Flap switch and FCS behaviour

[Brun]

The infamous 'nose-up' when slowing to approach speed has been done to death, but I'm still not sure everything's as it should be.

 

From the NATOPS (I-2-55):

Below 250 knots, leading edge flaps are scheduled as a function of AOA. Trailing

edge flaps and aileron droop are scheduled as a function of airspeed to a maximum

of 30° at approach airspeeds

 

That's for the flaps switch set to HALF. It's similar for FULL but maximum extension is 45 deg (and 42 deg aileron drop).

 

Annoyingly it doesn't specify what 'approach airspeeds' are, but you would expect some sort of proportionality as speed drops below 250 knots which is not at all what I experience in DCS.

 

In level flight at just over 250 kts, setting the flaps to HALF or FULL does nothing - as expected. They behave as in AUTO, which results in 5-6 degree extension (presumably this is dependent on AOA and therefore aircraft weight - fuel was low in this case). However, when *slowly* reducing speed there is a dramatic change in configuration when passing below about 240 kts. The flaps extend very suddenly, which causes the pitching up which everyone is familiar with. If you combat the pitch-up using the stick - without touching trim - the required forward deflection slowly reduces with airspeed until the aircraft will maintain level flight again at 165-170 kts.

 

A similar thing happens when accelerating with flaps down. They remain extended and suddenly retract at about 240 kts, which results in the nose dropping.

 

I've also noticed that the rudders are commanded to maximum toe-in (30 deg) a lot of the time when the flaps are selected, and again their movement seems very sudden. I found some mention of it in the NATOPS, but haven't done enough testing to see whether they're behaving as described.

 

 

I don't have the knowledge or experience to post this as a bug, which is why I'm posting it here. It may be the case that the aircraft actually does behave like this, but it seems very strange when it's so straightforward to fly in every other respect.

 

Would be very appreciative of any insight from an actual pilot.

[G B]

 

Would be very appreciative of any insight from an actual pilot.

 

 

What exactly is your question?

 

 

 

I've never had the flap switch in anything other than auto while above 250 knots. When configuring to land straight and level (such as a CV-1 at the ship or a visual straight in), as soon as the jet is below 250 I set gear down flaps full and the aircraft decelerates smoothly without any crazy pitching moments. And again, when taking off, doing a touch and go, or waving off, the gear and flaps are raised immediately after a positive VSI is established and I am not ever near 250 with the flap switch down.

 

 

Also, come ask your questions on our Discord: https://discord.gg/QJRCM6

Edited April 10, 2019 by G B
Forgot something

[darkman222]

I don't know where I heared it, it might be in a Matt Wagner tutorial, where it is told that you have to counter that with trimming the nose down. Like you said, it does feel strange to do so in a straight forward plane like the F18. Also the flight behavior gets very unstable which is strange when you have fly precisesly for an apporach. An interesting thing is, when you take off again after you landed with a trimmed F18, the FLCS seems to reset the trim after take off. I was expecting that I had to trim the F18 back to normal. Would be really interesting how actual pilots think about it.

[G B]

I don't know where I heared it, it might be in a Matt Wagner tutorial, where it is told that you have to counter that with trimming the nose down. Like you said, it does feel strange to do so in a straight forward plane like the F18. Also the flight behavior gets very unstable which is strange when you have fly precisesly for an apporach. An interesting thing is, when you take off again after you landed with a trimmed F18, the FLCS seems to reset the trim after take off. I was expecting that I had to trim the F18 back to normal. Would be really interesting how actual pilots think about it.

 

I was referring to the real Hornet, not DCS.

[Brun]

My question is whether the behaviour of the flaps in DCS - which is described in the NATOPS as "scheduled as a function of airspeed" is the same as the real aircraft.

 

The only reason I mention moving the flap switch from auto above 250 kts is to demonstrate that that's consistent with the behaviour described in the flight manual - i.e. it has no effect. The important thing is what happens between 250 kts and 'approach speed', which seems distinctly odd in that it's very abrupt.

 

I'll try to get a track or video recorded, but it might not be before the weekend.

[Wizard_03]

My question is whether the behaviour of the flaps in DCS - which is described in the NATOPS as "scheduled as a function of airspeed" is the same as the real aircraft.

 

The only reason I mention moving the flap switch from auto above 250 kts is to demonstrate that that's consistent with the behaviour described in the flight manual - i.e. it has no effect. The important thing is what happens between 250 kts and 'approach speed', which seems distinctly odd in that it's very abrupt.

 

I'll try to get a track or video recorded, but it might not be before the weekend.

 

Yes it is the same.

 

It seems abrupt because all the FCS gains change. When you select half/full flaps the FCS switches from flight mode to take/landing mode. Control surfaces now have much more authority, and the computer seeks on speed AOA instead of 1G. But the new gains only take effect once all the requirements are satisfied, (below 250 knots, flap switch in half/full ect.) It is not the flaps that cause the pitch up, it’s the increased authority from the stabs at higher speeds ~200 to 250 knots and the computer seeking on speed instead of 1G. Once your closer to on speed the control will be much less aggressive as computer starts seeing the AOA it wants.

Edited April 10, 2019 by Wizard_03

[Strikeeagle345]

Yes it is the same.

 

It seems abrupt because all the FCS gains change. When you select half/full flaps the FCS switches from flight mode to take/landing mode. Control surfaces now have much more authority, and the computer seeks on speed AOA instead of 1G. But the new gains only take effect once all the requirements are satisfied, (below 250 knots, flap switch in half/full ect.) It is not the flaps that cause the pitch up, it’s the increased authority from the stabs at higher speeds ~200 to 250 knots and the computer seeking on speed instead of 1G. Once your closer to on speed the control will be much less aggressive as computer starts seeing the AOA it wants.

 

It doesn't seek on speed AoA right away. It seeks 4.0 deg of AoA. You have to trim it to 8.1 deg (on speed AoA):thumbup:

[Wizard_03]

It doesn't seek on speed AoA right away. It seeks 4.0 deg of AoA. You have to trim it to 8.1 deg (on speed AoA):thumbup:

 

 

Yes that's right my mistake, that's correct! Thank you, I'm not 100 percent sure but I believe that's to account for different gross weights, but your right the pilot will have to trim to on speed every time.

Edited April 10, 2019 by Wizard_03

[3mta3]

So I’ve flown with guys who have flown the Hornet and they were a little baffled when I asked if the aircraft pitched up as the flaps extended. They said it does not. But they weren’t current and recollections fade with time - but to me, it still does not feel right. I just can’t imagine a FBW system that instantly changes gains with no dampening. That’s the whole pint of FBW, to ease these transitions.

 

Any current Hornet guys who want to chime in and say hell yeah that jet pitches for the moon as the flaps extend?

[Deano87]

So I’ve flown with guys who have flown the Hornet and they were a little baffled when I asked if the aircraft pitched up as the flaps extended. They said it does not. But they weren’t current and recollections fade with time - but to me, it still does not feel right. I just can’t imagine a FBW system that instantly changes gains with no dampening. That’s the whole pint of FBW, to ease these transitions.

 

Any current Hornet guys who want to chime in and say hell yeah that jet pitches for the moon as the flaps extend?

 

Its weird, I've never had an issue with the pitching up when deploying the flaps in the Hornet, Both at the boat and on land, I just push forward on the stick a little till the speed decays to where I can start trimming for speed. It really doesn't bother me, I don't even consciously think about it. It just feels normal to fly the aircraft through the transitionary period like I would with any aircraft IRL. I'm not saying its right, but I guess I've never had an expectation that the FCS is going iron that transition out seamlessly. Its all about being ahead of the aircraft, you don't sit there, select flaps down and then suddenly get surprised by the pitch up. You should be ready for it and attentively flying the aircraft.

[Wizard_03]

Don't touch the flap switch until your closer to 200, and it won't be so violent. Usually you hit the switch when your coming out of the break. The increase in pitch is not really felt since your already yanking on the stick and by the time you roll out on downwind your much closer to on speed and everything has stabilized.

 

I don't notice the pitch up at all, by the time I extend flaps I'm already about on speed and it feels smooth as butter.

 

But if your level at 240 when you select full flaps, your gonna spill your drink all over yourself :D

 

 

*edit* The break is there to slow you down. If your still well above on speed when you come out, either you entered the pattern too fast, or you didn't pull hard enough. Use boards when you enter the break if you feel like your not slowing down enough.

Edited April 11, 2019 by Wizard_03

[G B]

So I’ve flown with guys who have flown the Hornet and they were a little baffled when I asked if the aircraft pitched up as the flaps extended. They said it does not. But they weren’t current and recollections fade with time - but to me, it still does not feel right. I just can’t imagine a FBW system that instantly changes gains with no dampening. That’s the whole pint of FBW, to ease these transitions.

 

Any current Hornet guys who want to chime in and say hell yeah that jet pitches for the moon as the flaps extend?

 

It is a smooth transition in the real Hornet. As I described earlier.

[David OC]

 

*edit* The break is there to slow you down. If your still well above on speed when you come out, either you entered the pattern too fast, or you didn't pull hard enough. Use boards when you enter the break if you feel like your not slowing down enough.

 

Yep,

 

You can also play with the height/ballooning in the break by changing the bank angle.

 

Victory205 talks about this in his Case I Landing Tips found here.

 

QUOTE

"If the nose sags below the horizon, then roll right to underbank as necessary. If climbing, then roll left to overbank, holding a constant back stick position initially. You are managing the lift vector to control altitude." END QUOTE

 

You can see me do this here

and roll out almost on speed, as I was adding some trim while in the bank.

[Svsmokey]

I too find the pitch-up to be exaggerated , even setting flaps initially to half .

[CBStu]

A couple of thoughts. I come from the automotive repair world and have seen computer controlled engine fuel injection and emissions control parts evolve from on/off function to finely graduated movement simply because computers have advanced tremendously. Could there be some of this in an FA-18? I have no idea. Flying the DCS FA-18, I have done a little from a chase view to watch flaps action. My controller setup has a down and an up button for flaps. From up, 1 push on down gets me half flaps and a second push gets me full flaps as indicated by the lights on the panel. The reverse from full to up. In chase view I can see the flaps, after takeoff, retract w/ no button input from me. So there is some auto mode stuff happening but I don't count on it. I take off at 1/2 flaps and hit gear up and flaps up buttons nearly at the same time. So nothing unusual there. Slowing for an approach, I hit 1/2 flaps at about 200, and full flaps at 170-180. At these speeds there is nearly no ballooning on flap extension. The plane just slows really rapidly. Those speeds work fine on straight in approaches either to land or boat. Once is a while I will try a regular pattern carrier landing and, per recommendations from helpful guys here, I deploy flaps AFTER I roll into a bank for the 180 degree upwind to downwind turn. That greatly reduces the balloon effect

[Wizard_03]

The switch does not control the flaps, the FCS does.

 

It can and will operate the flaps independently of the switch position. The switch just tells the FCS which mode to be in, and allows for full/half range flap deployment. Auto for normal flight, Half for take off, and Full for Landing. It's just labeled that way to be familiar to pilots. You need to first wrap your head around the fact that you only indirectly control the hornet. The computer does all the leg work so you can just sit back and make decisions.

 

let me illustrate: Pulling back on the stick in the hornet does not mean the stabs go up, pulling back on the stick means I want to pitch up. The computer then determines how much you really want to pitch up based on the force applied. It then decides which control surface(s) to move based on the current situation it sees with its data.

 

Likewise the Flaps are not directly controlled by that switch, it merely tells the computer what you would like to do.

 

 

Also on landings you go right to full, you don't stop at half first. Half is for take off. People are getting into trouble with pitch because they are not following procedures correctly/staying ahead of the aircraft.

Edited April 11, 2019 by Wizard_03

[G B]

The switch does not control the flaps, the FCS does.

 

It can and will operate the flaps independently of the switch position. The switch just tells the FCS which mode to be in, and allows for full/half range flap deployment. Auto for normal flight, Half for take off, and Full for Landing. It's just labeled that way to be familiar to pilots. You need to first wrap your head around the fact that you only indirectly control the hornet. The computer does all the leg work so you can just sit back and make decisions.

 

let me illustrate: Pulling back on the stick in the hornet does not mean the stabs go up, pulling back on the stick means I want to pitch up. The computer then determines how much you really want to pitch up based on the force applied. It then decides which control surface(s) to move based on the current situation it sees with its data.

 

Likewise the Flaps are not directly controlled by that switch, it merely tells the computer what you would like to do.

 

 

Also on landings you go right to full, you don't stop at half first. Half is for take off. People are getting into trouble with pitch because they are not following procedures correctly/staying ahead of the aircraft.

 

Unless you are landing at the ship in high winds ;). Then half flap landings and 33k max trap is the name of the game!

[Wizard_03]

Unless you are landing at the ship in high winds ;). Then half flap landings and 33k max trap is the name of the game!

 

 

Yes that's right above 40kts you can use half for landing.

 

And try not to settle in close. :)

[Svend_Dellepude]

AFAIK that part of the FM is gonna be revisited once an engineer is available. According to an F-18 pilot, the AoA is a little too sticky and the 'pitch up' shouldnt be so pronounced.

[Sergeant_Hamlet]

Annoyingly it doesn't specify what 'approach airspeeds' are, but you would expect some sort of proportionality as speed drops below 250 knots which is not at all what I experience in DCS.

 

Approach speed is not specified because it changes depending on your weight (loadout, fuel state, etc). "Approach speed" is whatever speed you need to travel in order to maintain optimum AOA, as indicated by the indexer lights. That's why the lights are labeled as Fast, Slow or On Speed.

Edited April 13, 2019 by Sergeant_Hamlet