F-15 FM

[lunaticfringe]

Fancy-pants calcs - I like performance graphs :thumbup:

 

As do I. But while I do this sort of thing these days for pay, I'm the first to admit that I'm self-taught on the subject, unlike those I work with, and that I also am not the fastest on mathematical concepts when dealing with the conversations that come up.

 

So I do the math, since it gets me in the right mindset and keeps me honest.

 

And Darkfire- you're welcome. I'd rather go through the motions, especially since, as Frostie mentions, we've seen some heated conversations regarding other situations like this (see: Su-27 damage, AoA effect, etc) and I've done the same proofing there- that anyone can follow along with, so it's only fair to do it here.

 

When folks can follow along with the numbers, and see that it's within two knots versus the chart relative to the ICAO standard day based on unknown weight, and the aircraft likely had a bit more lift given the average temperature, they can be confident that things are pretty well done.

[OnlyforDCS]

A simplified way of saying what LF has wonderfully described, from a physical point of view is that just looking at airframe/airfoil stall is not enough.

 

There are any number of forces always acting on the airframe at any one time, and lift is just one of them. All of these combined over time give the airplane it's momentum.

 

If the net vector of the forces active on the airframe at any one time is pointing towards the sky, thats where the airframe will want to move at that point in time. Thats why, depending on the airframe's momentum, if you remove lift it doesnt automatically mean the airframe will fall out of the sky.

Edited August 19, 2017 by OnlyforDCS
more information

[Exorcet]

If the net vector of the forces active on the airframe at any one time is pointing towards the sky, thats where the airframe will move at that point in time.

A small clarification, force determines acceleration and not movement. You can move in the opposite direction that a force points just by carrying momentum.

 

Going back to wing rock, it'll probably be pretty hard to figure out exactly how the F-15 would perform in the exact situation in the track. Something that can be done instead is to try to find out the typical amplitude in roll that DCS's flight model will show and see how that compares to either the real F-15 or DCS's manual. The manual seems to state that 45 degree oscillations shouldn't be too hard to get. I don't think I've ever seen them grow to this magnitude.

 

Going through all of LF's linked document on wing rock (a very nice document to provide by the way) and comparing with the experimental data could also provide some general trends that let us see if DCS's predicted wing rock is more or less than the real plane.

 

EDIT

 

Decided to make a quick informal flight (track attached). I didn't really notice any rock until way past 30 units. I think I did hit 45 degrees roll a couple of times, but even past 40 units nothing seem unmanageable. I only tried to maintain somewhat level flight.

1.5.6 F-15 Wing Rock.trk

Edited August 19, 2017 by Exorcet

[OnlyforDCS]

A small clarification, force determines acceleration and not movement. You can move in the opposite direction that a force points just by carrying momentum.

 

 

You are indeed correct. Momentum is a huge part of the equation too.

 

Newtons Third Law: force x time = momentum

 

I have ammended my post to correct this oversight.

Edited August 19, 2017 by OnlyforDCS

[IASGATG]

Some serious standardized math from the back end of the simulation modelling environment I develop for. :)

 

Okay, kinda hoped for an answer like that. :)

[Bulldog_1]

What manual or document is this page from? :book:

 

[GGTharos]

The manual seems to state that 45 degree oscillations shouldn't be too hard to get.

 

Does it?

 

http://www.dtic.mil/dtic/tr/fulltext/u2/a256613.pdf

 

PDF page 157.

 

Decided to make a quick informal flight (track attached). I didn't really notice any rock until way past 30 units. I think I did hit 45 degrees roll a couple of times, but even past 40 units nothing seem unmanageable. I only tried to maintain somewhat level flight.

 

Same resource as above ...

 

40uAoA observed wing rock +/-15 deg under a 1g stall. Keep in mind that the presented scenario does not feature a 1g stall.

 

After the tables are presented, there is a description of the flight techniques used. Note that a lot of data is captured with ROLL CAS OFF.

Edited August 31, 2017 by GGTharos

[lunaticfringe]

July 1991 version of "Gray Matter"- the F-15 RTU declassified study guide, Instructor edition.

 

What manual or document is this page from? :book:

 

[Exorcet]

Whoops,

 

I forgot about this thread.

 

Does it?

 

http://www.dtic.mil/dtic/tr/fulltext/u2/a256613.pdf

 

PDF page 157.

Sorry, I meant the DCS F-15 manual.

 

 

 

 

40uAoA observed wing rock +/-15 deg under a 1g stall.

Alright then, I did a couple more test flights and +/-15 sounds comparable to what I saw. I can post the tracks later when I get chance.

 

After the tables are presented, there is a description of the flight techniques used. Note that a lot of data is captured with ROLL CAS OFF.

In some of the later tests I did disable CAS, but it did not make a huge difference in DCS.

[Exorcet]

Since I said I'd post my track, here it is. Around +/- 15 degrees in roll. CAS off at around 3:00 and back on at around 6:00. The end has some attempts at maneuvering rather than level flight.

1.5.6 F-15 Wing Rock attempt 2.trk