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Aircraft engine torque specs.

 
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kpiller



Joined: 06 Jan 2014
Posts: 3

PostPosted: Wed Nov 22, 2017 21:18    Post subject: Aircraft engine torque specs. Reply with quote

I have often wondered why the torque measurement is not given for aircraft piston engines-only the horsepower rating. After all the name of your magazine is "Torque Meter". Can anyone explain why torque values are not given. Thank you,
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kmccutcheon



Joined: 13 Jul 2003
Posts: 198
Location: Huntsville, Alabama USA

PostPosted: Thu Nov 23, 2017 10:27    Post subject: Reply with quote

Average torque values do not appear in aircraft engine specifications because they are of little value to aircraft designers, who get nearly everything necessary for aircraft design and operation from power, rpm and manifold pressure at altitude. These are the things shown in aircraft engine power curves. Please see
http://www.enginehistory.org/members/aehsdc/AEHSDCscans/2512.php
If aircraft designers need average torque values for, say, engine mount design, these can be calculated as follows:

Torque = (Power * 5,252) / RPM

Engine designers rarely work with average torque. They are more interested in power at given engine speeds and manifold pressure, which comes from the formula:

HP = PLANK / 33,000, where HP is either brake or indicated horsepower, P is either the brake or indicated mean effective pressure (derived from single-cylinder indicator diagrams), L is the piston stroke length in feet, A is the piston area in square inches, N is the number of power strokes per piston per revolution (0.5 for four-stroke/cycle engines), and K is the number of cylinders.

Engine designers are more interested in instantaneous internal engine torques and forces, which dictate component sizes, bearing loads and vibration characteristics. These involve much more complicated torque metrics, the derivation of which is described by Joseph Liston in his book "Air craft Engine Design" thusly:
http://www.enginehistory.org/Misc/TorqueDerivation.jpg

Engine torque becomes interesting to operators because they would like to know the ACTUAL engine power produced. The foregoing metrics are theoretical and sometimes do not take into account actual temperature, actual atmospheric pressure, actual engine condition, etc. If one knows the actual engine torque during operation, one can use the Torque formula above to calculate ACTUAL power. This is why engine torquemeters were developed. Even so, most U.S. aircraft engine torquemeters are hydraulic and, by convention, read in psi rather than in lb-ft. These measures allow operators to accurately lean their engines at altitude, and to detect and possibly diagnose engine problems during operation.

Below is a link to a short article on the development of engine torquemeters.
http://www.enginehistory.org/members/aehsdc/AEHSDCscans/2513.php
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kpiller



Joined: 06 Jan 2014
Posts: 3

PostPosted: Thu Nov 23, 2017 20:00    Post subject: Reply with quote

Thank you very much for your detailed answer to my question. I have worked most of my life in the heavy truck vocation so am familiar with big bore diesel engines that power the over the road vehicles. And always to me it was more important to know what the torque rating and at what rpm than just knowing the horsepower of an engine. Of course the engine families between truck and aircraft lead very different lives. When I think of how a fully loaded B-17 had to struggle its way to its target with the engines stressed to the limit under continuous full rated power-just wondered how the torque compared to the truck diesel engine of yesterday and even today. It amazes me of what we were able to accomplish back in the ww2 days with engine technology. Boosting engine manifold pressures past 45 psi and the complexity of designing the big radials etc. All by slide rule. Today you don't see the big bore truck diesel engines going much past 35 psi boost pressure. And if a tube from the turbo to the air to air cooler or intake should burst-scary for the driver as it sounds like a front steer tire blew and of course black smoke out the exhaust stack(s) and massive loss of power.
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kmccutcheon



Joined: 13 Jul 2003
Posts: 198
Location: Huntsville, Alabama USA

PostPosted: Fri Nov 24, 2017 08:20    Post subject: Reply with quote

kpiller wrote:
When I think of how a fully loaded B-17 had to struggle its way to its target with the engines stressed to the limit under continuous full rated power-just wondered how the torque compared to the truck diesel engine of yesterday and even today.


While I do not have specifications for the exact engines used in the B-17, I do have them for another Cyclone 9, the C9GC. This engine had a two-speed supercharger instead of the turbocharger used on the R-1820-97 that powered most B-17s, and had similar ratings using grade 100/130 fuel:
Takeoff = 1,200 hp @ 2,500 rpm
Normal = 1,000 hp @ 2,300 rpm
Cruise = 700 hp @ 2,100 rpm

Substituting these values into the torque equation from my previous post, we get torque values of:
Takeoff = 2,521 lb-ft
Normal = 2,283 lb-ft
Cruise = 1,751 lb-ft

The takeoff rating is only good for 5 min, but the engine can run forever at normal and cruise ratings. The difference is that at the normal rating gobbles a lot of gas, while the cruise rating (58% of takeoff power) is relatively efficient. Even at cruise the brake specific fuel consumption is 0.44 lb/hp/hr, which does not approach that of a diesel. Even at 1,400 rpm and 500 hp (42% power) the bsfc is only 0.405 lb/hp/hr.

I have posted another document that details how aircraft engine ratings are calculated, measured and specified. Please see
http://www.enginehistory.org/members/aehsdc/AEHSDCscans/2514.php
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kpiller



Joined: 06 Jan 2014
Posts: 3

PostPosted: Fri Nov 24, 2017 20:28    Post subject: Reply with quote

Thank you for the further information and providing the torque ratings using the formula you provided in your first reply. As a comparison to the Cyclone 9 radial engine which I believe is 29.88 liters to a modern day heavy truck diesel engine: I will use the Detroit DD16 engine which is a 15.6 liter engine and can put out continuous 600 hp/2050 lb ft torque at its highest setting. And again I realize we are not comparing apples to apples but now I can better appreciate the power needed to turn the large 3 and 4 blade propellers along with all the parasite accessories bolted to the engine. The pace of development from the early 1900's to the 1940's is just amazing I think. Just so sad that it has to take wars to expedite these advancements.
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