Explanation of jet behaviour under trim at low speed please

[markturner1960]

Hi, I was hoping someone could explain the the behaviour of the hornet at slow speed landing configuration for me, to better understand how to quickly and effectively get the jet under control and on speed. I have been doing some experimenting, but am still not clear exactly what happens and why. It is tricky to consistently get the exact same thing to happen so I can understand what’s going on...... Set this in the context of rolling out onto the downwind leg from the break, I am at approx 160 kits, gear down and have just set full trim and have some forward stick input

 

So when trim is introduced, the jet starts to want to rise or fall dependant on the throttle opening.

I want to know what is going on here and why, with the aim of minimising the vertical oscillations and getting the smoothest transitions to trimmed on speed flight.

 

Can someone explain exactly the relationship between trim, throttle opening and stick position for me please.....

 

Thanks!

[kengou]

In landing config, the flight control system tries to maintain the trimmed angle of attack. When you trim, you are setting the angle of attack for the computer to hold. Thus increasing throttle increases speed, which decreases angle of attack, So the computer compensates by pitching up slightly to maintain the angle of attack you set by trimming. Thus it pitches up and increases altitude. Same when you decrease throttle, it increases angle of attack causing the computer to pitch down to compensate, causing you to reduce altitude.

[Blech]

Hi,

 

[markturner1960]

Hi guys, thanks, perhaps I did not get across exactly what I meant. I entirely get the basics here of what is happening and the principles of on speed and the basic theory of landing.

 

What I was trying to understand was the seemingly more complex relationships between throttle position, flaps and stick at that transition to trimmed on speed flight.

 

For example, if you fly at approx 150-160kts, level and with throttle steady, say around 85%, then only set flaps to full, the jet will slowly go nose down and speed will drop. Position of the throttle is really important in that speed range of between 135 kts and 170 kts. Adding full flaps will have quite differing effects depending on your throttle position, if the throttle is closed, the nose will dive quite steeply and the spool up time means you will have quite large oscillations as you try and trim to bring the bracket level but at the same time add throttle to increase the nose up. There is a lot going on and understanding more how the variables work with or against each other would help enormously.

 

I have just spent an hour exploring what happens with differing throttle, speed, etc and basically to keep vertical oscillations at a minimum, you need to chop the throttle until speed drops to approx 170, then add the full flaps, then pick throttle back up to maintain the nose fairly level, but not too much, then as soon as it’s relatively stable, add trim to bring the bracket down. The trick is to have everything as close as possible, IE, throttle in approx the right -lace, nose level as you can and speed on or near 140....that way, all changes are smaller and easier / quicker to make.

 

That said, I would like to know WHY all that happens as it does, as understanding the principles is better than simply learning where you do stuff by rote.

[Cepheus76]

Hello Mark,

 

Another reason for the pitch excursions may be that by lowering flaps will generate various pitching moments. If you set flaps to half, the wing normally experiences a large increase in lift and a comparatively small increase in drag. Even so the center of pressure moves forward when lowering the flaps, this increase in lift creates a pitch down moment. When Lowering the flaps to full, lift is still somewhat increased, but now the drag increase is much larger. Apparently, these relationships hold true for most wings.

 

Now, in the Hornet, the flaps/ flaperons seem to make up a sizeable portion of the wing, so I guess that when they're lowered fully that the drag increase is tremendous. So, the decrease in speed isn't much of a surprise. Since the flaps seem to be quite low on the airframe, I also suspect that the added drag is effective below the aircrafts center of gravity, thus adding another nose-down moment.

 

However, there's another element to consider, which isn't rooted in aerodynamics, but in the flight model of the DCS F-18: It is said that in the low-speed range, the drag experienced by the Hornet is much, much larger than it is in the real thing, thus requiring the large throttle movements- which in turn can create their own pitching moments again. If the thrustline is, e.g. over the center of gravity, an increase in power will result in a nose-down moment. As far as I can see, the developers are also looking into the flight model, so perhaps we see an improvement soon.

 

In any case, I feel for you, the Hornet's behaviour in the slow flight regime used to (and actually still does) bewilder me as well. At times I have to go to almost full military power, just short of the afterburner, to control the descent rate when turning from downwind to base. This will happen even at low weights and still makes me at times scratching my head.

 

I hope that these explanations help you.

 

Happy virtual landings,

 

Cepheus

[markturner1960]

Excellent Cepheus, that is what I was looking for....it would be good to be able to make some definitive descriptions of what to expect when changing the variables at various points, for example, I have spent so much time battling the jet as I figure out why it is doing what it does and exactly which input had the undesired effect.... When you are in the break, there is so much going on, in such a short time, its very hard to understand exactly what you did and what the result was.

[PoorOldSpike]

I don't know whether flaps play a big part in this thread, I'm still only a relative newcomer to the sim but I think there's an "auto-flap" option where the flaps set themselves according to speed etc, and I still haven't figured out whether to leave them on auto or lower/raise them manually for the best flight experience.

Does the real-life Hornet have auto-flaps?

[Odey]

The Hornet does have auto. The FCS has 2 basic modes. “Up and away” and “powered approach”. Up And away trims pitch automatically to maintain 1G easing the pilots work load under normal flight. The approach mode caters to landing. The Hornet was designed around carrier operations. Landing on carriers is a different process than normal. AOA is the critical factor. AOA dictates the relationship the hook and main landing gear have to the ground/deck. Too shallow it may glide over the wires too steep it may bounce. No matter how fast or slow your approach needs to be depending on fuel and ordnance wight approach mode sets your AOA constant. That’s the whole reason for the flap switch.

Edited May 12, 2020 by Odey

[markturner1960]

The Hornet does have auto. The FCS has 2 basic modes. “Up and away” and “powered approach”. The approach mode caters to landing. The Hornet was designed around carrier operations. Landing on carriers is a different process than normal. AOA is the critical factor. AOA dictates the relationship the hook and main landing gear. Too shallow it may glide over the wires too steep it may bounce. No matter how fast or slow your approach needs to be depending on fuel and ordnance wight approach mode sets your AOA constant. That’s the whole reason for the flap switch.

 

How does that relate to the 3 settings we have in the jet ( Auto / half / full) ?

[tweet]

The normal position of the switch is AUTO. Think of half and full as an override of the FCS automatic control of the flaps. Half is used for takeoff and full is used for landing. I suspect half is also the position for a single engine approach. I haven't checked the manual but that is definitely what I would use. Going from half to full is almost all drag that you really don't want in a performance critical situation like an engine out.

Edited May 19, 2020 by tweet

[Odey]

The normal position of the switch is normal. Think of half and full as an override of the FCS automatic control of the flaps. Half is used for takeoff and full is used for landing. I suspect half is also the position for a single engine approach. I haven't checked the manual but that is definitely what I would use. Going from half to full is almost all drag that you really don't want in a performance critical situation like an engine out.

 

This is correct. Sorry I wasn’t very clear. Auto is up and away(maintains 1G in normal flight 100knots or 600knots the jet flies level). This is also the reason when you climb at a 45 degree attitude or greater you have to push the nose down to keep a specific attitude. Hands off in a steep climb the jet pulls to 1G. Half and full are the approach settings. The 1G logic is replaced by AOA. That’s why speed has a profound affect on pitch and altitude. 8 degrees of AOA at 100 knots and you sink. 8 degrees at 300 knots and you climb like a rocket.