Realistic torque curves for the turboprops

[MAFman]

I read engines usually output a changing amount of torque at different rotational speeds. What would a realistic torque vs. RPM curve look like for the turboprop engine?

[18Watt]

Most turboprop engines that I’m familiar with are operated within a very narrow RPM range.  Usually 97-100%, regardless of the output required.  Torque is controlled by the amount of fuel fed into the engine, engine speed is controlled by a governor which adjusts the propeller blade angles to maintain a constant speed.

Turboprop engines use a gas turbine to drive a propeller, often via a reduction gearbox.  Gas turbines are most efficient when compression is highest, which is also when the compressor is spinning the fastest.  That’s why you would normally want to operate one at its design RPM, even when you don’t need all the power output it’s capable of.  Even at low or mid power settings, I still want the engine to be using the fuel as efficiently as possible, so I want the compressor and turbine to be spinning as fast as they can.

Another thing to keep in mind is that the RPMs displayed are for the output shaft, not the compressor and turbine.  In real-world turboprops the turbine is spinning much faster than the output shaft.  For example, a typical propeller RPM would be ~1,500 RPM, but the turbine driving the propeller might be spinning at 20,000-40,000 RPM.  KSP has difficulty simulating things spinning that fast, so I think the rotors and turboprops are limited to 460 RPM, and other physical aspects are adjusted to counteract that limitation.

Anyway, with actual turboprop engines, I wouldn’t expect to use a torque curve over a range of RPMs.  I would expect the engines to operate within a very narrow RPM range, and achieve torque from 0-100% within that narrow range by adjusting the fuel flow into the engine.

 

[MAFman]

On 7/17/2022 at 7:56 PM, 18Watt said:

Most turboprop engines that I’m familiar with are operated within a very narrow RPM range.

So, it'd be more realistic to have a torque limiter curve as a function of throttle, with the RPM limiter locked at 460 RPM?

[18Watt]

3 hours ago, MAFman said:

So, it'd be more realistic to have a torque limiter curve as a function of throttle, with the RPM limiter locked at 460 RPM?

Basically yes.  I don’t think it’s possible to set up turboprops in stock KSP to function exactly like they would be operated in real life.

460 RPM works well, but due to how KSP handles the physics of the propellers, the most efficient RPM is usually lower than that (in KSP).  The ideal speed for KSP props varies depending on your goal- Max Speed, or Max Efficiency.  But in general I’ve found RPMs of about 390 give me the best performance.

Here’s how I normally set up propellers, either turboprops or electric rotors.  This isn’t the only way to do it- just what works for me..

• Main throttle is tied to engine torque.
• I tie Max RPM to an action group, or the translate controls.  This won’t be used much, because I control RPM with propeller blade angle.
• I tie the propeller blade angle to another action group or set of controls.

Then, during flight-

• I control power (torque) with the main throttle.
• I normally leave Max RPM set to 460.
• I control actual RPM with the propeller blade angle.
• I would normally set the desired power, and adjust blade angle for maximum speed, which usually is near 390 RPM.
• Every time you change power (torque, main throttle) you’ll need to adjust blade angle to maintain the desired RPM.

With a real turboprop a governor would maintain the desired RPM for you, by adjusting the prop blades.  In stock KSP I don’t see a way to make that work, you’ll have to control the prop blades yourself, in order to keep your RPM at the desired speed.

The reason 390 seems to work well is not due to the engine, but rather how the propeller blades work.  This mimics real life- a real propeller blade works best when the tips are perhaps just barely supersonic.  Any faster, and more of the propeller blade is supersonic, which decreases efficiency.  I’m not sure how the KSP model works, but that’s the basic concept, which applies in real life too.

When set up the way I described, KSP does model fuel flow fairly realistically.  An increase in torque equals an increase in fuel flow.  I couldn’t say how realistic the fuel flow to torque relationship is in stock KSP, but in real life there are a lot of different turboprop designs, which each have different fuel-torque properties.

One additional problem with running your props at 460 is how KSP handles extra torque.  Keep in mind torque equals fuel flow.  Say you are at half-throttle, and the engine is at 460.  If you increase throttle (to full), you have added torque, and added fuel flow, but the prop is still spinning at 460.  So you are using more fuel, but getting exactly the same thrust as before.  (Assume you left the prop blades as they were..).  So running the props at max speed (I think it’s 460), you don’t know if you are wasting excess fuel to provide excess unused torque.

Again, with the stock KSP props I usually find about 390 RPM works well, sometimes as low as 330.  Wouldn’t surprise me if some designs do best at even lower RPMs, you’ll have to experiment.