View Full Version : Mach Tuck

blue six
December 18th, 2011, 11:17
I wonder if any of the old hands here could shed some light on how to go about moderating the "mach tuck" effect in FSX? I'm tweaking a WW2 prop fighter - in a steep dive from 25,000 feet, a very strong nose down pitching moment develops at approx 300 mph IAS / 18,000 feet. It rapidly builds to the point where recovery is impossible, and just past 400 mph IAS the elevator control actually reverses. I'd like to neutralize or at least postpone the onset and severity of this nose down pitching moment, and if possible eliminate the control reversal.

The two .air file variables I've been adjusting in an effort to improve things are Cmo in Primary Aero/Pitch, and record 433, Delta Cmo(M). From what I've seen so far, it seems that inserting negative values in Primary Aero / Cmo gives various degrees of nose up pitching moment and moderates the tuck, and that increases to the Y values in the curve at record 433 work against this moderation. I suspect that successful juggling of the two could eventually deliver a partial solution, but am working in the dark here. In particular, I don't have a good understanding of the horizontal axis at record 433 - presumably the increments here are various Mach values, but I'm not certain of the actual values in use. I'm also concerned that changes to the Cmo value in Primary Aero will have undesireable effects elsewhere in the flight envelope.

Am I on the right track? Advice would be much appreciated. Thanks.

blue six

December 19th, 2011, 12:26
The best program for manipulating the .air files is Aircraft Airfile Manager. It is indeed posible to modulate the pitch moment change with Mach. Generally the Mach variable graphs are set in 0.2 M increments. Perhaps not optimal spacing for most sub-trasonic aircraft, but workable.

Another program: AirEd, is useful for say copying a desireable entry directly from one plane to another. Quite useful for experimentation.

The CM0 in pri aero is not too useful fro this as it merely shifts the overall effective aerodynamic Center of lift/CG of the plane relation. About the same effect as just shifting the CG. Should be set for normal flight.

Several other entry tables also might affect the behavior you seek to modify. Some aircraft do experience a control reversal at high Mach.... Check out all of the table that have Mach in them, including the effect of Horiz Stab vrs mach and elevator effectiveness vrs dynamic pressure. AAM had some useful explanation notes in the upper right hand corner box, along with typical values.

Good Luck! T

blue six
December 19th, 2011, 12:51
Thanks for your assistance, yet again, fliger747. I think AAM v2.2 will be just the ticket - to this point, I've only been using airEd and have been quite happy with it. The problem I encountered with Mach tuck and record 433, however, was that I couldn't figure out how to drive the delta Cmo curve below zero, using airEd. This seems quite easy in AAM, and will offer a much more direct solution than what I proposed in my first post (manipulating Cmo in Primary Aero and record 433 simultaneously, to arrive at a negative Cmo at high Mach). And as you've mentioned, the comments and explanations offered by AAM will be very helpful.

Thanks again, and best wishes,

blue six

P.S. Record 420, Delta Cmde due to Mach, looks to be my best bet to address the control reversal issue.

December 19th, 2011, 20:44
Amazing how some much vaunted aircraft such as the A2A P47 do not Mach tuck. Not so
Ong ago there were discussions about how FS aircraft could not even be made to spin.

Good luck with your efforts!


December 23rd, 2011, 10:05
Hi Blue Six.

I was working on the same thing a while back and I believe I was fairly successful. Not having flown a real Thunderbolt recently, this is all just my opinion: (Note that this was for a CFS Aircraft: My P-47D-27)

I tried to get the nose tuck effect to happen just outside the normal operating range of the aircraft because I did not want to affect the trim in level flight. I experimented a bit to find the initial value and the speed at which it should start happening. I then adjusted the curve using plotted values and a graph using a spreadsheet. Next, I wrote a very simple C program to convert the raw numbers in a text file into a binary value in the format of a AIR File record and used AirEd to replace the existing record in the AIR file. AirEd works to tweak the values, but isn't convenient to use for gross adjustments of the curve.

BTW, This method on one of the other mach records also works to limit the terminal velocity of the aircraft.

- Ivan.

December 28th, 2011, 23:54
If you use Aircraft Airfile Manager you can adjust the curves very easily without resorting to creating your own software. I used this to adjust the mach diving of Milton Shupe's P47J which is still on a back burner. I was dissapointed (shouldn't have been) that A2A didn't model this in their P-47D. Not one of the more difficult sleight of hand manuvers in airfile work.

Cheers: T

January 1st, 2012, 06:17
Hi Flieger747,

I was a C programmer for many years. For me, spending a couple hours to write a program to do exactly what I want is quicker than figuring out what software package does this and then finding the download and then figuring out how to use it. It also helps me understand the exact binary representation in the AIR file. I figure if I can write a program to decode the record to a CSV file, examine it, and then encode it back to a AIR file record without any changes, it was successful. I just wish I could remember where I left the source after I was done. I'll probably write another program when I need something like this again.

BTW, By "Mach Diving", do you mean Terminal Velocity? I had loads of fun with that trying to find the proper drag curves to figure out what the values should be at their extreme. I eventually found a graph which I believe was from a NACA report. In an absolute ceiling test for one of my Thunderbolts, I ran out of fuel fairly high up and decided to see how fast it would go straight down (without power). It broke the speed of sound which was just wrong and that is what started the messing around with the mach tables.

- Ivan.

January 1st, 2012, 07:14
My major beef with the mach drag table is that the steps are too far apart at M 0.2 intervals. Might be OK for the really fast jets but not as useful as it might be for most sub/transonic aircraft. Fairly easy to adjust the table to make a pretty steep Mach Drag "Wall".

AAM is useful as it has examples and explamations for most parameters.

Good Luck! T

January 2nd, 2012, 05:05
You are correct, it is very easy to make a steep "Wall". The trick is first to find out what the max value should be rather than just pick an arbitrary value. That was where the NACA / NASA report came in. The next trick is to make a nice, smooth curve leading to that wall which is where MS Excel came in. Then comes the minor adjustments to shift the curve one way or another to adjust when the effects begin so that they do not affect normal level flight. Again, Excel was used for that. C Program comes in to convert data back and forth, and of course there is flight testing.

For normal flight, the peak of the curve doesn't really affect much, but I believe in getting the curves correct even past Mach 1 so that even though the aircraft can't fly that fast, the curve still looks right. Maybe I got it right, maybe I didn't, but I can say I didn't take any shortcuts, and it is as correct as I know how to make it.

There is actually a fair amount of room to shape and move the curve as you might want it. It won't ever be nice and smooth, but there is room to fit to something "Realistic".

- Ivan.