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  #16  
Old 04-04-2005, 04:39 AM
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David Staten David Staten is offline
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Default ..BUT....

Quote:
Originally Posted by martinkh
Torque and HP are NOT the same thing.
No, they are not... but the two values are directly related.. And if I remember correctly, horsepower is calculated. Torque and RPM are directly measured to calculate HP.

Horsepower = Torque X RPM/5252


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  #17  
Old 04-04-2005, 01:18 PM
martinkh martinkh is offline
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Thanks for including the missing factor of 1/5252

But the methods of maximizing torque or HP in an engine are different.

To max torque, you go for max torque(not being a smart a$$) which comes at moderate rpm levels.

To max HP, you push your peak torque WAY up the RPM scale, so that the high torque and high RPM multiply. But then you get a very sudden rise in torque, with lousy low to mid RPM performace. This is why sport bikes have more gears than cruisers, their engines make little power outside of a very narrow powerband.
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  #18  
Old 04-04-2005, 01:40 PM
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Default Torque/hp

John's takeoff/ climb performance is related to the propeller/ engine match and the fact that he can pull only around 4500 rpm for takeoff, limiting hp considerably. Hp determines acceleration on the runway and rate of climb. There is a good chance that the prop is not operating at efficient AOAs at low speeds, sucking up even more available hp being partially stalled. Such is the compromise of fixed pitch props on higher revving turbo engines. John knows this and accepts this for a good match at cruise speeds which is where the aircraft spends most of its time.

The beauty of a turbo engine is that you can increase torque and hp at any rpm simply by running more manifold pressure.

A variable pitch prop on an auto engine is generally much more important than on a slow turning aircraft engine to a get max performance everywhere because of the much wider rev range. This is amplified on turbocharged engines at high altitude and high airspeeds where fixed pitch stuff can overspeed easily.

The only real sloution without using a VP or C/S prop, is exactly what John has done- apply lots of boost with a high pitched prop for takeoff and climb. Once the speed is up, the prop comes into its efficient AOA range and works well with less power applied.

Our RV6A would be useless with a fixed pitch prop. If I leave the IVO in roughly neutral pitch, takeoff and climb are impressive but lower the nose to even 90 knots at climb power and the rpm starts increasing rapidly. Try taking off with the prop in cruise pitch and you can tell from the noise that the blades are stalled. I only get 3200 engine rpm at 38 inches in full coarse on the ground. Acceleration is really poor and the climb is about 1/3 rd of what it is at best climb pitch.

Above 12,000 feet, we are fully coarse to limit rpm to 4600 rpm using 35 inches in cruise so we are under propped a bit still. We can only use 30 inches at 17,000 feet without exceeding 4600 rpm. Ideally, we could use a higher reduction gear ratio with this prop/engine/airframe combo to extract more performance as we don't use much of the fine pitch range of the prop for takeoff/climb and need more pitch in cruise.
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  #19  
Old 04-04-2005, 10:10 PM
martinkh martinkh is offline
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I was wondering if you lowered the RPM's at which peak torque occurs, would that not improve take off.

Questions for the pilots:
Starting at a dead stop, how do you add throttle until you are clearing the trees and retracting nose gear? Do you feather it or get a roll started then pour it on, or do you "step on it" from the start?

When you give it takoff throttle on the runway, and watch the tachometer, is there a point where the engine acceleration increases it's rate? You know, linear for a while, then suddenly the RPM's start to really increase, or visa-versa? If you get linear acceleration and then finally a big increase in speed at the end, it could be that you don't have enough power in low to midrange RPMs.

Brain teaser:
Ever notice when you watch drag racing, sometimes the winner of the race has the lower speed at the finish line? When getting a plane off the ground, the point is not to beat the next guy to the 1,300 foot line, but to have liftoff speed earlier. I wonder exactly what engine/drive ratio factors might cause this to happen, and would there be a lesson for plane builders in it.
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  #20  
Old 04-05-2005, 01:24 AM
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Quote:
Originally Posted by martinkh
Questions for the pilots:
Starting at a dead stop, how do you add throttle until you are clearing the trees and retracting nose gear? Do you feather it or get a roll started then pour it on, or do you "step on it" from the start?
Depends. On a normal take-off with more than enough field, come off the hold short line rolling, turn toward the center line and gently and evenly roll on the throttles with quick glances to the RPMs to keep the engines somewhat synced. On a short runway, or if there is a need to clear obsticles... hold the brakes, throttle up, wait for prop RPMs to come up to almost redline, and let it go.... wait for positive rate climb and for there not to be enough runway left and pull in the gear... if I were following the textbook. Normally the minute there is positive rate, i pull the gear in (I've picked up some bad habits. I'll have to remember not to do this when I have my checkride ).... around 1000-2000 feet AGL, pumps off, come back on the throttles, props and mixture to cruise climb.

Quote:
When you give it takoff throttle on the runway, and watch the tachometer, is there a point where the engine acceleration increases it's rate? You know, linear for a while, then suddenly the RPM's start to really increase, or visa-versa? If you get linear acceleration and then finally a big increase in speed at the end, it could be that you don't have enough power in low to midrange RPMs.
I havent really noticed. I'll get back with you again on this tommorow after I go fly. If I had to guess, I'd probably say they come up pretty quickly on the bottom end and start slowing down toward the top of the RPM range... but I'll be sure to pay attention to it tommorow.

Quote:
Brain teaser:
Ever notice when you watch drag racing, sometimes the winner of the race has the lower speed at the finish line? When getting a plane off the ground, the point is not to beat the next guy to the 1,300 foot line, but to have liftoff speed earlier. I wonder exactly what engine/drive ratio factors might cause this to happen, and would there be a lesson for plane builders in it.
Sure... STOL kits get somewhat the same effect.... lower take off speeds, shorter take off roll, but your cruise performance normally suffers a bit with fixed kits. STOL flap setups normally help in that area a bit since they can be retracted when they are not needed. They still are not perfect however... you can get a shorter take-off roll with a STOL flap equipped plane by not extending the flaps until you are above flaps extended flying speed (which you reach faster due to less drag since the flaps are not out), then throw them out and hope they go out, or you have enough runway to get enough more speed to take off without them, or stop. You get about the same feeling as getting a Cherokee with parking brake style flaps up to no flaps take off speed and yanking in the first notch of flaps.

The best solution however is probably an engine, with a CS prop, that is well matched to the airframe. You can force any plane to fly sooner, or faster, etc... but there is always a trade off... airframe fatigue, lowered cruising speed, more fuel consumption, a big expensive engine that you can't run at full power once in the air, increased airframe weight, etc.

It is kinda like a Cessna 150 with a PT6 on the nose, sure... it would get up and climb like a homesick angel within the first few hundred feet of runway... of course you would probably still be sitting at the numbers holding what is left of the control yolk in the back half of the airplane watching the turbine and prop climb to the heavens without you.. if you didnt run out of gas taxiing to the runway.

If you want a fast plane, you will normally have to settle for longer take-off rolls. If you want a plane that has a short takeoff distance you will normally have to settle for slower cruise speeds... or RATO bottles, but those might be hard to get.

Disclaimer: This information is not based on Canard flight experience. It is mainly based off light twins as well as research I have done into aerodynamics.... so it might not be directly relatable to a single engine, or a single engine canard, or any airplane for that matter if I misunderstood the information. For some reason I have 10 hours in a single engine, 40+ logged in twins, and over 250 unlogged hours in twins (the guy letting me fly wasn't an MEI) and out of all that.... 0 hours in a canard aircraft.
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  #21  
Old 04-05-2005, 01:19 PM
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Default hp/torque

Torque is force, hp is work. The torque figure is meaningless when it comes to takeoff and climb performance. Best TO and climb is obtained at max power rpm. On a Lyc/Cont with a very flat torque curve and narrow rpm operaing limits, you'd encounter less degradation in performance by using a few hundred rpm less (torque peak) on takeoff than on an auto engine which has max torque at 4000 and max power at 6000 rpm.

The shortest takeoff would always involve running up to max hp rpm on the brakes and releasing.

Turbocharging permits a large boost in torque and hp at mid to high rpms making a fixed pitch prop on auto engine much more viable than a heavily cammed or ported atmo engine which usually has a peaky torque curve and cannot develop decent hp at mid rpms on takeoff. No torque, no hp.
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  #22  
Old 04-05-2005, 01:33 PM
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well spoke mr rv man, just what i thought, only with words
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  #23  
Old 04-05-2005, 10:11 PM
martinkh martinkh is offline
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Quote:
Originally Posted by rv6ejguy
Torque is force, hp is work. The torque figure is meaningless when it comes to takeoff and climb performance....

Turbocharging permits a large boost in torque and hp at mid to high rpms making a fixed pitch prop on auto engine much more viable than a heavily cammed or ported atmo engine which usually has a peaky torque curve and cannot develop decent hp at mid rpms on takeoff. No torque, no hp.
Torque is force*distance (of the lever arm) Work is force times distance (travelled), by definition. The force is the prop pushing air, the distance of the lever arm is the centroid, and the distance travelled is the radial distance covered by the spinning prop calculated from the radius at which the centroid of the prop loading is found.
Notice that speed has nothing to do with work, unless you are calculating work done in a given amount of time, which then shows how HP helps, because you can speed up an engine faster with higher HP. But if you are running at a constant speed, you do the same amount of work if the engine is making 200hp or 300hp at that RPM. But the higher your torque cabablity at that speed, the less heat you generate and the less fuel you use(caeteris paribus).

As far as the peakiness of HP goes, turbo engines are very peaky, about as peaky as any race engine. The difference is, a race engine has high compression, =/>10:1. A turbo engine has a very low compression, usually around 8:1. They make very little bottom end power, as the turbo has not spooled up yet. Hence the term "turbo lag". The popular solution to that has been twin turbos, with one turbo designed to generate low RPM boost, and the other designed to generate high RPM boost.

Keep in mind that a engine built for high low-midrange torque will have a migh higher peak torque than an engine built for high RPM's. Look at ads for crate motors from folks like Summit Racing.

My questions regarding what happens to RPM vs Throttle Position vs acceleration is intended to help me see if my theory about low midrange torque(& HP) is what might cause a high pitch cruising prop to not get you airborne as quick as one may like, and/or keep him from reaching his goal speed. I was thinking that in a case like John's, a broader power curve with less peak would be better, and if the power curve could be broadened enough that with a more agressive prop, perhaps he could get his 220nmh at a lower RPM. I am thinking area under the curve may be more important than all around performance.
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  #24  
Old 04-05-2005, 11:19 PM
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Quote:
in a case like John's, a broader power curve with less peak would be better
I think I follow all of that. I'm just not sure how you go about modifying a specific engine to get a broader power curve.

Interesting you should mention compression ratio. Based on advise from a couple of "professionals" and contrary to the advise of some other "professionals" , I used the non-turbo 9.7:1 rotors.
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  #25  
Old 04-06-2005, 12:08 AM
martinkh martinkh is offline
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John,
A turbo, supercharger or ram will compress the air going in, the vacuum or EFI will draw more fuel to keep the mixture. The volume of mix in the cylinder is pre-compressed, then when the piston goes to top dead center, the effective compression ratio is elevated.

with 9.7 to start with, you end up with monster compression when the turbo spools up. Just to ballpark, I THINK you can do a simple ratio to figure it out. Assume that your airport has 12PSI air pressure. If you add another 50% of that in boost, I think that would boost you to effective 15:1 compression. If it were a piston engine, you would damage valves and beat up your bearings and blow your rings, melt the turbo and overheat the engine. Other than that, you would be fine.

My intercooled turbo 2.2 probably went from 8:1 to 11or11.5:1 at top end. But it was engineered by Carol Shelby.

On a 350 chevy engine, one that I would drive as a street performance or off-road mud bogger, I would go for max torque by switching the crank for a longer stroke 400 crank. This raises it to 383cubic inches. The increased stroke length would build mondo torque by increasing the distance that the piston travels under pressure, and also increasing the amount of fuel burned in each stroke. But then it takes more energy (HP is a unit of energy, like Watts) to change the rate of rotation (speed up or slow down, which is why big engines engine break better).

Sport bikes often lower stroke and increase the surface area of the piston to raise peak HP for a given discplacement, but that kills torque and reduces the area under the curve.

I would also do mild porting as too much porting wastes fuel, and put in a "tow truck" type cam, one with medium-low valve duration, but with good valve lift. The area under both the HP and torque curves would be huge, and the peak HP would be much better than stock. It would also run cool and last forever. It would not win serious drag races, but man would it be a joy to drive.

How you would do this with a rotary engine I have no clue, although if the RX-8 has no turbo and 250hp, they may have figured it out. Porting would be good, but how to increase the torque would have to involve the shape of the rotors or the force during the stroke. As the rotors run on an offset cam, increasing that offset would boost torque, but reduce peak HP. If you could increase the amount of fuel charge in each cycle (without going too far and causing inefficiency) at lower RPMs, you would increase torque. If it would be possible to have your turbo kick in sooner, but never exceed 3-4 PSI, that would work. You would need a variable vane turbo to do that, to prevent overspeeding at higher RPMs.
***As I typed that last sentence, I realized that is the answer!!***
If you are in the market for a new turbo, the variable vane is it. If you damaged your rotors, I would go back to the turbo rotors, although with way lower boost, it may work well as is. See if there is any machine work that can be done to the faces of your rotors to increase torque. Machininng the 9.7 rotors would bring the comp ratio back down to sane.

Last edited by martinkh : 04-06-2005 at 01:03 AM.
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  #26  
Old 04-06-2005, 01:16 AM
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Interesting reading. Keep in mind that the turbo rotors are 9.3 which isn't much different from my 9.7. The rotary is a whole different ball game to recips in a lot of areas. The way it was explained to me is that I'll get the power with "hardware" rather than "software" by using the higher comp rotors and letting the turbo work less hard. Apparantly race guys do this at crazy rpm and boost levels without damaging thier engines. I have a lot of faith in the knowledge of a guy called Leon in AU who tells me that, at my rpm, I'll be fine with the current setup provided I stay under about 9 or 10 PSI boost.

According to the compressor map I'm not likely to overspeed this new hi-flow turbo, even at 46MAP and 12,000'. It's balanced to 120,000 rpm and I should be way below that. Time will tell. Before I settle down to a stately economical cruise I would like to know what it can do in a pinch. (I have Kwatchas at stake here). Two nice things about these experiments 1. the engine is very unlikely to stop running. Even if I blow the seals it'll get me home. 2. Worst case I can get a totally rebuilt replacement engine for < $5k.
Obviously I'm not about to start modifying the engine at this stage. If I seriously break it by "over experimenting", then I might do a bit more porting during the rebuild, but not much because the idle gets too high and this can be a problem on approach with a big prop. I'd also get the rotors ceramic coated.

Quote:
If you could increase the amount of fuel charge in each cycle (without going too far and causing inefficiency) at lower RPMs, you would increase torque
Isn't that exactly what the turbo is doing now?

I think the biggest change I can get for the buck is to cut the prop back a bit, thus getting me higher up the power band for take-off and climb. I need to do some more experiments before I decide on this. I'm tempted to get a 2 blade back-up prop made, then decide what to do with the 3 blade based on this. I'm just not sure if they can make a 2 blade for me that'll be short enough. Without a constant speed prop its a matter of compromise. So far I'm pretty happy with what I've got. It gets off the ground as fast as an IO360 powered Cozy, and it goes like a bat outta hell at altitude. So long as it doesnt break I'm tempted to leave well alone.
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  #27  
Old 04-06-2005, 01:38 AM
martinkh martinkh is offline
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The turbo is not kicking in at low rpm. If you are in the market for a new turbo, and the one I thought you fried (I may have misread) is not variable vane, then a variable vane would broaden your torque range. It works like a variable pitch prop.

Sounds like Rotaries are pretty darn tough if they can handle 9.7 and 10lbs boost.
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  #28  
Old 04-06-2005, 09:24 AM
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Quote:
The turbo is not kicking in at low rpm.
Huh? What do you mean by "kicking in"? A turbo produces boost. Right? Obviously I don't need boost at idle. My static (where I need the power) is currently 4050 rpm at 46 MAP. There's plenty of boost left to be had, but that's all the engine can take without detonating. I have a blow-off valve set at about 9 PSI.

Quote:
If you are in the market for a new turbo
Not until I break this one. How would a variable vane (I never heard of this option) help if I can already get more boost than the engine can use at take-off rpm?
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  #29  
Old 04-06-2005, 01:57 PM
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Default Torque/ turbos

Quote:
Originally Posted by martinkh

As far as the peakiness of HP goes, turbo engines are very peaky, about as peaky as any race engine. The difference is, a race engine has high compression, =/>10:1. A turbo engine has a very low compression, usually around 8:1. They make very little bottom end power, as the turbo has not spooled up yet. Hence the term "turbo lag". The popular solution to that has been twin turbos, with one turbo designed to generate low RPM boost, and the other designed to generate high RPM boost.
Sorry to disagree but turbo engines do not need to be "peaky". Audi puts out a turbo engine with torque peak at 1900 rpm and this is virtually flat to 5500. Matching the compressor and turbine housing is the key to where boost comes in and how. With modern ball bearing turbos and proper matching, turbo lag is not an issue on auto applications and has never been an issue on aircraft applications. The wastegate will dump excess exhaust to prevent overboosting. Twin turbos usually refers to non-staged turbos. Staged turbos like used on the last Supra and RX7 models were pretty much a failure and have been replaced by a single ball bearing turbo, reducing weight and complexity while boosting performance.

The compressor density ratio, not pressure ratio, multiplied by the static CR would give you the effective CR under boost.
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  #30  
Old 04-06-2005, 05:51 PM
martinkh martinkh is offline
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Quote:
Originally Posted by John Slade
Huh? What do you mean by "kicking in"? A turbo produces boost. Right? Obviously I don't need boost at idle. My static (where I need the power) is currently 4050 rpm at 46 MAP. There's plenty of boost left to be had, but that's all the engine can take without detonating. I have a blow-off valve set at about 9 PSI.

Not until I break this one. How would a variable vane (I never heard of this option) help if I can already get more boost than the engine can use at take-off rpm?
Then I suppose the question is, when you give the engine throttle on the line, does it take a long time to get the engine up to speed?

If it takes a long time, then that is the bottom end torque. High HP at 4050rpm may get you off the ground, but does it take a coons age to get to 4050rpm? I'm asking, not assuming. I have no clue if the engine gets up to rpms quick or if the air resistance keeps this from happening.

A turbo uses the exhaust gas pressure to spin a vane. At low RPM, the turbo will not boost much (this is turbo lag, and if they got rid of it in the last 10 years, hurrah)because the vanes on the turbo are designed for higher RPM. Variable vane turbos are designed to spin more at low RPM, and then the vanes flatten out at higher RPM so they dont overspeed. In 1988 shelby went from the intercooled turbo that I had, which was very peaky and had plenty of turbo lag (which was very common) to a variable vane turbo on the shelby shadow. It did wonders to broaden the powerband.
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