Is Trimming Needed When Landing in A10C?

[bluepilot76]

Guess what, its not a simple subject. There are thousands of posts on forums like this where people, all of which can land aeroplanes, argue about how its done.

 

The way I understand it at the moment PPL flying, most schools these days teach elevator for speed, throttle for rate of descent. Basically this is simple and safe for the small stuff with propellors.

 

Its not what you are then taught for bigger stuff though, then its elevator for rate of descent (ie following the glideslope on the ILS, or moving the TVV onto the start of the runway) and throttle for speed.

 

Two things you need are speed and pitch.

 

Personally I fly the TVV onto the start of the runway with pitch. I hope to be trimmed out for the majority of the approach, I control the speed with throttle. I stop trimming on short final.

Looking forward to being flamed!

Edited June 4, 2011 by bluepilot76
remove smiley mistake

[Fish]

Check this trk out if it works for you. Look at the stick and throttle movement for the last 4 miles.

(ignore touch down, i was descending too guickly so had to flare too much)

 

VFR Landing Clean

VFR Landing Laden

 

Not sure how many pilots will do this way from 5 miles. Its generally very slow cause you are just a little over stall speed for the finals. I generally come in at maybe 160, and slow for last mile or less, and get AOA. Wrong way. But quicker. But I guess if you learn the 'the right way' from the start its better.

 

Updated.

Better track, with a more accurate decent path!

Edited June 5, 2011 by Fish

[bluepilot76]

Hi Fish,

There is something quite interesting in what you posted : "ignore the touchdown, I was descending too quickly so had to flare too much" I have been told many times a simple truth; a good landing is the end result from a good approach.

 

I watched your track, thanks for posting.

 

The fact you were descending too much is because for the majority of the approach you were overshooting - getting relatively high. The perspective of the runway changes throughout your approach. It is easy to see why because the TVV (the crash point you can also think of it) is nearly always hovering beyond the runway.

 

I have attached my track of the same mission. I flew this approach primarily using the TVV to point at the runway threshold. Then all I had to do was operate the throttle to maintain an appropriate speed. its incredibly simple, and is exactly how I would fly an ILS. Only there instead of pointing the TVV at the threshold you are chasing the glideslope needle with pitch, its effectively the same thing. Anyway I hope you will agree that its relatively stable, and once established on the glideslope I really had very little to do. I think I stopped trimming a few seconds before crossing the threshold, as I started the flare.

 

I have enjoyed many of your videos by the way, and you have taught me several things about the aircraft systems. Thanks!

bluepilot76_Landing.trk

[Fish]

Cheers Bluepoint,

Your point is well made, and I should have remembered the significance of the FPM from my Falcon days. I have redone the track, and from about 4 miles out kept the FPM on the threshold. (much easier!!), with green doughnut all the way!!

 

The point of the track, for the poster, is to demonstrate the 'properly trimmed' approach. I.E. from the 4 mile point, there is little or no adjustment of the 'pitch' through the stick (like yours). The FPM is kept correct by increase / decrease of throttle, and the AOA is constant throughout, once trimmed correctly.

 

The track is best viewed with the stick and throttle visible as well as the HUD.

 

VFR Landing

Edited June 5, 2011 by Fish

[Machinist]

You aren't really supposed to use the stick at all (except for roll). You pitch trim for the desired speed, then use the throttles to stay on the glide slope.

That's for good pilots. What about the rest of us? :joystick: :harhar:

[Qazme]

That's for good pilots. What about the rest of us? :joystick: :harhar:

 

Just get over the airfield and *EJECT EJECT EJECT* :pilotfly:

[nemises]

I get to a nice airspeed (say 140) and trim for AoA (as per the AoA lights left of HUD) , the rest is just rolling to compensate for windage and throttle to shift the TVV around

[bluepilot76]

Bruins> Are you getting anywhere with your landing? One thing that occurs to me is if you are flying with dynamic weather and a bit of wind, that did have strong turbulence that can cause effects like you describe. Im not sure if this has been toned down in 1108. Practice with nil weather and see how you get on.

 

Fish, and all you other alpha guys/gals, I tried it with trim to speed and using the throttle for touchdown position, of course that works as well. Some points though, the throttles take longer to respond than a pitch input, so fine adjustment of the glideslope is not as straightforward. Takes a bit of practice to get the anticipation correct. Not impossible, but not easy for the beginners. Anyway I think this is the method the navy guys use where it is essential to have the right amount of energy (ie not too much) to allow the wire to stop you.

 

Fish, in your 2nd landing you do still have quite a hard touchdown, the navy guys apparently had a saying..."Flare to land/squat to p*ss"..Not sure if they still say that now the ladies are flying onto carriers as well. Anyway the F18 etc have very sturdy undercarriage so that a flare is not required, but any footage I have seen of A10 landing it is flared nicely for a gentle touchdown.

 

Anybody know what the optimum touchdown pitch is, to avoid tailstrikes?

 

Using the Alpha method should provide the optimum approach speed for short field landing. You can forget the airspeed then as the AoA will tell you what you need to know (margin above stall), which should be useful in the A10 as the pitot tube seems to come off anytime there is a bit of battledamage.

 

For beginners or those having trouble, I still say the easiest way to learn landing would be fly the TVV onto the start of the runway whilst maintaining a reasoneable approach speed. I usually fly the approach at 140-160, and bleed off on short final to 120-130, round about this time I usually get the green donut. Other advantages of this method; you are not "hawging" the approach for 5 minutes whilst all the F15 guys run out of gas...

[Qazme]

Fish, and all you other alpha guys/gals, I tried it with trim to speed and using the throttle for touchdown position, of course that works as well. Some points though, the throttles take longer to respond than a pitch input, so fine adjustment of the glideslope is not as straightforward.

 

Actually that's the whole point, throttle will give you finer input, that isn't so reactionary to keep you on target easier. If you do it all with the stick you will be all over the place. The throttle adjustments are much more fine grained and easier to estimate where you will end up.

 

Takes a bit of practice to get the anticipation correct. Not impossible, but not easy for the beginners. Anyway I think this is the method the navy guys use where it is essential to have the right amount of energy (ie not too much) to allow the wire to stop you.

 

This is why on page 501 of the manual it tells you to use the (TVV) Total Velocity Vector, in the heads up display to best estimate your touchdown position. Obviously in bad weather the usefulness of that goes down and is when you would rely upon ILS.

 

 

Fish, in your 2nd landing you do still have quite a hard touchdown, the navy guys apparently had a saying..."Flare to land/squat to p*ss"..Not sure if they still say that now the ladies are flying onto carriers as well. Anyway the F18 etc have very sturdy undercarriage so that a flare is not required, but any footage I have seen of A10 landing it is flared nicely for a gentle touchdown.

 

Anybody know what the optimum touchdown pitch is, to avoid tailstrikes?

 

10 degrees is what I use, not sure if you can give it more but if you are setup right on approach with you AoA and speed, speed brakes out 40%, once you chop throttles and touch down you will already be slightly flared and you are generally trying to maintain that attitude until the speed come down enough to drop the nose.

 

Using the Alpha method should provide the optimum approach speed for short field landing. You can forget the airspeed then as the AoA will tell you what you need to know (margin above stall), which should be useful in the A10 as the pitot tube seems to come off anytime there is a bit of battledamage.

 

For beginners or those having trouble, I still say the easiest way to learn landing would be fly the TVV onto the start of the runway whilst maintaining a reasoneable approach speed. I usually fly the approach at 140-160, and bleed off on short final to 120-130, round about this time I usually get the green donut. Other advantages of this method; you are not "hawging" the approach for 5 minutes whilst all the F15 guys run out of gas...

 

Yup pitot like to come off if you get hit lol. I make sure mines there when I come back.

 

As far as learning putting your TVV onto the runway is the right way to do it. From page 501 of the manual under "Landing" the very first thing is mentions in the key points:

 

You can also use the Total Velocity Vector(TVV) symbol on the HUD to best estimate your touchdown position.

 

Remember guys if you want to learn it, read about it first. The try it. Then dig up video's and tutorials lol.

Edited June 5, 2011 by Qazme

[Bruins]

Thanks very much to all of your suggestions. I've learned a bunch and am getting a hang of it now. As many of you've suggested, using trimming for pitch is much easier than the stick. And of course, practice makes perfect.:)

[bluepilot76]

Congratulations, keep at it and they get easier and easier. I expect you will be doing ILS approaches in snowstorms in no time. Have fun!

[effte]

A lot of the problems people are having seem to stem from an incomplete understanding of what trim is, what it does and how it does it. I take the liberty to copy & paste a post I wrote on the subject in another thread.

 

But first, a few notes.

 

You trim for an angle of attack. No ifs, no buts.

 

You then use the stick to adjust the angle of attack of the aircraft away from the trimmed angle of attack. Depending on other parameters, this will result in a climb or a descent, but stick and trim are two parts of an angle of attack selector.

 

Generally, I like to trim a wee bit nose heavy when on the approach, i e for a slightly lower AoA than I want to use on short final. I'll be holding positive elevator control pressure, or back on the stick/yoke, throughout the approach. That way, if it all goes pear-shaped and the neuro-opto-mechanical control actuator (that'd be me) loses focus, relaxing on the yoke/stick will lead to a speed increase and increased stall margin.

 

Stalling out on final is all too easy if distracted, and it's a killer. If you are distracted, the natural reaction is to pull on the yoke. If you are trimmed neutral for hands off flight at the approach speed... well... inadvertently pulling back is bad. Real bad. However, if you speed up and go below the desired glide due to getting distracted, you are at least still flying and can either save the approach or go around and try again.

 

Do not argue the "stick for speed" vs. "stick for rate of descent" issue. It's a moot point. That's for getting the students through early flight training. In addition, the best way to think about it differs between different aircraft. Eventually, you learn to fly and how to fly a specific aircraft, and you realize how it is interconnected and how you need to coordinate stick and throttle in the particular aircraft you are flying.

 

----------------------------------------------------

 

This turned into a bit of a paper. Important points italicized. The main focus is pitch control, but lateral (aileron) control is essentially the same, just in a less changing context.

 

First off, Chops is the one who got it right.

 

Use the flight controls and power settings to put the aircraft into the orientation that you want. For example - level flight at 10,000 ft and 250 kts. Then use trim to relieve any control pressure that you feel.

 

Think of trim as a helper to relieve the pilot of large and/or constant control forces. On many gliders, the trim is simply a bungee which can be made to pull on the stick with a variable force. Early implementations of trim surfaced during WWI. The planes were built for low speed flying and manoeuvring, which meant you had to push the stick forward to stay level at high speed during cruise. Pilots solved this with a bungee cord attached to the panel which they could hook to the stick. Voila! Instant trim!

 

This, however, has its drawbacks. The elevator position, and hence the stick position, effectively determines the angle of attack the aircraft will seek. Google for "static stability" for more background reading - there's plenty.

 

The stick (and the connected elevator) is an angle of attack selector.

 

A constant angle of attack means that for a constant airspeed, lift will remain constant. Another way to put it is that as lift is (more or less) constant due to the non-changing weight of the aircraft, an aircraft will when stick-fixed maintain a constant airspeed. It will do this through adjusting the rate of climb, either at once or through a series of climbs and descents at decreasing maximum climb/sink rates (google "dynamic stability")*.

 

A constant angle of attack means an aircraft which will seek a constant indicated airspeed.

 

This stick-fixed behaviour is desirable.

 

However, the stick force required for a given elevator deflection varies with airspeed. You need a larger force to keep the stick forward in a constant position at a higher airspeed. With the bungee cord solution, the trim force is constant. Hence the stick position will vary with varying airspeed. Not really what you want for trouble-free hands-off flying.

 

Ingenious minds considered this problem and trim evolved. The trim tab was invented. Rather than pushing the stick forward manually to make the elevator go down, or have a bungee attached to pull the stick forward, someone decided to use the same aerodynamic forces which pulled on the bungee cord to replace the bungee cord. If you have a trim tab at the trailing edge of the control, it will apply a force to the control surface as it is deflected. This force will be proportional to the airspeed - just as the force on the main control surface. As the trim tab is deflected, the position the control surface returns to if left to move freely in the airflow is changed. Remember that the elevator position determines the angle of attack. The trim tab changes the position the elevator will return to if you take your hands off the stick.

 

Trim tab deflection changes the stable hands-off angle of attack of the aircraft.

 

Of course, to make the nose go up (increased angle of attack), the elevator has to go up (stick aft) and the tab has to push the elevator up. This means the trim tab has to deflect down in order to make the elevator go up. This is, as we have seen in this thread, counter-intuitive to some people.

 

To raise the nose through elevator up deflection, the elevator trim tab deflects down.

 

Now, as aircraft got bigger and faster, the control forces increased beyond what humans could achieve through control wires and pushrods. One way of countering this was to introduce servo tabs. These are similar to trim tabs, but automatically move as the control surface moves. As the elevator goes up, the servo tab always goes down in order to assist the pilot to move the control surface against the air flow. There have also been light aircraft where anti-servo tabs, operating in the opposite fashion, were used to increase the control forces and achieve better control harmony. Some aircraft have no direct connection between the controls in the cockpit and the control surface, relying on servo tabs to fly the control surface which then flies the aircraft.

 

This too has drawbacks, so eventually powered controls were introduced. Hydraulic actuators move the control surfaces in accordance with commands from the pilot.

 

All fine and dandy, except for the fact that the amount of control surface deflection needed, desired and allowable changes with aircraft speed. This didn't use to be a problem. The pilot's limited muscle force automatically limited control surface deflection at higher airspeeds, meaning x Newtons of control force created a smaller control surface deflection at high speed than at low speed. The physics work out so that the anticipated aircraft response to a given applied control force is reasonably similar across the speed range, albeit with a much smaller control deflection at the higher airspeed**. The controls are loose at low airspeed, and as speed increases they feel as if they are stuck in a concrete block. The aerodynamic forces on the control surfaces stop the pilot from applying large control deflections which would overstress the aircraft.

 

With powered controls, the pilot could move the stick full aft at any airspeed and the elevator would go full up. At high airspeed, this would mean instant disaster due to violently over-controlling the aircraft. The pilot also lost the feel for the aircraft and airspeed normally given through the feel of the forces required to move the controls.

 

This was solved through artificial control forces, or q feel (as the letter q is normally used to designate dynamic pressure, a fancier name for the part of air pressure felt on the deflected control surfaces due to airspeed) was introduced. Actuators are used to increase the forces required to move the controls proportional to the airspeed, just as they would be increased by air pressure acting on the control surface in a non-powered control path.

 

(Another method, commonly seen in FBW aircraft, is to have a spring loaded control, with constant control force for a given deflection, but to limit the deflection of the control surface for a given control deflection depending on the airspeed.)

 

Obviously, as powered controls negate the forces created by the airspeed on the control surfaces, they also negate the forces created by the trim tabs, rendering them useless for trimming.

 

With powered controls, trim has to be introduced by somehow biasing the control system actuators instead.

 

This is what we have in the A-10. Powered controls, with a trim system to change the control surface deflection at which the control force felt by the pilot is zero.

 

However, pilots still need to be able to fly the aircraft if the hydraulics go out. Thus, the trim tabs are still there but are only used for trimming if the hydraulics are lost. In normal operation, they augment the hydraulics which also means they will be in more or less the right position if a transfer to a non-powered mode has to be done.

 

On the A-10, the elevator tabs will act as trim tabs in manual reversion (lost hydraulics) mode, while the aileron tabs will act as servo tabs.

 

No trim tabs on the rudders, as they should normally never be out of center for normal flight. Lose an engine with unpowered rudders and you can expect to have one tired leg upon landing, and try to find a runway without too much of a crosswind.

 

You trim for an indicated airspeed, which the aircraft will then essentially maintain if left to its own devices (hands-off flight).

 

Cheers,

Fred

 

*) Some aircraft are dynamically unstable and will climb/descend ever more violently. That's not a nice behaviour at all, and beyond the scope of this discussion.

 

**) What you are really looking for is essentially a constant G load for a given control force, which makes things nice and predictable. You know what happens when you apply a given force to the stick, regardless of airspeed.

[Fish]

 

Fish, in your 2nd landing you do still have quite a hard touchdown, ..........Anybody know what the optimum touchdown pitch is, to avoid tailstrikes?

 

Yeah felt like it too. Problem is at 3.5deg glide, 130knots descent rate is 800fpm,(its higher with a full load out). and this will give a fair bump. I suspect a safe touch is more like <300f/m (5ft/sec).

 

With the AOA fixed, its necessary to throttle up to reduce the VV, and this pushes touch down further down the runway. Flare increases AOA, with risk of tail damage. What I find myself doing now, is making sure the VVT is just off the end of threshold, and then when i power up a little, the VV goes down, and i get wheels down close to threshold. I suspect this is unsafe, and a combination of a 'little' flare, and power is a safe compromise. I too would like to know the AOA limit for touch down.

[bluepilot76]

Fish, Yes thats the impression I got watching your landing, I was sort of thinking, as you came nicely over the threshold, well whats he going to do now? I think as a carrier landing it was spot on. When I tried the same method I had the same thing. Following the philosophy of using throttle for glideslope is all very well, but it falls down at the flare. Pilots piling on the throttles just before touchdown are usually correcting a mistake, of course the flare is normally some combination of pitch and reduction in throttle. Perhaps you can land with much more alpha in the A10 though. I havent got a good internet connection at work so cant check YT for the typical A10 touchdown attitude.

 

Anyway, good post from Effte too, I do think the discussion is useful for people struggling with landing though, breaking down what is going down to a few variables, not all of which need be adjusted at the same time.

[effte]

As a method of teaching, "power for sink rate, elevator for speed" is not bad. Gets students out of the "point the nose up or down depending on where I need to go.... ooooh, what is the ASI doing now?!" thing I guess. I just don't think it is of any use discussing the relative merits of the two ways of describing what is really the same thing in internet forums. :)

 

As for landing without a flare - do not do it! You need to retard the throttles and pull that nose up in order to land safely! No aircraft, except carrier based ones and the Saab 37*, will withstand being slammed onto the runway at three degrees and approach speed. Lift the nose three degrees or so and reduce power, being very careful not to balloon up to a higher altitude with decreasing airspeed. Easiest is to leave a little power on and ever so gently let the airspeed reduce, but you will increase landing distance by floating down the runway. The manual calls for power being retarded to idle once landing is assured, but I find that this requires careful timing or you risk hitting hard - at least in DCS.

 

Powering up is obviously bad and will easily have you floating forever. Besides, you simply cannot time it. The spool-up time of the engines is upwards of 10 seconds, and the rate of onset of thrust is likely to vary depending on the engines installed and how worn they are.

 

I can't find a good reference for tail clearance at the moment, but the recommended rotation on take-off is 10 degrees pitch up so that's probably a good maximum figure. Shouldn't be a problem - if you adhere to procedures and approach speeds - and if you need more than 10 deg pitch for landing you are doing something wrong anyway. Probably holding off in the flare too long, going for the greaser. Just arrest that sink rate and then plant the mains. She's not fragile and she's not a taildragger.

 

*) Now go dig up the few remaining types :D

[hassata]

I always flare as well, but as far as I know, Viper pilots are taught to land without flaring. Some still 'walk' the VV down the runway though.

 

Re floating, I was told the the F-104 was a challenging plane to land because you have to nail a precise speed to avoid floating or slamming.

[effte]

I always flare as well, but as far as I know, Viper pilots are taught to land without flaring. Some still 'walk' the VV down the runway though.

 

Oh no, that bird has a fragile gear. The normal procedures specifically caution that you must arrest the sink rate prior to landing or you may suffer structural failure. While I don't have hard numbers I trust on the limitations, the requirement for other (non-carrier) aircraft is probably a good indicator. 10 fps when light, 6 fps when at max landing according to a quick google of the FARs/JARs. 360-600 fpm. At 120 knots, you are doing 600 fpm down a three degree glide slope...

[hassata]

Looks like you're right:

 

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