Engine power output and adjusting it.

[fliger747]

An interesting variation on this graph could be displayed in three dimensions, with the Z variable giving a depth and form. Any sorts of variable that affects the shape of the curve, weight, altitude, temperature etc could be added, giving a much better idea of how the two dimensions we are looking at here are just a slice of a particular specific condition. Then time could be added and we are venturing from vectors to tensors.....

The main and simple thing to be derived from study of such curves are where and how optimal operating airspeeds are obtained for various regiemes of flight. The second thing to be understood, a bit more complex, is how a change in conditions changes the shape of this curve.

FS generally does an OK job of simulating these curves.

Cheers: T

 

[Brett_Henderson]

An interesting variation on this graph could be displayed in three dimensions, with the Z variable giving a depth and form. Any sorts of variable that affects the shape of the curve, weight, altitude, temperature etc could be added, giving a much better idea of how the two dimensions we are looking at here are just a slice of a particular specific condition. Then time could be added and we are venturing from vectors to tensors.....

The main and simple thing to be derived from study of such curves are where and how optimal operating airspeeds are obtained for various regiemes of flight. The second thing to be understood, a bit more complex, is how a change in conditions changes the shape of this curve.

FS generally does an OK job of simulating these curves.

Cheers: T

Agreed.. and if go way back to where this thread got started.. it was about modeling an ENGINE/PROP performance curve first, regardless of how that rendered V-speed stuff while in-flight.

FSX handles the "3rd dimensions" accurately enough to make chasing after them (and skewing V-speed stuff) a veritable excersize in futility (been there many times). It even does a relatively good job of modeling the differences between fixed-pitch, and constant-speed, props.

We all (me too), got a little turned around while trying to super-impose an actual thrust-curve, onto a graph that only allows for situational, fixed, relative thrust.

Actual thrust can be graphed in many ways... ala; at X newtons, Y airspeed can be achieved... all the way out to an engine/prop's max airspeed (where max airspeed could not be where there is less than max-thrust).. but this has to be at level flight (or a constant RoC, and to stay on 2 dimensions, we'd have to pretend altitude was a constant).

 

[Ivan]

The Y axis is AVAILABLE thrust. It is consumed as airspeed increases, so less is available for climbing..OR the inverse is true too. As you use the thrust for climbing, airspeed goes down. Perfectly logical, and displayed perfectly on the graph.

If the Y axis were available thrust, the shape of the purple graph would be different: At some speed above Zero, the available (excess) thrust would drop to zero, because below this speed, the drag would exceed thrust (behind the power curve). The shape of the curve does not appear that it has this characteristic. Also, at maximum speed, available (or excess) thrust would also be zero because all thrust is used to overcome drag. Again, the purple curve does not show this.

What it does show is an intersection between thrust and drag at maximum speed and a termination before zero speed. It would make sense if the left endpoint were the stall speed of the aircraft.

Also, please note that the label "Maximum Excess Power" is shown as the DIFFERENCE between the purple curve and green curve which is labeled as "Power Required". If the purple curve represented available power as thrust minus drag, then Vy would be at the highest point of the purple curve and it is not.

A graph that would show actual engine power curves would have to have something like manifold-pressure, RPM, or altitude as one of the axes.

As I understand it, the actual engine POWER is not being graphed here. It is the engine thrust that is being generated. The manifold-pressure, RPM, and altitude are constants as far as this graph is concerned. If any of those factors change, you would get a new curve for engine thrust.

- Ivan.

 

[Brett_Henderson]

You're still lost to the idea that this generic graph cannot display thrust the way you're implying..

If your red-curve were applicable (it's like an outside variable as un-applicable as TAS would be on a graph set up for IAS).. then my C206 would have different Vr and Vy for different power settings.

And even if this graph did allow for thrust as you want to represent it, your curve would be an engine so different as to make it a new airframe (as I mentioned earlier in this thread).

We're gonna have to just respectfully agreee, to disagree

 

[Ivan]

Hello Brett_Henderson,

I can see your argument, but just don't agree with you. Respectfully agreeing to disagree sounds like a reasonable outcome.

Thanks.
:salute:

Hello Anthony31 (The Fellow with the ROTAX who started this discussion)

Need a couple details about your installation:
What is the propeller diameter?
What is the expected maximum level speed?
The propeller is a fixed pitch, but is it optimised for climb or level speed?

- Ivan.

 

[Ivan]

Hello Folks,

I spent a couple hours last night unbolting the rotary engine from my Fokker E.III Eindecker and bolting in a Rotax 912 ULS - D.C.D.I engine. Not everything fit all that well. Had to do some minor modifications to the airframe to make the thing fly straight. Also had to add some trim tabs so that the autopilot would work, but here is the result with a constant speed propeller.

From the Rotax site, here is how I read their graph:
RPM Power
5800 98
5500 95
5000 90
4500 80
4000 67
3500 55
3000 45

Here is output from the Rotax mounted in a Fokker Eindecker.
Propeller diameter is 63 inches
Idle speed is 594 RPM
Tested at 500 feet at a speed of around 83 mph TAS:

RPM HP (Actual RPM)
5800 100 (5799)
5500 95 (5518)
5000 87 (4987)
4500 78 (4487)
4000 71 (4018)
3500 61 (3487)
3000 52 (3018)

It isn't an exact match, but isn't all that far off either. I didn't try to tune the propeller tables, but scrounged Table 512 from the stock CFS P-47D Thunderbolt because its power coefficient was closer than the one from the stock CFS P-51D. I also adjusted the pitch limits so that the engine could develop full RPM at a reasonable airspeed.

Anthony31, if you PM me, I can email you the resulting AIR file.

Regards.
- Ivan.