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cmbaviator

Too easy to makes greasers? Questions on the ground effect

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Hi guys.

 

Normally the ground effect has mainly 2 effects:

 

* huge amount of lift created, therefore killing some vertical speed. I'm not used to heavy aircraft, been flying mainly the a320 and 737cl, it looks like this effect takes place around 200 feets agl (for the b747) until touch down.

 

* negative torque. As the aircraft comes closer to the ground, a negative torque is applied on the aircraft, well, after 6-7 landing I haven't been able to feel that pitch decreasing, it only decreases when I retard the throttle between 30-40 fts agl.

For me the first effect is greatly implemented but is the second effect implemented ? Or maybe it has marginal effect on a heavy aircraft like the b747??

 

With the IXEG 737cl on Xplane, you can really feel the need to maintain more back pressure to maintain your attitude when you are < 20 fts

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FSX and X-plane has flawed ground effect. FSX has too much while Xplane has too little. As you stated, ground effect will cause a gradual decrease of sink rate about 200 to 300ft per minute loss. An aircraft enters ground effect at a height of half the wing span. Another effect you mentioned is that ground effect will cause the nose to drop. The aircraft when flown properly will be trimmed at REF+5 on approach. Losing the 5 knots as you cross the threshold along with ground effect and speed lost in the flare requires some back pressure to maintain the flare pitch. The IXEG 737 you mentioned had the ground effect issue of xplane engineered out. On another note, the 747 is known to have a cushion effect due to its wing design. My buddies who flew the E-4B said you could grease it with ease.

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FSX and X-plane has flawed ground effect. FSX has too much while Xplane has too little. As you stated, ground effect will cause a gradual decrease of sink rate about 200 to 300ft per minute loss. An aircraft enters ground effect at a height of half the wing span. Another effect you mentioned is that ground effect will cause the nose to drop. The aircraft when flown properly will be trimmed at REF+5 on approach. Losing the 5 knots as you cross the threshold along with ground effect and speed lost in the flare requires some back pressure to maintain the flare pitch. The IXEG 737 you mentioned had the ground effect issue of xplane engineered out. On another note, the 747 is known to have a cushion effect due to its wing design. My buddies who flew the E-4B said you could grease it with ease.

IXEG has tweaked the xplane ground effect model so it should be pretty accurate.

 

 

you can see that when i flare to the correct attitude (5°), the vertical speed is cut rapidly but then i need to hold some back pressure to try maintainning that attitude which is not the case in the B747 in my opinion, currently the B747 behaves the same but without the need to hold some back pressure (in order to maintain that pitch attitude).

 

So i  want to know form PMDG devs if they have modelled that negative torque or not and if not if they plan to do so.

 

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I have never heard anyone complain that an airplane is too easy to land. Theres a first for everything i guess...

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FSX and X-plane has flawed ground effect. FSX has too much while Xplane has too little.

Ground effect in FSX can be modified in the air file, you can make it zero if you want. The problem in FSX is that ground effect is simplified. It only affects lift. There is no pitching moment effect (the " negative torque" Camille refers to). Nor is there a reduction in induced drag. So unless PMDG has added ground effects outside the air file then the pitch effect can't be present in the 747v3.

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I have never heard anyone complain that an airplane is too easy to land. Theres a first for everything i guess...

I didn't said that it is in really life but the way the ground effect has been modelled with the B747 is very easy to land, at 100-50 fts, put yoke at neutral, the ground effect will reduce the V/S to -550-500 and you juste need to retard slowly the throttle at 30 fts and start pitching up slowly and here comes a greaser. For me itsd much harder to land the ixeg B737Cl than the B747, you can see that the pitch is less stable when entering into ground effect compared to a very stable pitch on the B747.

Ground effect in FSX can be modified in the air file, you can make it zero if you want. The problem in FSX is that ground effect is simplified. It only affects lift. There is no pitching moment effect (the " negative torque" Camille refers to). Nor is there a reduction in induced drag. So unless PMDG has added ground effects outside the air file then the pitch effect can't be present in the 747v3.

 

that's why i just want to here a statement from a PMDG Devs. I flied the NGX a lot before the 737CL wnad it was the same thing, looks like there was no negative torque applied to the aircraft. If it's not implemented, it will be nice to have it ^^

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I didn't said that it is in really life but the way the ground effect has been modelled with the B747 is very easy to land, at 100-50 fts, put yoke at neutral, the ground effect will reduce the V/S to -550-500 and you juste need to retard slowly the throttle at 30 fts and start pitching up slowly and here comes a greaser. For me itsd much harder to land the ixeg B737Cl than the B747, you can see that the pitch is less stable when entering into ground effect compared to a very stable pitch on the B747.

 

that's why i just want to here a statement from a PMDG Devs. I flied the NGX a lot before the 737CL wnad it was the same thing, looks like there was no negative torque applied to the aircraft. If it's not implemented, it will be nice to have it ^^

You're comparing to very different airplanes. you're also comparing tow very different simulators.

Saying one plane is easier to land than another and then saying it's modelled wrong doesn't make much sense.

 

The CRJ 200 is much much easier to land than the CRJ 900. They are two different airplanes just like the 747 and 737 which means they will have different techniques.

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Comparing the B737 with a B744?? Pitch and negative torque?  Man, I want some of what you are smoking.

 

Torque is twisting force..., what negative twisting are you talking about?  If the aircraft is demanding slightly more back pressure to hold a pitch while landing it is likely due to the movement of the center of lift vector as ground effect comes into play...so I guess it is a stretch but you could call a pitch axis moment a torque, I guess.

 

To be honest, I don't measure how many degrees of pitch I am holding in a flare and I usually only end up holding a constant pitch angle when i've messed up the landing.  I flare, i land, no holding flare.

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You're comparing to very different airplanes. you're also comparing tow very different simulators.

Saying one plane is easier to land than another and then saying it's modelled wrong doesn't make much sense.

The CRJ 200 is much much easier to land than the CRJ 900. They are two different airplanes just like the 747 and 737 which means they will have different techniques.

I do know that the ground effect affects the planes the same way, there is in my opinion something wrong with the ground effect of the B747 making it easier to land.

 

I only used the comparaison to show the negative torque of the ground effect which is obviously modeled in the IXEG, I'm updating another video of a greaser with the b747, i only had to make very small input to flare, i almost feel like I had to push forward the yoke to avoid floating too much.

 

Anyway I'm just waiting for dev's answer

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I've got a DVD of Cargolux receiving their first 747-400 as it's landing in Luxembourg. Believe me, you won't see it any softer than that, and it didn't look like that was a fluke. I suspect this airplane is very easy to land for quite a number of reasons.

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Anyway I'm just waiting for dev's answer

 

You realize this is a users forum?  For direct contact with PMDG please visit the PMDG Product Support portal at their web site.

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You realize this is a users forum?  For direct contact with PMDG please visit the PMDG Product Support portal at their web site.

Ah okay, thanks for the info

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negative torque

It's a nose down pitching moment. Calling it "negative torque" confuses things.

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Well a former real pilot of the 744 has made a series of tutorials  on youtube for the Aerowinx 744 precision simulator, and one of those videos is a landing tutorial,  and he indeed talks about the reduction in vertical speed below 200ft RA,  but a pitching down moment due to ground effect is not mentioned nor is it visible in the live presentation of the running simulator, the only down pitching moment is due to thrust reduction.

 

Marcel van Santen

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In ground effect the downwash from the wing is reduced. That increases the angle of incidence, and therefore the upward lift, of the stabiliser, which in turn causes a nose down pitching moment. It's all happening dynamically during flare. Speed, pitch and height are all changing. If you need to apply more elevator to flare as the aircraft gets nearer to touchdown it may not be that obvious to the pilot what contributes to what. However the correlation between thrust change and pitch certainly will be.

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It would seem so :wink:

 

greaser_zpsvucdo86j.png

 

Joking aside, I'd contend that if it's relatively easy to pull off a greaser, then it probably indicates PMDG got a lot of things right, as it shouldn't be difficult to get the thing down in one piece so long as you're on the numbers on your approach. After all, companies such as Boeing don't design aircraft such as the 747 to deliberately be hard to land, otherwise none of us would get on the things!

 

Not forgetting of course that one of the difficulties with creating something like a 747 for a desktop flight simulator lies in the fact that there's only so much you can do to simulate actually sitting in the real thing, so any notion of whether ground effect 'feels right', or anything else that is a bit 'seat of the pants', is going to be somewhat subjective when we've not got the luxury of a full motion platform, realistic views out of the side windows etc. All a company such as PMDG can do, is try to get the numbers right, because when a real 747 touches down and you're sat in that left seat (or the right seat), the reality is that you're the best part of fifty feet off the deck when those main wheels touch, not to mention sat at an angle looking down over that nose, and doing about 130 miles per hour. In that situation, as with landing any aeroplane for real, in addition to referencing the instruments, you're using a certain amount of 'feel' from your numerous senses to ensure you get it right; peripheral vision to help you keep it level, judging the flare off the ground rush, and so on, plus your experience of course.

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when a real 747 touches down and you're sat in that left seat (or the right seat), the reality is that you're the best part of fifty feet off the deck when those main wheels touch, not to mention sat at an angle looking down over that nose, and doing about 130 miles per hour.

 

Indeed -- I was chatting to a friend who is a former 747-400 Training Captain recently and he mentioned how odd many pilot find it when they first take off in the Jumbo because, of course, as the pilot when you pull back you go up rather a long way -- but it is possible to end up with your backside some 70-100ft in the air, with the main wheels still on the deck.

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IXEG has tweaked the xplane ground effect model so it should be pretty accurate.

 

 

you can see that when i flare to the correct attitude (5°), the vertical speed is cut rapidly but then i need to hold some back pressure to try maintainning that attitude which is not the case in the B747 in my opinion, currently the B747 behaves the same but without the need to hold some back pressure (in order to maintain that pitch attitude).

 

So i  want to know form PMDG devs if they have modelled that negative torque or not and if not if they plan to do so.

 

The 747 is NOT a 737.   The huge wing surface means that the aircraft handles vastly differently to the 737 when entering ground effect.   There is a huge cushion of air as it settles onto it-  you cannot compare ground effect of the much smaller 737 wing to that of the 747, that is comparing apples to oranges.

 

Have a look at the voluminous amount of vids on the internet about 747's landing - they all hover in ground effect for quite a bit of time before settling down.   Only in rather nasty whether, bad turbulence or some windshear will you notice a bit of a bump, or unless the pilot really gets it wrong.   I have read many posts about pilots commenting on this very aspect.

 

Regards

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Indeed -- I was chatting to a friend who is a former 747-400 Training Captain recently and he mentioned how odd many pilot find it when they first take off in the Jumbo because, of course, as the pilot when you pull back you go up rather a long way -- but it is possible to end up with your backside some 70-100ft in the air, with the main wheels still on the deck.

 

Of course the reverse of that is when someone used to flying a 747 does it the other way around. There used to be a guy who flew with me at Camphill in the UK (a notoriously tricky glider airfield in Derbyshire with lots of windshear). He was a 747 pilot, for BA I think, and you could always tell if it was his glider coming in for a landing, because he'd be the one coming in at 95 knots who'd commence flaring whilst still 75 feet of the ground. Good job those things don't start giving you some pre-stall warning buffet until you're down to about 40 knots, cos you can't hit TOGA and go around in a glider lol.

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The 747 is NOT a 737.   The huge wing surface means that the aircraft handles vastly differently to the 737 when entering ground effect.   There is a huge cushion of air as it settles onto it-  you cannot compare ground effect of the much smaller 737 wing to that of the 747, that is comparing apples to oranges.

 

Have a look at the voluminous amount of vids on the internet about 747's landing - they all hover in ground effect for quite a bit of time before settling down.   Only in rather nasty whether, bad turbulence or some windshear will you notice a bit of a bump, or unless the pilot really gets it wrong.   I have read many posts about pilots commenting on this very aspect.

 

Regards

I know that a b737 is different compared to a b747 but the ground wffect physic is the same gor any aircraft. Therefore we should feel a pitching down moment on the b747. The video i posted is to only show that the ixeg had the negative putching moment and not te B747 ( maybe ?)

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Therefore we should feel a pitching down moment on the b747.

 

You are ignoring many good observations made by others. I think you are a little too much invested in what you think should happen.

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I know that a b737 is different compared to a b747 but the ground wffect physic is the same gor any aircraft. Therefore we should feel a pitching down moment on the b747. The video i posted is to only show that the ixeg had the negative putching moment and not te B747 ( maybe ?)

 

I don't think I've ever felt a pitching down moment from the 74...in fact if anything I've had to force the nose down once your get in ground affect.  Honestly the 74 is a lot easier than those stubby wing 73s and MD-80s.

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I don't think I've ever felt a pitching down moment from the 74...in fact if anything I've had to force the nose down once your get in ground affect.  Honestly the 74 is a lot easier than those stubby wing 73s and MD-80s.

 

Indeed, I've got quite a few hours in a Level-D 744 sim thanks to come airline connections and I never felt that it wanted to pitch down...  forcing the nose down was always a common occurrence. 

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The 747 is insanely easy to grease on. I flew the 737 for over 8500 hours and recently left the 320 after 3300 hours. The 737 (300 and 500) were fairly predictable. The 320, I could make a number of smooth landings then bam! The 747 landing pretty mechanically, I listen to the radio altimeter call outs. At 50' start pulling the power off, at 30' start flaring, but not a whole lot, at 10' feet, hold the site picture, which requires a tad bit of back pressure. What I noticed in the sim was you knew you touched down when you heard the spear brake moving back. It's has a loudish servo. I've done 8 landing so far in real life. It was just like the sim. 7 greasers and one so-so. The so-so was my fault. I landed flaps 30 instead of the normal 25, it was windy and the runway was SFO 19L. All a bit non standard. I fixated on the aiming spot and didn't change my focus down the runway in the flare.

 

What I'm trying to say is this simulation is pretty realistic. I think real life is a bit easier to grease it on.

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So far all of my landings have been pretty good apart from one where I bounced it (my own fault). I have never flow a 747 for real, but the ground effect does seem to be what I would expect, given how ground effect works.

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