Derated Engines?

[westwind]

I was just wondering what the purpose is for taking an an airplane engine (eg. the Lyc. 0-360 in the C-172R being derated to 160 hp or the Lyc. 0-320 in a Cherokee 140 derated to 140 hp) and limiting it's power. If you (the pilot) want to get better economy, set the engine at a lower cruise rpm and you will still have full power available in an emergency situation. These engines are made to run at the higher rpms anyway, are they not? These were the two examples that came to mind. Are there others out there similarly "Derated"? Any thoughts?

 

[KigAir]

By derating an engine, is it's TBO increased?

 

[averyrm]

The 172 has 160 or 180hp because of the prop. One prop has a lower max RPM, one can turn faster, hence: more horsepower.

 

[Lead Sled]

This is a very interest topic for me. There are two terms that come to mind anytime you start limiting aircraft engine power - "De-Rating" and "Flat-Rating" The differences between “Flat-Rating” and “De-Rating” engines can be confusing. De-rating an engine means arbitrarily assigning a lesser output to an engine than it was designed to have. There are many ways to achieve the power output reduction on piston or turbine engines – mechanically (for example a different fuel computer, or as was previously mentioned - unstalling a different propellor) or “on paper” (charts and graphs which limit output).

Many turbine powered aircraft use derated engines. An engine derated to a particular thrust a SL will also be limited to that thrust at 40 000Ft. In other words, when they take away the power, you don’t get it back. There are many reasons to install de-rated engines. One of the major reasons is that they “loaf” – they’re operating at a certain reduced percentage of their design capability. This usually makes for increased engine life. Back about 30+ years ago, a race team (I’ve forgotten the driver’s name - I want to say it was A.J. Foyt) installed a P&W PT-6 in an Indy car and proceeded to eat everyone’s lunch. If it weren’t for some bad luck, he would have won the Indy 500 the first time out with it. The following year the race officials forced them to de-rate the engines – by adding restrictor plates to the air inlet – to the point that they lost all of their previous advantage. By the way, there is a company in Texas that has an STC to put the big Continental 0-520 (yes, they take an injected engine and refit it with a carburetor.) I think the engine ends up with a TBO of 2400 hours. Way back in the 60's, Piper decided that they needed an airplane to compete with Cessna's 150. Rather than design another aircraft, they simply removed the back seats from their Cherokee 150 and placed a restriction on the engine RPM - bingo... The New and Improved Cherokee 140. The first thing most folks did is get an STC to restore the RPM and get the 10 hp back. That’s de-rating.

Now, for flat-rating…
Generally speaking, it must be remembered that (in very simplistic terms) turbine engines are not supercharged, but rather normally aspirated - in other words, they lose power with altitude just like a Cessna 152. The percentage of N1 (on most turbofans) or EPR (many turbojets) required to obtain the engine's full rated thrust will vary significantly depending upon airport elevation and outside air temperature. For example, on one of the aircraft that I'm typed in, on a cool day at a sea level airport the engines will develop their maximum rated thrust with an N1 somewhere in the upper 80's say for example 88.7%. Go to a higher elevation airport on a warm day and the N1 will be higher, for example 93.4%. (As I type this, I'm looking at the Static Takeoff Thrust Setting Chart for our aircraft. Depending on the airport elevation and outside air temperature, the N1 settings vary from a low of 84.2% to a high of 96.1%.) These numbers will, of course, vary from engine to engine, but you get my point. On most older generation engines, the flight crew is required to come up with a takeoff power setting from a set of charts or tables. In later generation engines with DEECs (Digital Electronic Engine Controllers) or FADECs, the pilots only have to set the power levers into the takeoff detent and monitor things while the computer takes care of the rest.

Just to make things a bit more interesting, some aircraft have larger engines installed than they were designed for. These engines are "Flat-Rated" back down to what the airframe was designed to handle. In other words, say for example, an airplane was designed to use a pair of 40,000 LB thrust engines, the aircraft designers might specify a pair of 50,000 LB thrust engines and limit their thrust to 40,000 LBS. Why would they want to do this? Simple, remember that turbine engines are "normally aspirated" and start loosing power the moment they start to climb. By using a larger engine, the aircraft can operate at higher altitudes or temperatures before it runs out of power. The engine never produces more than the “airframe-rated” thrust (in this example 40,000 lbs), it’s just able to do it to a higher altitude.

Turboprop engines are similar, only instead of N1 or EPR, they usually measure their power output in Percent Torque. For those guys it's a bit simpler, they simply advance the power levers until the engines reach either their torque limit or their temperature limit. Typically, with flat-rated engines, they will "torque" out at lower altitudes, then as the aircraft climbs higher they "temp" out as the max operating temperatures become limiting.

I hope that this isn't more information than you wanted, but I'm bored. Sorry.

Lead Sled

 

[phishn@daves]

Lead Sled, that was so beautifully put I'm sobbing.


Really, that was the best explaination I have heard on this topic! Hey, next time you are really bored do you mind tossing in some diagrams?

 

[mzaharis]

Quick detail - it was teamowner Andy Granatelli that did the most famous fitting of a turbine to a race car at Indy. Parnelli Jones drove it in 1967, and got to within 10 miles of winning the race when the proverbial $6 dollar transmission piece broke, allowing AJ Foyt to win.

There were some other attempts to race turbines at Indy around that time, but USAC put prohibitive restrictions on them, effectively banning them.

http://8w.forix.com/altpower-turbines.html

 

[Lead Sled]

Quick detail - it was teamowner Andy Granatelli that did the most famous fitting of a turbine to a race car at Indy. Parnelli Jones drove it in 1967, and got to within 10 miles of winning the race when the proverbial $6 dollar transmission piece broke, allowing AJ Foyt to win.

There were some other attempts to race turbines at Indy around that time, but USAC put prohibitive restrictions on them, effectively banning them.

http://8w.forix.com/altpower-turbines.html

Thanks for the clarification. I remember AJ Foyt was in there somewhere.

Lead Sled

 

[westwind]

Thanks for the info, guys. I have a couple of follow-up questions.

Rather than design another aircraft, they simply removed the back seats from their Cherokee 150 and placed a restriction on the engine RPM - bingo... The New and Improved Cherokee 140. The first thing most folks did is get an STC to restore the RPM and get the 10 hp back. That’s de-rating.

What did the STC change on the Cherokee? It wasn't putting on a different prop or re-pitching the original was it? I can see changing props for better climb or cruise (depending on your needs), but for limiting HP? I guess that I was envisioning a governor on the carb of the Cherokee (that would be removed or reset by the STC), and something similar on the throttle body (does the C-172R even have one?) or maybe in the computer of the Cessna, as was previously mentioned. Not trying to belabor the previous posts, just trying to learn. Thanks.

 

[Lead Sled]

What did the STC change on the Cherokee? It wasn't putting on a different prop or re-pitching the original was it? I can see changing props for better climb or cruise (depending on your needs), but for limiting HP? I guess that I was envisioning a governor on the carb of the Cherokee (that would be removed or reset by the STC), and something similar on the throttle body (does the C-172R even have one?) or maybe in the computer of the Cessna, as was previously mentioned. Not trying to belabor the previous posts, just trying to learn. Thanks.

I learned to fly in 1966. Back then, the Cessna 150 was the big D) trainer. In those days, Piper's trainer had been a 2-seat, 108 hp version of the Tri-Pacer. In 1962, Piper developed the Cherokee 160 to replace the Tri-Pacer; but they still needed an inexpensive entry-level trainer. In 1964, the Cherokee 160’s rear bench seat came out and they added a climb prop that effectively de-rated the engine to 140 HP by limiting RPM. (It was basically the same idea that later resulted in the last downmarket PA-28 variant, the Cadet: Offer a stripped-down, low-cost version of an airplane already in production and pitch it to flightschools.)

The results were mixed. Yes, the extra space in back was nice but the 140 didn’t do what the Cessna 150 did. For one thing, the 150 HP engine was more expensive to operate and maintain than the Cessna’s O-200. Second, the 140 didn’t handle like the ideal trainer. It had a mushy stall and less-than-crisp slow flight qualities.

In 1965, Piper changed to a different prop which allowed the full 2700 RPM and restored the missing power. A gross-weight increase was allowed and a removable bench seat was installed, yielding, well, a Cherokee 150, more or less. The rear seat was only good in a pinch but the competing Cessna 150 didn’t have that option at all. If you had an early Cherokee 140 you could install the newer prop under an STC to get back the 10 HP. I understand that most owners opted to do so.

Lead Sled

 

[westwind]

Thanks again LeadSled. PM me when you are headed my way.