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Turboshaft Engine Efficiency Tips [1.7.3]


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This post is to share some tips at improving the efficiency of turboshaft engines, driving propeller blades (airplanes) or rotor blades (helicopters).  The concepts should apply equally to electric rotors driving propellers or heli-blades.  I'm using 1.7.3, the electric rotors, turboshaft engines, and propeller/heli blades are new to stock KSP, so in later versions some of this info might become outdated.

It's easy to focus on either the Turboshaft Engine, or the Prop/Heli blades when troubleshooting 'why doesn't my vessel work'.  The reality is you need to look at both elements to make a functioning prop-airplane or helicopter.  The engine produces torque, which spins a shaft.  This is important, torque produces zero thrust.  Add all the torque you want, it will not produce any thrust- until it is coupled to a lifting device, such as a propeller blade or heli blade.

On the other hand, the prop/heli blade also produces no thrust unless it is moving.  You can angle the blades any way you want, they will produce no thrust unless they are moving. The movement is provided by the Turboshaft Engine (or electric rotor).

As of 1.7.3, the stock engines can rotate at a maximum rate of 460 rpm, which is evidently a Unity limitation.  Adding torque will increase rotational speed (rpm) up to that limit of 460 rpm.  What happens if you are already at 460 rpm, and increase torque?  That's an important question.  What happens (in 1.7.3) is you increase the Fuel Flow (or EC draw), but do not get any increase in rpm.  Beyond 460 rpm, adding torque is simply wasting fuel.

In the following examples, we're going to experiment with turboshaft engines.  There are two variables we are going to adjust, and unfortunately they are on two different parts.  One is the Torque of the Turboshaft Engine, the other is the Authority Limiter of the prop/heli blades.  To make things easy, I like to assign these values to action groups.  I assign Engine Torque to the Main Throttle, and the Blade Authority Limiter I assign to Translate Forward/Back (usually 'H' and 'N').

Okay, lets get started.  Here's a turboshaft engine we're going to play with.  It's in the 'propeller' configuration, but the idea is the same for helicopters.


Engine test- Probe Core, LF Tank, Turboshaft Engine, and a Battery.  Use Launch Clamps to make sure it stays put.


Couple of things to note.  I've changed 'Attach Nodes' to Quad.  Rotation Direction is 'Clockwise'.  Invert direction is 'Normal'. 

Next shot is after adding the blades, Propeller Blade Type 'B'.


With the blades, note the following:  Pitch, Yaw, and Roll are Inactive.  Set 'Deploy' to Extended.  Blade Variant (at the bottom, and important) is Clockwise.

Almost ready to launch.  But first, let's set some action group functions.  I like to set the engine Torque to the Main Throttle group, and I set the Prop/Heli Blade Authority Limiter to the Translate F/B keys.  Use whatever keys you want though.  Next photo is me setting the Prop Blades to the Translate F/B keys.


After building the engine on a stand, let's go play with it.

Ok, we have an engine built on a stand, let's play with it to see how Torque, Blade Authority, RPM, Fuel Flow, and Thrust are related.


First, let's bring up the PAW windows for the Turboshaft Engine and the Propeller Blades (one blade is plenty..). Couple of things to notice in the next photo.

1. The PAW info windows are open for both the Engine and one Blade.  (the blades should all work together and be the same..)

2. The Engine Torque is zero (if not, make it zero).  I have mine tied to the Main Throttle via action groups, so it starts matched to a zero throttle.

3. The Prop Blades start Extended, because that's how I set them in the SPH.  If yours aren't extended, extend them now.

4. I've rolled back my Prop Authority Limiter to 0 (zero) using the 'H' and 'N' keys, which I set in action groups.  


Next up, we'll play with Torque and Blade Authority Limit, and watch the rpm and Fuel Flow.

In the next spoiler window, I'll play with Torque and Blade Authority Limit, to see what happens to rpm and Fuel Flow.  Keep in mind that Thrust is only dependent on rpm and blade angle.  If my rpm and blade angle stay constant, but my Fuel Flow increases, I have not increased my thrust.  I'm just wasting fuel at that point.  So, one goal is to find the minimum Fuel Flow which will maintain a specific rpm at a specific blade angle.  Here goes.


Ok, let's increase torque to 100%, and blade limit to 100%.  The blades start spinning.  Note the Current RPM is ~460.  We're not looking at the RPM Limit, the Current RPM is displayed just below the RPM Limit.  Note the Fuel Flow (for LiquidFuel) in 375mu/s in the engine PAW.  This is also displayed in the Resources window, showing a LF consumption of (0.38).


Alright!  We're producing thrust!  However, we can do a lot better in terms of efficiency.

Let's start reducing the Torque, bring it back to about 30%.  Notice the Current RPM is still 460, and the Prop Authority Limiter is still at 100.  That means our thrust is still the same.  But now look at our Fuel Flow.  Fuel Flow is now about 115mu/s, about 1/3 of the fuel flow at 100% Torque!  We are now burning 1/3 of the Fuel as before but still producing the exact same thrust!


Wonder how far we can take this fuel savings?  Here we go!  I'm going to reduce Torque to about 4.0%.  That's about as low as I can go and still maintain 460 rpm with the Blades set to 100.  So again, I'm making exactly the same thrust as when I had 100% Torque.  Only now, my Fuel Flow is 15.0mu/s.  My fuel burn is now 4% of what it originally was, but I'm still making the exact same thrust!!


That means I'm burning about 1/20th the amount of fuel, and making the same thrust.  Or, to look at it another way, I can go the exact same speed, but now go 20 times farther than I could before.

There's a lot more tweaking we can do, I'll touch on that next with a demonstration of an aircraft that flies instead of an engine on a stand.

In the next spoiler window, I'll demonstrate an actual aircraft, making adjustments to torque (and Prop Authority) to reduce fuel burn.  Ideally I'd use a single-engine airplane to keep things simple.  However, counter-acting the torque effects of a single-engine are difficult to design for, and fly efficiently.  It's easier to just make a plane with two counter-rotating engines and propellers.  Then, it flies quite easily just like a jet, with no nasty torque effects.


Here's a really simple plane, with two Turboshaft Engines.  The engines are counter-rotating so the torque effects cancel each other out.

Note the Left Engine (right side of screen) is set to Clockwise, Normal.  The Right Engine (left side of screen) is set to Counterclockwise, Normal.

The Left Propeller (right side of screen) is set to Deploy Direction Normal, and is the 'Clockwise' variant (at bottom of the PAW).

The Right Propeller (left side of screen) is set to Deploy Direction Inverted, and is the 'Counterclockwise' variant (at bottom of PAW).


Let's go fly it, and look at Fuel Flows at various settings.

Next photo, we are cruising at ~2,000m altitude.  Torque is 100%, Prop Authority is 100.  Speed is 130m/s, and Fuel Flow is 370mu/s, or (0.75) total.  It's really burning a lot of fuel.


We can improve on that though, right?  Let's reduce Torque to about 7%. That's about as low as I can go before rpm starts to drop below 460.  So, what happens at 7% Torque?  RPM is still 460.  Fuel Flow is 26.3mu/s (0.05 total).  And the speed is the same, 130m/s.  I get the same thrust, the same speed, but Fuel Flow is about 7.1% of what it was at 100% Torque!  The other 93% was just being wasted!


Ok, so far we have adjusted Torque, but kept Blade Authority Limit and everything else constant.  We can still get a lot more out of this plane though.

In the next shot, I tried decreasing Blade Authority to 75%.  That caused my rpm to drop, so I added just enough torque to maintain 460 rpm.  In this case, I increased torque to about 19%.  

So, at 75% Blade Limit and 19% Torque, my speed increased to 210m/s.  Fuel Flow also increased, to 71.3mu/s (0.14 total), but that's still far less than the 370mu/s (0.75 total) I was getting at 100% Torque.

The takeaway here is that to get the most out of the turboshaft engines and propeller or helicopter blades, you need to look at more than just the engine or just the blades.  Both elements need to be adjusted for optimum performance.  Ideally, you need to be able to adjust both Torque and Blade Authority Limit in flight, to adjust for varying conditions.  Regarding efficiency, the big takeaway is that adding Torque beyond what is required to maintain rpm is just wasting fuel, and lots of it.

As of 1.7.3, I believe the concept is the same for electric rotors driving propellers or heli blades- adding torque beyond what is required is only wasting EC.

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My biggest issue right now is with the helicopter blades; I can't get them to produce pitch torque in the right direction, despite there being no issues in the roll axis. I'm using a 6 blade coaxial rotor (2x3, like a Ka-50) to counteract the torque (which works like a charm), but no luck being able to fly it without reaction wheels. Upon further examination, it seems the bow and aft blades do not rotate as needed, probably due to them being programmed as wings (they only work when at the sides of the craft), leaving only the starboard and port blades to do a job which they never could.

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