. these guys may have something. Water cooled engines are way more efficient and you have equal cooling at around 180 to 200 Deg F. compared to a head at 450 F. det can be controlled at a lower temp. I think all things equally, one small water radiator is no more than a big oil cooler, and take away all the cooling fins and the cooling drag may be a wash. what do you think?
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7A Slider, EFII Angle 360, CS, SJ.
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03-21-2012, 06:51 PM
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Join Date: Mar 2009
Location: Virginia
Posts: 37
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Another way to look at it...
I've often wondered how the general consensus is that auto engines aren't 'reliable' when we get in our cars and drive for hrs and miles without nearly the prep and nurturing we give our airplane engines and more times than not, with success. If we gave as much attention to our auto engines, were they installed in an airplane, as we do our airplane engines, they'd WOULD be reliable and they WOULD last despite not working as hard, etc, etc. A lot of the fear is centered around the unplanned landing using an auto engine in an airplane vs just pulling over to the side of the road. Design for redundancy (ignition, fuel, electrical, etc) and the reliability will be there. I'm not planning to install a Lycosaurus (yet) and am convinced there is something 'better' (for me anyway) out there! Fortunately Mistral has venture capital investors interested in the way forward...that would be my installation of choice. Just my $0.02 but a good thread nonetheless.
Last edited by GusRV8 : 03-21-2012 at 06:54 PM.
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03-21-2012, 07:10 PM
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Join Date: Jan 2012
Location: Canada
Posts: 40
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Quote:
Originally Posted by Remag
2. They are a heavy, hugely inefficient, slow revving, noisy, dirty engine (that is part of the romance of the engine for me).
Mark.
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Again, we are getting at the same point that were mentioned before in this post...
-Slow Revving... --> You can't turn a propeller too fast or the tip will break the speed of sound and will then destroy your prop... You have to be slow revving but have more torque so you can increase the pitch instead!
No point in using a 10000rpm engine then... Even with a gearbox, you would be adding weight and reducing reliability to get almost the same result!
-Heavy... --> To reduce weight, you would have to use different metals, but then again, it would cost more to produce and add to this all the required test it would need to get the new parts FAA approved...no way a homebuilder or small private plane owner could afford a 40K$ engine for a such a small increase in gas consumption/performance!
-Noisy --> To cut down on noise, you would need to install mufflers... This means, more weight and performance loss... Who would want that!
As for the dirty engines, we could improve a little on this with better ignition system and fuel injection systems...but again it means more R&D and FAA approved modifications which means you pay more for your engine!
Not trying to be rude here, but just trying to explain why it is that we can't really improve on these points when we have something that is already "Reliable" for an already high price!
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03-21-2012, 07:30 PM
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Join Date: Mar 2005
Location: Calgary, Canada
Posts: 5,766
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Quote:
Originally Posted by frankh
You can build as an aerodynamic cowling as you want, but the problem is you still need to cool the engine.
The fact is that liquid cooled engines need more cooling air flowrate which equals more drag (yes I know the famed Mustang overcame this at 400mph, but no one has ever done it for a 150mph homebuilt)
This means: liquid cooling = more drag= less fuel efficency.
Basically the reciprocating engine is not THAT much more efficient n matter how you slice it, diesels are better but the fact you have to discharge heat to a cooling airstream at 200F (ish) in a water cooled motor, vs 350F in an aircooled one..Well the only way to dump more cooling horse power is more flow.
I.e BTU/Hr = 1.09 * flowrate in CFM * Delta T.
For an outside air temp of 100F
delta T is 200-100F = 100F, in an aircooled engine 350 -100 = 250F
Thus you can see for the same BTU/Hr flowrate goes up by 2.5 times for the same HP in a lquid cooled motor.
if you want to find out how much.. 1kW= 3413 BTU/Hr and 1Hp = 0.746kW.
Frank
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Way more variables involved than just these to say that liquid cooling causes more drag- mass of water vs. air, specific heat of water, thermal conductivity, radiator efficiency vs. those of thick cooling fins, duct design, momentum recovery, etc.
There is no evidence to suggest that a well designed liquid cooled installation has more drag than an air cooled one, in fact there are various examples which I've brought up before numerous times showing just the opposite. The fuel burn vs. TAS on the STI RVs is comparable or better than those powered by Lycomings and these don't even have optimized radiator layouts- just stuffed in the cowlings and using the stock Vans inlets.
With regards to new engines, I just don't see anyone being able to design, test and produce a clean sheet design in the small numbers used by this industry and be any cheaper than a Lycoming. While you might, maybe, possibly get 10% better SFCs with a really modern design, would all that trouble, risk and liability exposure be worth it? Doesn't seem like it to me.
The diesel is intriguing but nobody has demonstrated comparable reliability, pricing and longevity in any aero diesel yet and some have been pretty poor. With time, I believe we will see some of these improve and/or prove themselves and if so, these might make more sense in this application of relatively constant rpm/ high power than spark ignition engines.
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03-21-2012, 07:31 PM
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Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
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Has anyone read up on Mikes post #36 a V6 smaller than an IO 360 and lighter, up to 320 HP.

__________________
7A Slider, EFII Angle 360, CS, SJ.
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03-21-2012, 07:34 PM
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Join Date: Sep 2006
Location: Douglas Flat, CA
Posts: 589
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Quote:
Originally Posted by CMBAero
...You can't turn a propeller too fast or the tip will break the speed of sound and will then destroy your prop...
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A minor quibble: When the prop tips go supersonic, it is not necessarily particularly damaging to the prop itself. There is no magical power to a shock wave that immediately rends asunder anything it touches.
What is bad about pushing the tips transonic is that it is a lot draggier than when they're subsonic. The shock waves pretty much hemorrhage energy by converting it into noise and neighborhood complaints.
In fact, prop tips go transonic and create shock waves pretty regularly. The tip speed doesn't even need to be supersonic; when the tips get to between 60% and 85% (depending on the blade section), the curvature of the airflow around the blade section causes enough local acceleration in the air to get it to go super sonic and rip out some shock waves.
I don't know what happens when the prop is forced solidly into the supersonic realm. I can imagine that a prop runaway caused by a bad governor or something like that would cause some damage. But probably mostly to the engine.
__________________
Bob Kuykendall
HP-24 kit sailplane
EAA Technical Counselor
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