Topic Author
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

How To De-rate An Engine

Sat Nov 08, 2003 5:51 pm

G'day folks

I've thought about this a few times but haven't had a chance to post it. Say I was designing and building a C-182 size aircraft, and I'm working over the performance figures for the rate of climb I hope to achieve in my new toy, and I figure out that I want to use an IO-540 in my aircraft putting out 230shp. So I talk to Mr. Lycoming and he's got an IO-540 which generates 300shp but it can be de-rated to 230shp for me. How exactly is this done? What does an engine manufacturer do when he/she develops an engine generating 300hp but wants to derate it so it can be used in other applications?

The thread Who Flies The Trent800 And/or GE90 At Full Power? also got me thinking about this for gas turbines. It mentions operators buying the engines then permanently de-rating them. How is this done?

Thanks in advance

Rob.  Big grin
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Joined: Tue Oct 26, 1999 11:34 am

RE: How To De-rate An Engine

Sat Nov 08, 2003 8:03 pm

On an EEC controlled engine you just change the rating plug.

The rating plug is a multipin plug that is plugged in the EEC and depending on the postion of the pins tells the ECC what rating the enging should be operated at.
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: How To De-rate An Engine

Sun Nov 09, 2003 2:02 am

I think, but don't quote me on this, that on some piston aircraft, like the Robbinson R-22 that there is just a placard that says "do not operate the engine above XXXX rpm"...

Is there really anything more than that needed?

I suppose that you could have an A&P make you a stopper for the throttle and get it certified...

RE: How To De-rate An Engine

Sun Nov 09, 2003 3:14 am

Just limit either the RPM - if its got a fixed pitch prop, or the manifold pressure if you're planing to use a constant speed.
T prop
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Joined: Tue Apr 03, 2001 4:33 pm

RE: How To De-rate An Engine

Sun Nov 09, 2003 3:34 am

I can only tell you about smaller gas turbines that I work on. A derate would call for a change of data plates, engine instruments, and a change to the POH and MM's. The engine instruments are different in that they are remarked, the data plates show a change in engine model and build spec.

An uprate of the same engine, if possible, is a little different in that you pay a lot of money to get the data plates from the manufacturer. The engine itself does not change, the only change is how far you are allowed to push the power levers up, higher torque/temp limits.

T prop.

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Joined: Mon Apr 23, 2001 10:34 am

RE: What With Thrust Reverse Prior To Touchdown.

Sun Nov 09, 2003 11:21 am

Well...what about the following techniques:

-lowering the compression ratio on piston engines(sometimes all you need to do this is a thicker cylinder head gasket or replacing the pistons and the rods). OK, this might be problematic, since with lower compression, also a lower octane gas is needed, using "normal" gas can end in burning away the exhaust valves

-restricting the intake airflow and/or the maximum supercharge pressure on supercharged/turbo engines(at least for automotive engines, such parts are not a rarity...or were not a rarity in the past)
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RE: How To De-rate An Engine

Sun Nov 09, 2003 2:53 pm

Ok... forgive me for being stupid and dumb...

Why would you want to derate an engine? I would think that, if you are going to use 300-hp engine, why not get all 300 out of it? It takes the same amount of space anyway. If you want 230 hp, get a smaller engine, not a bigger one that is restricted.

Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
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RE: How To De-rate An Engine

Sun Nov 09, 2003 5:57 pm

Manufacturers usually derate engines as a way to increase reliability. Derating gives you a product that is 'overbuilt' for the purpose, therefore stronger and more reliable. Mooney now uses the same basic lycoming engine in all three aircraft they produce. They limit the maximum propeller RPM to reduce the maximum horsepower. Somebody correct me if I'm wrong, but I believe this pushed the TBO from 2000 to 2200 hours in Mooney's aircraft. May not sound like much, but this will save the owner of the aircraft time and money in the long run. Same engines also means same parts. The manufacturer, and consumer can enjoy lower costs because there is more commonality between variations and less model-specific parts can be stocked.

I never really understood this until one of my professors explained it in this manner. An automobile is rarely using more than 30% of it's available power. That's why their engines can be used for longer periods with less failures. Now consider an aircraft engine(piston). These engines are almost always operated in the upper band of their available power. Most economy cruise settings are 65% or 75% power. And they are operating over a wider range of temperature and atmospheric conditions at a higher level of performance. Becuase of these facts it's not such a bad idea to have an engine that's designed for more.

RE: How To De-rate An Engine

Mon Nov 10, 2003 11:25 pm

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 – mechanically (for example a different fuel computer) or “on paper” (charts and graphs which limit output).

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, as has been explained previously, 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, the 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 decied that they needed an airplane co 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 the aircraft that I fly, 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.

It sounds to me like you may want to consider your chioce of powerplants for your C-182.

Topic Author
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RE: How To De-rate An Engine

Tue Nov 11, 2003 5:25 pm

G'day Jetguy

I hear what you're saying regarding flat-rated engines - I reckon a good analogy is climbing a turbocharged piston engine at max rated power and progressively closing the wastegate until full throttle height, at which point climbing further and the engine acts as normally aspirated.

I guess what I was really asking is how two engines of the same capacity have different outputs. For example at our flight school we have an IO-360-L2A in the 172's (160bhp) and 2 x IO-360-A1B6 (200bhp) in the Partenavias. Give the relatively simplistic design of aircraft piston engines (compared to cars) i've just considered capacity and power to go hand-in-hand (unless superchargers or turbochargers are bolted on) - more capacity, more power - that simple. I've got a reasonable understanding of piston engines and the otto cycle and all that, we covered it in some detail at uni but very briefly - no chance to learn it thoroughly.

I've read you post a couple of times and hear what you're saying. Now with a bit more of an understanding of de-rating and flat rating I think what i'm really asking is what are the differences in the engins above that produce a different max power?

Thanks for your input - appreciate your posts. & thanks everyone else for your input.

Cheers, Rob.  Smile
T prop
Posts: 930
Joined: Tue Apr 03, 2001 4:33 pm

RE: How To De-rate An Engine

Tue Nov 11, 2003 7:57 pm

Hello Rob,

You asked: what are the differences in the engins above that produce a different max power?

This would be hard to answer exactly without manuals for both engines.

Usually to make 40 more horsepower in the case of the 2 engine examples, there would be changes made to increase it, and changes to support the increased power reliably. Typically compression ratio is raised, better cylinders with angle valve crossflow heads are utillized, different camshaft, tuned intake system, fuel servo differences are likely and other things I have forgotten. To support the increase in heat and stress, there may be a heavy duty case, crankshaft, conrods, bearings, piston cooling nozzles, rotator valves and more.

T prop.


RE: How To De-rate An Engine

Wed Nov 12, 2003 12:25 am

T prop hit the nail on the head. In VERY simplistic terms, aircraft engines are comparable to automotive engines - the factory takes a basic engine and "soups it up" or modifies it by using all of the methods T prop mentioned and more.

Topic Author
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

RE: How To De-rate An Engine

Wed Nov 12, 2003 8:48 am

Beautiful....thats cleared things up nicely - thanks T prop and Jetguy for your help.


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