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Tommy:
Torque is a non-issue. A given horsepower at a given RPM requires a given amount of torque be produced. Torque is a static measurement of force. Horsepower is the ability to do work. An example would be carrying water up the stairs by the bucket. Torque would be the amount of water you could carry per trip, and horsepower would be the amount of water you could carry carry per hour. If you can carry two five gallon buckets of water per trip you have twice the torque as someone who can only carry one five gallon bucket, but if the guy with one bucket can make twice as many trips he does the same amount of work as you do. A million pounds of torque at zero RPM does no work at all.... it just sits there.
Horsepower can be defined as Torque * RPM / 5252
If a 13B produces 140 horsepower @ 6000 RPM 140 * 5252 / 6000 = 122.54 lbs torque @ 6000 Put that through a 50% redrive and you end up with 245 foot pounds at 3000.
You must think in terms of generating horsepower...... Horsepower is the ability to do work..... Oops I'm repeating myself.
GENERATING TORQUE:
There is only one way to increase torque....(aside from gearing down) that is to increase air / fuel flow PER REVOLUTION. This can be accomplished by improving induction design which generally assists only in a limited range.... most such measures work primarily at high RPM. The reason for this is that the resistance to airflow increases as the airflow increases. You will note that peak torque is found at low RPM where maximum airflow can be achieved. This can also be achieved by supercharging an engine..... which has other undesirable effects including increasing the compression to the extent that detonation is an issue, and of course weight, and higher stresses.
GENERATING HORSEPOWER
To increase horsepower we need to increase airflow PER UNIT OF TIME. I use cubic feet per minute as a standard measure of airflow. It takes very close to 1.62 cubic feet per minute to produce one horsepower. We can increase horsepower by increasing RPM. Simple...... open the throttle. By using the appropriate gear reduction we can run our RPM up to where we have the horsepower we need....... there's your increase in torque. The extremists resort to such measures as peripheral ports and turbocharging to achieve much higher than rated horsepower, and much less than normal life expectancy and greatly reduced reliability.
There are a lot of pie in the sky claims out there, and a lot of mis-conceptions....... unfortunately we must operate in the real world with real world limitations. You have about 140 horsepower to work with..... all you need to do is determine what your optimum prop RPM is based on diameter which you determine from gear clearance and your intended operating speed...... do you want lots of low end thrust or a high cruise speed? High cruise speed does not require nearly as large of a prop as good climb out, static and low speed operation. You then spec out your reduction to match the desired prop RPM. Don't bother your head about "torque".....
Interestingly you can convert horsepower directly into thrust at a given forward speed if you know prop efficiency. Assuming a prop efficiency of 80% which is realistic from around 100 mph upward through most of our cruise ranges, you can use the following formula to work out your thrust. (note that it does not work much below 100 mph because efficiency falls off increasingly)
Thrust = horsepower * (300/speed in mph). Thus your 140 horsepower should give you 280 pounds of thrust at 150 mph
Actually Thrust = horsepower* { (375*prop efficiency) / speed in mph } Horsepower has to be real honest American Society of Automotive Engineers Brake Horsepower for this to work. Note that at very low speeds prop efficiency drops to near zero values, and in fact at zero forward speed efficiency is at zero for all intents and purposes.
Hope this helps clarify the issue...... there are way too many people out there trying to cloud it.
H.W.
*********** REPLY SEPARATOR ***********
On 12/5/02 at 6:06 PM Tommy James wrote:
>Hi Guys and Kathy, Happy Christmas!
>
>Does anyone have ideas about how much torque@rpm the typical aviation
>installation makes with a NA 13B? Also some elementary principles on the
>variables of generating torque would be helpful.
>
>TIA
>Tommy James<><
>
>
>>> Homepage: http://www.flyrotary.com/
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