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  #1  
Old 04-02-2005, 02:21 PM
martinkh martinkh is offline
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Default How to take advantage of HP- Cozy

Newbie question here.

Please feel free to lecture me on any factual or theoretical errors.

There is a given prop specified for the standard 180HP Lycoming. If one goes with a renasis engine, you get what, 250HP? Would you then chose a prop with more pitch in order to get higher speed with a given RPM? I would assume that the higher power engine would have enough torque to give you just as much power on takeoff as the lower powered engine with a lower pitch prop.

Also,in a car, you get your best MPG by driving at the peak torque RPM. If one was math deficient and wanted to plan trips based on 200mph cruise, the engine, reduction and prop would be set up to provide peak torque between 190 and 200mph. but then this would leave alot of HP above this speed for play, but then the question is, is there enough power at low RPM to spin up and get you clear of the runway and into the wild blue yonder.

I'd love to see a torque and horsepower chart for the IO-360 and the turbo 13B or Renasis and get a clue regarding prop selection. No, I haven't even bought a package yet, but I'm excersizing my brain.

Last edited by martinkh : 04-02-2005 at 03:14 PM.
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  #2  
Old 04-02-2005, 05:34 PM
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Quote:
is there enough power at low RPM to spin up and get you clear of the runway and into the wild blue yonder
Well, I can answer that part. Yes.

My prop was designed to achieve 6300 engine rpm @ 2.17 ratio with 280HP at 10,000' and 220 kts. I haven't been there yet, but I got close with 6100 on one occasion. The engine needs a little more tuning before I can be sure I'm getting best power. There IS enough power to get off the ground, but I'd like more. I can only turn 4050 static which is 1000 rpm below where I'd like to be and well down on the power curve. The airplane still gets airborne in 1200 - 1500 feet and climb's at over 1000 fpm. I expect to be cutting the prop back a little soon to get slightly better take-off results. One option I'm considering is ordering a second, possibly 2-blade (shippable) prop based on the numbers I have, then using the numbers I get on this one to help fine tune what I want done to the original 3 blade.

Does that help, or provoke more questions?
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  #3  
Old 04-02-2005, 06:18 PM
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Default Steve Brooks 13BT

Anyone know how Steve Brooks is making out with his 13BT powered Cozy?
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  #4  
Old 04-02-2005, 09:35 PM
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Last I heard he was making progress. He's keeping the boost under 5 PSI for fear of blowing the stock turbo. Steve has to travel 300 miles each way to fly because he moved state during the flight testing. I think Steve is running an IVO adjustable prop.
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  #5  
Old 04-02-2005, 10:41 PM
martinkh martinkh is offline
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Is it due to the peaky nature of the turbo engine that engine speed has to be so high to get that speed? If a 180HP Lycoming can pull 200mph, it seems odd that you require 280HP to get 20mph more. What is the redline of a turbo 13B? What is the peak torque RPM? Is the rotary engine fairly low torque compared to a std engine?

From this newbie's perspective, it seems like either the engine lacks torque on the bottom end(per your comment about desiring more power for takeoff) or the gear reduction/prop combo is sub-optimal.

Please don't think I said you have an ugly baby, I learn through questions!!
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  #6  
Old 04-02-2005, 11:33 PM
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Quote:
Is it due to the peaky nature of the turbo engine that engine speed has to be so high to get that speed?
Not really. 6000 -6500 is typical for peak torque from an "automotive" engine. I think the torque curve is pretty much a straight line to this point.

Quote:
If a 180HP Lycoming can pull 200mph, it seems odd that you require 280HP to get 20mph more.
I'm sure I'm nowhere near 280 HP yet. As someone said recently, that last few rpm on the prop takes a lot of power to get.

Quote:
What is the redline of a turbo 13B? What is the peak torque RPM?
They're run up to 9000 regulaly by the race guys. Maybe Rusty knows this stuff more than me, but I think 6500 - 7000 is peak.

Quote:
Is the rotary engine fairly low torque compared to a std engine?
Depends what you mean by a std engine. If you mean a Lycoming, the peak torque on that is more like 2900 rpm, hence the 2.17 ratio redrive we use.

Quote:
From this newbie's perspective, it seems like either the engine lacks torque on the bottom end(per your comment about desiring more power for takeoff) or the gear reduction/prop combo is sub-optimal.
Well yes, the prop IS sub-optimal for take-off. It was deliberately designed to be optimum in cruise. There is a 2.85 redrive I could use, but you need a longer prop than I can handle to take advantage of it.

Quote:
Please don't think I said you have an ugly baby, I learn through questions!!
You've fully entitled to call it an ugly baby right now - it's in desperate need of a paint job. I'll get to that once the flight testing is done.

Taking best advantage of the rotary is really matter of balancing the ratio, rpm and prop for acceptible performance on take-off & climb and good cruise performance. A constant speed prop should give the best of all worlds, but they're expensive and heavy. The IVO Magnum might be an option, but they're said to be a bit weak at the top end. My approach has been to optimize for cruise first, then take whatever steps I need to tailor the take-off power. I have the new auto tune chip installed (and the EC2 computer came back today) so hopefully I'll soon be seeing what power I get with "perfect" tuning. Then I'll know where to fo from here.
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  #7  
Old 04-02-2005, 11:33 PM
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Quote:
Is it due to the peaky nature of the turbo engine that engine speed has to be so high to get that speed?
Not really. 6000 -6500 is typical for peak torque from an "automotive" engine. I think the torque curve is pretty much a straight line to this point.

Quote:
If a 180HP Lycoming can pull 200mph, it seems odd that you require 280HP to get 20mph more.
I'm sure I'm nowhere near 280 HP yet. As someone said recently, that last few rpm on the prop takes a lot of power to get.

Quote:
What is the redline of a turbo 13B? What is the peak torque RPM?
They're run up to 9000 regulaly by the race guys. Maybe Rusty knows this stuff more than me, but I think 6500 - 7000 is peak.

Quote:
Is the rotary engine fairly low torque compared to a std engine?
Depends what you mean by a std engine. If you mean a Lycoming, the peak torque on that is more like 2900 rpm, hence the 2.17 ratio redrive we use. People speak of 'Torque' and HP interchangably. Its my understanding that HP is a direct function of torque and rpm.

Quote:
From this newbie's perspective, it seems like either the engine lacks torque on the bottom end(per your comment about desiring more power for takeoff) or the gear reduction/prop combo is sub-optimal.
Well yes, the prop IS sub-optimal for take-off. It was deliberately designed to be optimum in cruise. There is a 2.85 redrive I could use, but you need a longer prop than I can handle to take advantage of it.

Quote:
Please don't think I said you have an ugly baby, I learn through questions!!
You'd be fully entitled to call it an ugly baby right now - it's in desperate need of a paint job. I'll get to that once the flight testing is done.

Taking best advantage of the rotary is really matter of balancing the ratio, rpm and prop for acceptible performance on take-off & climb and good cruise performance. A constant speed prop should give the best of all worlds, but they're expensive and heavy. The IVO Magnum might be an option, but they're said to be a bit weak at the top end. My approach has been to optimize for cruise first, then take whatever steps I need to tailor the take-off power. I have the new auto tune chip installed (and the EC2 computer came back today) so hopefully I'll soon be seeing what power I get with "perfect" tuning. Then I'll know where to from from here. And yes, Mike's right - the prop can easily be adjusted "down", but not "up", so its better to err on the high side.
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  #8  
Old 04-02-2005, 11:35 PM
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Well, he hasn't got what he wanted and designed the prop for yet. Still workin on it. But as an aside, he had the prop made so that it can be adjusted later on if it needs be to get optimum rpm and hp.
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  #9  
Old 04-02-2005, 11:44 PM
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Quote:
Originally Posted by rv6ejguy
Anyone know how Steve Brooks is making out with his 13BT powered Cozy?
Steve has an IVO Magnum prop with a 2.17 reduction drive. I have not heard any recent news of his test flights.
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  #10  
Old 04-03-2005, 12:59 AM
martinkh martinkh is offline
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My old Shelby GLHS, intercooled turbo 4 banger had peak 200ft*lbs torque at 3600, peak HP of 186 at 4700, redline at 6000. It would be considered a peaky engine, as the HP curve is steep, and the torque peak is pretty high up there. If I were using this engine in a plane, I would want to be doing about 3600rpm at cruise. This would give me best milage and probably longest life on the engine. I have no idea how much poop I would have to get off the ground though.

As you soup up an engine you usually raise the rpms at which peak torque and HP occur, thus getting the best out of your HP=torque*rpm formula. But in severely tweaked engines, you sacrifice area under the curve. You constantly shift gears to keep in the power. In a plane with no transmission, you don't have that option. So when you soup up your engine, you can get into a situation where you lose your bottom end torque, and have a heck of a time spinning up. If I were to use a turbo engine, I would be trying to increase the area under the HP curve. If you have a 230hp turbo motor and you souped it up to 280, I wonder what would happen if you backed it off to about 210-220. If it was a piston engine, like a small block chevy, you could do a mild porting, gasket matching and put in an RV cam and have a very broad HP curve. You could make a torque beast by increasing the stroke with a different crank. On a rotary engine, I don't know what performance mods you can make, no cam to swap, no way to increase "stroke" or displacement. I would talk to some wankler racers about what your HP and torque curves look like, and if there are options for increasing bottom end and smoothing out the HP curve. Doing this would increase engine life and reliability conciderably, hopefully without killing the top end.

Am I missing something?
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  #11  
Old 04-03-2005, 01:40 AM
martinkh martinkh is offline
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Quote:
Originally Posted by John Slade
Not really. 6000 -6500 is typical for peak torque from an "automotive" engine. I think the torque curve is pretty much a straight line to this point.
Peak HP in a dragster or twin turbo beast maybe. Torque is going to be way lower. A chevy street-strip car with a 350 may have peak torque as high as 4500 rpm, HP peak near 6000, but a torqued out daily driver or tow truck motor would be closer to 2000-2500 for torque, 4500 for HP. The daily driver won't bog down at low speed under load like the strip car will, but it will pull hard all the way through.
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  #12  
Old 04-03-2005, 01:25 PM
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Torque, love the term as it has the exact same meaning as HP. You are gonna spin the prop at 2200 to 2400 rpm - with 140 hp and at 26 to 3000 rpm at 180 to 2??? HP.

you have to have the hp at those rpm's and since the rpm and the torque and the hp are DIRECTLY, why mention torque? It just confuses many

Just my pet peeve
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Old 04-03-2005, 01:49 PM
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Default Implied meanings

Quote:
Originally Posted by Dust
Torque, love the term as it has the exact same meaning as HP. You are gonna spin the prop at 2200 to 2400 rpm - with 140 hp and at 26 to 3000 rpm at 180 to 2??? HP.

you have to have the hp at those rpm's and since the rpm and the torque and the hp are DIRECTLY, why mention torque? It just confuses many

Just my pet peeve
Mike,
you are partly right, after all: power = torque x RPM, that's all. What people imply, however, when they talk of an engine with plenty of torque, is an engine with plenty of torque from low down in the operating range of RPM's. This again implies an engine with a fairly flat torque curve and a smoothly rising power curve, something like a diesel, or the perfect example, an electric motor. Gas automotive engines on the other hand tend to have "peaky" torque curves. When it comes to turning a propeller, our application of interest, an engine with a peaky torque curve does not have the torque (or power if you want) low down to turn a steeply pitched cruise propeller to the RPM's needed for top power for takeoff, it's like trying to climb a hill in fifth gear. Of course, with an adjustable prop it's a whole different ball game, it's like being able to change gears in a car. Flat pitch for takeoff, steep pitch for cruise.
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Old 04-03-2005, 01:57 PM
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yes - understand, but at the rpm's we are dealing with there just doesn't seem to be that much variability. i mean, all not adjustable props are usually combination cruise/climb props.

Now you can go to a variable pitched prop and make the take off roll much shorter, but it really is not needed as the ground roll will still be allot on landing and the plane with a cruise/climb prop will be able to take off and land at the same field.
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  #15  
Old 04-03-2005, 08:28 PM
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Torque and HP are NOT the same thing. Torque is measured in Ft*lbs or a similar unit. It is rotational force. 300 Ft*lbs means that at 2 feet from the axis of the crank you are exerting 150lbs of force. As the prop is biting the air, you can say that the air is exerting a force that acts against the prop. This is why the most efficient theoretical cruise speed should be peak torque, not peak HP. If the wind is applying an average resistance torque of 200ft*lbs and the engine has 300ft*lbs at that speed, then the engine is not working hard to maintain speed, thus way less fuel, stress and heat. HP is really the ability to accelerate quickly.

A tow truck motor cannot accellerate like a dragster, but a dragster cannot pull another heavy car up a hill, and if you did, you would blow the engine having to keep the RPM's that high. It would overheat like a MoFo.

Can I use an analogy of drive ratio on a car to compare to prop size/pitch? With a high HP motor, you use a huge gear reduction to take advantage of the thin power peak. You want to lower the torque load acting against the engine so you can "spin up" the tires faster. If you put an engine in that has mega torque and modest HP, you can put huge tires on it, reducing your drive ratio, and still have the power to accellerate up a hill (albeit not as quick). But the top speed increases, as does engine life. Engine temps are lower too, although redline is usually lower.

With a high torque motor, one usually gets off the line faster, but then the HP moter beats it before midway down the stretch. It also beats it into the shop for an overhaul.
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