View Full Version : Changing rate of climb

October 11th, 2014, 08:35
Hi all,

I am currently working on an FM for an early version of the Spitfire Mk.I for CFS3. This version is equipped with a De Havilland two-speed prop (pitch is adjustable, but this is not a constant speed prop). So far I have been able to get it working just fine and all the RPM and MAP gauge readings are falling pretty close to the flight test data I have in all different conditions of flight.

Everything seemed pretty well tuned, but my top speeds were about 30-40mph too slow and my rates of climb were roughly 1,000fpm too low. So I adjusted the thrust_scalar value in the propeller section of the aircraft.cfg up just enough to cancel the the friction loss on the engine in the .air file (this allowed me to avoid completely recalculating my propeller tables). Top speed now fell right in line with the test data (~367mph at 18,600ft at 3,000rpm and 42inhg MAP). However, my rate of climb was now about 1,000fpm too high at all altitudes (this is while conducting the climb in the same manner as described in the test data).

Is there something I can tweak that will drop my rate of climb down without messing too much with top speed? Or is there a better way than using thrust_scalar to adjust top speed and rate of climb?

As a note, this is being done in CFS3, so it uses an FM that falls somewhere between FS2002/CFS2 and FS2004 capabilities, but uses the same basic format as all the MS sims (aircraft.cfgs and .air files look and work pretty much the same).

Thanks for any help!


Dev One
October 11th, 2014, 23:08
I've never tried it, but first thought is max lift, maybe drop the air file max Cl vs alpha a bit? If you know your AoA in the climb you can then fine tune it better to that angle. From your other flight data are you getting correct stall speed & turning radius? If so then maybe Cl is OK.

October 12th, 2014, 16:39
Stalls are occurring within a couple MPH of published data for both flaps up and flaps down. Haven't found turn radius info yet, not sure how to test it either, but I'm guessing CL is Ok based on the stall testing.

Dev One
October 12th, 2014, 22:53
The only other suggestion then is to reduce engine HP a bit & then up the prop thrust a bit to compensate so you keep your level speeds OK. If it was in FS9 or FSX one could check the HP using AFSD, not sure in CFS
Good luck

October 13th, 2014, 07:01
Getting the proper rate of climb is achieving the correct balanced ratio between thrust and drag. For instance higher than normal thrust with higher than normal drag will give a correct top speed but an abnormally high climb rate. A low thrust and low drag, also giving proper speed will give a low climb rate.

In your case you need to both reduce the thrust and drag a bit. Climb rate is probably the most difficult to accurately measure, one really needs to use an autopilot to achieve a steady state.

Cheers. Tom

October 13th, 2014, 22:32
Thanks for the help guys! Flieger, your comment makes a lot of sense to me, valuable information. And yes, measuring rate of climb accurately has been a challenge, especially trying to hold something as twitchy as a Spitfire in a steady state climb by hand!

I came upon another solution meanwhile. I adjusted the propeller efficiency curves a bit, reasoning that my full coarse pitch (45 degrees) was possibly providing too much thrust at the lower climb speeds (185 mph) but was giving the proper amount to give the correct top speed (355 mph) at the high end of the curve. I made the curve a bit steeper and this brought my rate of climb into line while leaving the top speed in the right place.

But now I wonder if I shouldn't start over and set my propeller thrust scaler back to 1 and try to get rid of some excess drag and see what happens.

Dev One
October 13th, 2014, 22:48
well done I shall try to remember that. Now as an aside on prop curves, for my Miles Falcon Six over at cbfs, I steepened to almost vertical the curves after j=1.01 so that one gets aero braking from the prop on closing the throttle, with shallow curves one seems to get no braking at all.

October 14th, 2014, 11:28
Good idea for working it out in a sim. A prop that has a very fine pitch can really slow you down dramatically. It doesn't show much on the Spit as fine pitch is 32.5 degrees, and the prop reaches is greatest efficiency at around j=2.5, but on the 206 I fly in real life you really feel it!

October 17th, 2014, 17:30
The way I attempt to get a valid rate of climb is through the use of an autopilot, one for a plane such as the spit, in a popup. Set various v/s rates and see if you can maintain an airspeed, or if it accelerates or slows.

Two speed props are a bit of a challenge as it is neither a constant speed or fixed pitch per se. How did you set yours up?

regards. Tom

October 18th, 2014, 12:56
CFS3 has only the most basic of autopilots, so it really isn't an option for me. It all has to be done by hand.

The two-speed prop was pretty interesting to set up as there are only the fixed pitch and constant speed options to choose from, and like you say, this is neither. So I made it a constant speed and then went to Record 0512 in the .air file to make it work like a 2 speed. The pitch range on this prop was a fine setting of 32.5 degrees and a coarse setting of 45 degrees, with 38.75 as the mid point between the two. So I got my values for my coarse and fine pitch settings (I had some great RAE test data to go by so I knew exactly what RPM I wanted at a variety of airspeeds and altitudes at full power.) and I copied those lines for the other intermediate blade settings until they met in the middle. I then changed the values of the two middle blade settings to 38.750000 and 38.750001. This basically eliminated all the intermediate settings so now I had only full coarse and full fine.

I then went to the aircraft.cfg and entered the correct beta min (32.5) and max (45) settings for the prop and set the minimum governing rpm to 0. Since the minimum rpm the engine will make even at idle is around 600, this ensured that when coarse pitch was selected, the blades stayed pinned to the coarse pitch stops and didn't reduce pitch when the target rpm was reached because it was not possible to get the engine that slow. Then I set the max engine rpm to 3700 (max rpm on a constant speed Spitfire is 3000). This ensured that when fine pitch is selected the blade remains pinned to the fine pitch stop as the engine will never reach this rpm in normal flight. It also gives plenty of room for the prop to overspeed the official 3,000 rpm limitation so the pilot has to keep that in mind, just as in real life.

One final feature I added was a way to make the engine fail if the rpm limitation was exceeded by too much. The Spitfire I pilot's notes allow for up to 3,600 rpm in a dive so long as the engine isn't running above 3,000 rpm for more than 30 seconds. So I used Record 0509 in the air file to put a sudden stop to the engine above 3700 rpm by setting a normal value at 3700.0000 and a really high value at 3700.0001.

By the way, I really want to try making a fully variable pitch (manual, not constant speed) prop for an early Bf 109E but haven't get it figured out. The above method doesn't work. Any tips are appreciated!

October 20th, 2014, 11:20
Thanks for the info on your workaround. Haven' done any CFS files! Regards. Tom

October 21st, 2014, 11:29
No trouble. And all mention of the autopilot aside, all of the above ought to work just fine in FS9 and probably FSX.