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Excessive nose up attitude?


agathorn

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Note I'm not blaming the sim here. I'm sure its accurate, but I'm trying to understand it.

 

Also note that the majority of my sim and real world flight experience up to now is either civilian props or the A-10. I have very little flight time in high performance jets.

 

I am finding with my F5 training that I frequently have to put in a buttload of nose down trim for level flight. It seems very excessive to me.

 

Is this "normal" or am I doing something wrong?

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If going from take off trim to level trim then yeah its alot. I find once I'm off the ground I immediately push the stick and start adding loads of nose down trim. Once trimmed for desired airspeed its just a matter of continuously fine tuning it.

 

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Yeah that is pretty much what I meant. After takeoff I need to jam the stick probably 3/4 forward to keep from breaking pattern altitude while I trim it up. Just seems like a lot, but maybe that's normal for something so light and strong :)

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Guys, keep in mind that you trim nose up prior to take off up to 9 units... It´s normal that you have to trim the nose down again when airborne... If you don´t do that it´s pretty clear that you have to push the stick forward...

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Not sure if it’s the same thing, sometimes I’d come off the carrier in the hornet and I’d go into a 70-80 degree uncontrollable climb and no amount of forward stick would help. I’d noticed similar a few times in a few modules - Hornet, spitfire and the Hog, I can’t remember if others. And oddly what I’ve found is that if I cycle the flaps before launch/ take off - I don’t know why but it seems to rectify it. Hasn’t done it in a little while mind you so not sure if it was an update glitch.

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Guys, keep in mind that you trim nose up prior to take off up to 9 units... It´s normal that you have to trim the nose down again when airborne... If you don´t do that it´s pretty clear that you have to push the stick forward...
This. I should have been more clear and that I wasn't disputing the reasons have to trim after take off. I was merely stating that from take off trim to level trim there is a large transition. Keeps the thumb nice and limber. ;)

 

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Not sure if it’s the same thing, sometimes I’d come off the carrier in the hornet and I’d go into a 70-80 degree uncontrollable climb and no amount of forward stick would help. I’d noticed similar a few times in a few modules - Hornet, spitfire and the Hog, I can’t remember if others. And oddly what I’ve found is that if I cycle the flaps before launch/ take off - I don’t know why but it seems to rectify it. Hasn’t done it in a little while mind you so not sure if it was an update glitch.
It sounds like you may have been using something other than half flaps for the bug. If you take off in auto or full flaps you will have some adverse effects.

 

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Guys, keep in mind that you trim nose up prior to take off up to 9 units... It´s normal that you have to trim the nose down again when airborne... If you don´t do that it´s pretty clear that you have to push the stick forward...

 

Actually I haven't been trimming up for take off at all, unless the game is doing it for me due to the way I set the mission up.

 

Maybe I should read the -1 :)

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Actually I haven't been trimming up for take off at all, unless the game is doing it for me due to the way I set the mission up.

 

The mission editor cold starts the F5 at pitch trim setting 0. Starts with the engines running set the pitch trim in a semblance of correct setting for loadout.

 

As others have correctly said, large trim changes in takeoff are generally the norm for jets. Consider that recommended trim settings for takeoff on twin engine jets are generally formulated to assist the pilot in the worst case scenario; meaning that in the ENGINE FAILURE ON TAKEOFF / TAKEOFF CONTINUED situation, the aircraft will try to achieve a safe single engine takeoff speed with a minimum of pilot intervention. Different aircraft/services call this speed a different thing (often V2 for the civilian certified fleet, various names across the various militaries).

 

This just means that prior to takeoff you have the jet configured to try to achieve an airspeed that is much lower than you are going to desire 99.9% of the time, since engine failures are so rare. This being the case, once you are airborne with both engines running it will take a lot of trim to comfortably attain your desired airspeed and accompanying attitude.

 

It is also worth bearing in mind that aircraft are generally designed to have just enough pitch trim authority to maintain trimmed flight at Vmo / Mmo. I have never paid close attention in cruise at the F5's gauge, but I have a strong feeling that when the airspeed is at or near the barber pole, the trim will essentially be at its most nose-low position. All that said, as long as you have enough trim to fly at max certified speed, I don't know if I would say it is taking excessive trim, I would just say that it takes a lot of trim but you have enough.

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The mission editor cold starts the F5 at pitch trim setting 0. Starts with the engines running set the pitch trim in a semblance of correct setting for loadout.

 

I have been using a mission I set up with engines running, so sounds like takeoff trim is automatically dialed in for me, so good to know!

 

As others have correctly said, large trim changes in takeoff are generally the norm for jets. Consider that recommended trim settings for takeoff on twin engine jets are generally formulated to assist the pilot in the worst case scenario; meaning that in the ENGINE FAILURE ON TAKEOFF / TAKEOFF CONTINUED situation, the aircraft will try to achieve a safe single engine takeoff speed with a minimum of pilot intervention. Different aircraft/services call this speed a different thing (often V2 for the civilian certified fleet, various names across the various militaries).

 

Thanks for that explanation. I am familiar with V2 and that makes sense.

 

All that said, as long as you have enough trim to fly at max certified speed, I don't know if I would say it is taking excessive trim, I would just say that it takes a lot of trim but you have enough.

 

Very good point. Words have meaning and I should choose the correct ones.

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Very good point. Words have meaning and I should choose the correct ones.

 

 

Hopefully I didn't come off like I was criticizing your description, I certainly didn't mean to haha! I was merely trying to think of a different way to look at the situation. Good luck with the F5, enjoy.

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Hopefully I didn't come off like I was criticizing your description, I certainly didn't mean to haha! I was merely trying to think of a different way to look at the situation. Good luck with the F5, enjoy.

 

No you were not. You are 100% correct in what you said, and I appreciate you for saying it!

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When starting the mission with the engines running I have to trim a lot nose down before take off, since the automatic trim doesn't correspond at all with my usual loadout (clean + 50% fuel).

I usually trim like IRL so that I need only minor trim corrections for the initial climb.

I definitely don't want to fight an ever increasing pitch attitude after unstick.

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When starting the mission with the engines running I have to trim a lot nose down before take off, since the automatic trim doesn't correspond at all with my usual loadout (clean + 50% fuel)

 

To be fair to the programmers, DCS is almost dead on with the USAF in the situation you pose. Slick F5 with full cannon and 2255 lbs internal fuel (50%) yields a 13300 lb aircraft with a MAC of ~11.9%. A 10-13% MAC requires a takeoff trim setting of 8. DCS will start such an aircraft with the trim set at 7, even more nose down than specified by the USAF but close enough as they say. (This is all straight out of TO 1F-5E-1 but I am not professionally trained in the ways of the USAF so if anyone is arriving at different numbers here please point it out so I can fix it). Of course this is a sim so obviously everyone has their own way, but I think Belsimtek(now ED) is right in the ballpark with their defaults according to what the guys at Northrop and the AF laid out.

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The automatic trim setting results in slightly less than the obstacle clearance speed as a trimmed speed, which is under these conditions approx 160kts.

I don't know where Belsimtek got the info from that this is what should happen and if this is what Northrop designed, but looking at RW F-5 cockpit videos I don't see any significant or prolonged trim input after unstick until reaching climb speed.

That's also much more in line with my RW experience.

The amount of required nose down trim after unstick during acceleration seems to be excessive.

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The automatic trim setting results in slightly less than the obstacle clearance speed as a trimmed speed, which is under these conditions approx 160kts.

 

 

Sounds pretty close to me, to be honest. When I was going through your example I used KLAS (2181 MSL, 10C, 29.92). This yields a takeoff speed of 141 and a min safe single engine takeoff speed of 147. (As you said, just a little below 160). As the -1 indicates (page 3-7), 147 KIAS is your target in the worst case scenario (which was my point in post #9 of this thread) until obstruction clearance is assured. Again, if my calculations are wrong here on the numbers or the interpretation of how USAF does things, please anyone correct me.

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Again, the amount of trim you have to apply after unstick to accelerate from 150kts to only e.g. 250kts seems to be excessive.

If you rotate at 140kts, get the gear up and pitch up to 20-30deg, the F-5 will do a loop on its own if you don't re-trim.

I've never flown an F-5 IRL but none of the jets I've flown exhibited this behavior.

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I've never flown an F-5 IRL but none of the jets I've flown exhibited this behavior.

 

Fair enough. But what jet have you flown in real life that has a 153 KIAS difference between its safe single engine takeoff speed and its normal targeted climb speed? (147 to 300 KIAS in the case we have been discussing)? I mean, say you are flying a Boeing 757 for a large US flagged carrier; to my knowledge, their difference between V2 and their climb speed until clean up is about 15 knots. Big difference from 153 v 15 knots. What kind of RL climb profile as it relates to V2 are we talking about? I'm not trying to be a contrarian or doubt your RL experience, I am just trying to make sure it is an apples to apples comparison since I am quite sure DCS has the trim setting right according to the book - and as always, if I am reading the -1 wrong, please someone correct me.

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1. I mean, say you are flying a Boeing 757 for a large US flagged carrier; to my knowledge, their difference between V2 and their climb speed until clean up is about 15 knots. Big difference from 153 v 15 knots.

2. I am just trying to make sure it is an apples to apples comparison since I am quite sure DCS has the trim setting right according to the book

1. If you take e.g. the 767 (and most other Boeings) you have a minimum clean maneuvering speed of Vref +80kts which means that if e.g. V2 is 165kts, the minimum clean speed is 259kts and there's very little re-trimming required, even when acceleration to 330kts for climb.

It's similar in the Saab 105 and all the other jets I've flown (and the F-5 cockpit videos)

 

2. Again, the point I'm trying to make is that while the numbers might be correct, the trim effect maybe isn't and without a dedicated performance table speed vs stab trim, it's rather difficult to know how it should be.

 

Don't forget that the DCS F-5 isn't perfect, e.g. with the very strange and rather serious tailwind/headwind performance bug.

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1. All might be true but you are not trimmed for min clean maneuvering speed nor climb speed when you start the takeoff roll. If you are saying that every jet you have flown takes very little trim to transition from the start of the takeoff roll to an enroute climb configuration, we apparently have a vastly different understanding of what constitutes a large amount of trim.

 

2. I agree. I haven't seen any of the videos of which you speak, but without knowing the conditions / loadout / ect and getting a good look at the trim gauge before takeoff I'm not sure how great of a source they are either, although I would be interested to watch. That being said, since I have never flown an F5, I try to stick with what I can read in the manuals because when it comes to trying to point out potential issues hard data is all ED will consider. The "it doesn't feel right" argument has never gone anywhere towards a change with ED to my knowledge. As you say, I don't think the table we need to solve this exists or is available haha.

 

All that said, total agreement on the F5 not being perfect and the longstanding headwind/tailwind issue you have documented and raised numerous times. I wish you would get some acknowledgment there (at least the last time I followed it ED hadn't said anything about it, my apologies if I missed something).

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To be fair to the programmers, DCS is almost dead on with the USAF in the situation you pose. Slick F5 with full cannon and 2255 lbs internal fuel (50%) yields a 13300 lb aircraft with a MAC of ~11.9%. A 10-13% MAC requires a takeoff trim setting of 8. DCS will start such an aircraft with the trim set at 7, even more nose down than specified by the USAF but close enough as they say. (This is all straight out of TO 1F-5E-1 but I am not professionally trained in the ways of the USAF so if anyone is arriving at different numbers here please point it out so I can fix it). Of course this is a sim so obviously everyone has their own way, but I think Belsimtek(now ED) is right in the ballpark with their defaults according to what the guys at Northrop and the AF laid out.

 

After running a couple worksheets I think that the a/c weight given in the mission editor is incorrect; on the low side. I'm seeing a few thousand pounds difference with various configurations. But its close, and to be fair I'm not sure of some of the specifics of how the DCS sim relates to reality for doing the sheets for example which type of JP is used, the fuel load (according to the t/o it can be two different loads), as well as which rails and launchers are used. So my margin of error is probably also a few thousand pounds.

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you are not trimmed for min clean maneuvering speed nor climb speed when you start the takeoff roll. If you are saying that every jet you have flown takes very little trim to transition from the start of the takeoff roll to an enroute climb configuration, we apparently have a vastly different understanding of what constitutes a large amount of trim.

Since the F-5 and most other high performance jets are using a low moment wing section (NACA65A004.8 in case of the F-5) I don't know where the large moment changes would come from which would require large pitch/trim changes.

 

If you look at this video you will see that this guy applies just a few short clicks after unstick during the acceleration/climb out. Given the low pitch attitude, the speed increase must be rather high.

https://youtu.be/gBmL1DNgobE?t=28

 

That said. I never complained about the trim in the DCS F-5, it was just an observations that this might be incorrect and it can be easily 'corrected' by just trimming a few units nose down.

 

It's still the only module I regulary fly and really enjoy doing so.

 

If they could fix the new horrible lighting problem in the VC (and the engines) it would be great!

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Looks like he didn't trim for takeoff in the first place. Difficult to say since we cant see the meter, but he pulls the stick a lot during takeoff, and the stick ends up, what looks like, just a few inches forward of it's trimmed position, once he is airborne and climbing.

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Nope. That's extremely unlikely. IF there's a difference in stick position its' because he applies slight aft stick pressure before rotation and after rotation he releases the stick pressure.

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He did put some trim in before takeoff, but not a lot.

 

Just try to have a look at the stick position before and after takeoff. Not a big difference in position. On rotation he pulls the stick almost all the way in his lap.

 

At least thats what it looks like to me.

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