Theory About Seemingly Excessive Thrust on 757

[adamant365 28]

Hello All, I've noticed quite a few posts regarding what some believe is excessive thrust on the 757. I have noticed something through experimentation. With the P&W version of the aircraft, I can almost always derate the thrust enough so that climbs (especially initial V2+15 climbout) are very smooth and precise. I always attain the 250 knot limit during the <10000 foot phase. I cannot, however, say the same for the RR variant. I know that the RR's are generally more powerful but I agree that it is excessive. I tried US964 (KPHL to EGPF, a RR engined aircraft) a couple of days ago with a T/O weight of 253200 lbs and with CLB 1 and 38 degree assumed temperature. Even with these limitations, I only used up about 6000 feet of rwy 27R at PHL. I could barely manage the initial climb as I shot through V2+15 by about 100 ft. agl. And, I could never attain 250 knots. I was stuck at about 235-240 the entire way up to 10000 in VNAV, as others have reported. Even after 10000, the plane never reached it's ECON climb speed of 308. Always around 290-295. That seems odd to me, seeing as my T/O weight was very near MTOW. Now, my observation is that with the P&W variant, I get derated (CLB 2) EPR's of around 1.30-1.40. With the RR variant, I am seeing EPR's in the order of 1.50-1.60 even with derate. I don't know if this is how the RR's are supposed to function or not though, as I am no expert. I also find it interesting that the RR engines spool up very quickly. The first 10% of thrust that I add on the ground comes on almost instantly. I hve not noticed this with the P&W engines. I just thought I would make some observations and note that I am only having excessive thrust trouble with the RR engined aircraft. I love the P&W birds...very nice. Now, as I said before, I'm no expert on engine mechanicals so this may just be a long ramble on nothing. Thank's for listening for those who actually read this...-adam

[ghiom 2]

AdamAlthough I did not witness any difference between the two engine variants, I can confirm what you are saying:- shot through V2+15 at initial climb - stuck at about 235-240 the entire way up to 10000 in VNAV (at flaps 1, it takes very long for the plan to accelerate to 250, even though the vertical speed is >3500fps)- feel like no real de-rate with the RR variant (maybe)I am glad I am not the only one to report these problems.For me, it seems like the elevators are too much effective, forcing the plane into climbing vertically, rather that targetting the correct speed. I am not an expert as you though. As said in a previous post, I noticed that lowering the elevator effectiveness tends to help on the issue.It leads me to another question to PSS: does the A/P use the evelator/aileron/rudder or their respective trim?Thank you for your help.Adam, can you check if you play with the elevator_trim/elevator effectiveness?Regards,Ghiom

[taguilo 33]

Hi,Maybe you'd want to make this simple test with 757-200 RR versions and find whether it has excesive thrust or not:- Set Brake release weight (take off) 250,000 lbs.- Set Normal CLB (not derated)- Set MCP taget speed 290 IAS (or 0.78 Mach)- Set MCP Altitude at 35,000 ft- Environment 0 wind ISA condition 15

[FlyinIron406 3]

I always thought it was common knowledge the 757 was made with EXCESSIVELY way too much power/thrust. My grandpa had told me you always had to pull throttles way back to stay below 250 and such like that, even in climbs!

[ghiom 2]

To reorient the subject, the pb with this SIMULATOR plane is that it climbs too vertically, without maintaining the selected speed. It is not an overpower issue, it is a A/P issue.I won't mind if the A/C were climbing at 4200fps with 250knots strongly held!!!It does not do that: it struggles to meet the airspeed target while still climbing too vertically (especially during flaps retraction).I hope my english is not too bad for everyone to understand my concern. -:).I any case, Adam was very good in opening the topic and describibg the issue.Thank you and regardsGhiom

[adamant365 28]

Hello Again, Okay, I agree that the amount of power that both versions of this very nice 757 has is pretty much spot on. The flight I based my post on was performed at night (ETD 2045 local) on a day where the atmospheric conditions were very near optimum: low RH, temp ~16C, baro ~29.88 inHg, as in reality a sharp cold front had just passed through the eastern US. In this case, yes the "rocketplane" lived up to it's billing. What my problem is, however, is exactly as Ghiom states. In VNAV the plane should reduce pitch at flap retraction altitude (1000 ft. AGL) and accelerate to 250 up until 10000 feet or until ATC kills the speed restriction. During this time, flap retraction occurs. This isn't happening with the RR version. The PW version does attain 250 KIAS before 10000, but very slowly. I, with Ghiom, have no problem with very high vertical speed. Any pilot would love to have that much assurance for terrain/obstacle clearance. But if I want to go 250, as the plane is designed to do in VNAV up to 10000, than the bird should go 250. The closer to 308 that the plane can fly while climbing, the more efficient the climb. That's why the ECON climb is set for 308 up to mach changeover. All of that said, I have one other observation to make about this 757. This may be and FS issue because I might recall it in at least one other add-on I have. As the plane crosses 10000 feet, the EPR limiter goes up to near max continuous for about a thousand feet before settling back down to where climb thrust should be. I only noticed this when I heard the engines spool up quite a bit when passing 10000. Unrelated, but while we were talking about the powerplant, I figured I'd mention it. Now, Ghiom, if you are still reading and aren't asleep yet, what exact lines in the aircraft.cfg are you asking me to decrease for this little test? As soon as you tell me this, I'll give it a go. Thanks again for listening, everyone...-adam

[ghiom 2]

Hi AdamSorryI am living in malaysia, GMT+7.My theory is that the elevator (or most probabaly elevator_trim) are either:- too enough effective, thus forcing the plane into an uncontrolled vertical climbor- not enough effective, thus unable to counterbalance the power of the engines.I noticed that during these climbs the elevators are in the position to lower down pitch (did you notice that?). So I would tend too think not enough elevator_trim effectiveness. This could also eplain the V2+15 overshoot at take-off.The elevator_trim effectiveness can be changed in aircraft.cfg by +/- 0.2.The problem here is that it is all guess. Would be good PSS gives us some background about A/P.Can you try?1st increase from 1.0 to 1.22nd reduce from 1.2 to 0.8Thank you for your cooperation.Ghiom

[ghiom 2]

AdamDid you try?I tried last night and I tend to think TOO MUCH effectiveness. So try to reduce it slightly.actually, it takes time to accelerate between flaps 1 and flaps up.Maybe V/S is better for this phase of flight.RegardsGhiom

[adamant365 28]

Not yet, but I have been working (at my job) a lot lately. Now I am off for three whole days starting now. I am just about to make a little tweak or two and fire up the sim. Don't worry, I haven't forgotten. :)-adam

[adamant365 28]

Ghiom and/or anyone else who wants info on this, After only doing a couple of tests, adjusting the elevator and/or trim effectiveness this way and that, I've come to two hasty conclusions:1) Adjusting either elevator_trim_effectiveness or elevator_effectiveness just leads to poor capture of selected altitude and poor continuation of climb. These values are best where they are at by default. They in no way seem to effect the acceleration above 1000 AGL in VNAV.2) The problem lies (in my very humble opinion) in the coding of the VNAV logic itself. The FD is never commanding enough decrease in rate of climb to accelerate in the first place. This, I'm afraid, can only be corrected by the gauge designer themself. Since I have no knowledge of how the coding of this particular gauge works (besides the fact that modifying the .gau is technically illegal) I don't think we will be able to come up with any sort of quick fix. The key phrase here is: the FD is never commanding enough of a decrease in rate of climb to accelerate the aircraft properly. I just don't see it happen. The FD bars dip slightly, but most acceleration is coming from decrease in drag due to flap retraction. I think that this is just a case of "add it to the list" and wait for it to be addressed. I think at the current time, PSS are doing what they can to release the load editor/fuel planner and also work on correcting other little hiccups in this truly remarkable (with a few tweaks) add-on. I personnally had a busy week at my "day job" and only have had a total of about three hours to devote to FS. All of PSS are in the same situation. I know that PSS will correct the issues in due time. Alright, enough ramble, I hope this helps someone.-adam

[ghiom 2]

Hi AdamThanks for having reviewed this issue.I accept your conclusion. It is all in the hands of PSS now.PSS/Normal, have you considered (already) looking at this issue?Would it be tto much asking for a list of issues/bugs you are currenlty reviewing?Thank youGhiomPS: what about the link to Ryan Int'l repaint?

[Chris Willis BAW976 0]

OK, the RB211-535E4 757 goes up like a pocket rocket, no doubt about that.I have got some data on it, and will post it a bit later, other than that I have no real issues with it following the speed bug on the VNAV. She keeps to 250 below FL100 well, and accelerates well up to 305 over FL100, follwong both the FD and the required speed.Chris

[opherben 0]

Hello Chris,I see different aircraft behaviour, namely it would reach 250KIAS if leveling off or climbing through 10000 feet. It takes too long to reach climb airspeed and maintains target airspeed minus 5 knots. I saw the same things people had reported on various threads and concur with Adam's assessment below:2) The problem lies (in my very humble opinion) in the coding of the VNAV logic itself. The FD is never commanding enough decrease in rate of climb to accelerate in the first place. p.s. BAW1159, Senior Captain flying various aircraft fleets>I have got some data on it, and will post it a bit later,>other than that I have no real issues with it following the>speed bug on the VNAV. She keeps to 250 below FL100 well, and>accelerates well up to 305 over FL100, follwong both the FD>and the required speed.>>Chris>>

[Chris Willis BAW976 0]

climb data from fpa as promised, I have not chaged anything at allNORMAL CLIMB NCR.CLI 2 TOC TOW TMP NM TME FUEL 000002 209700 000000 000000 000000 000000 000010 209700 000000 000001 000000 000151 000020 209700 000000 000001 000000 000263 000030 209700 000000 000002 000001 000418 000040 209700 000000 000003 000001 000575 000050 209700 000000 000004 000001 000717 000060 209700 000000 000006 000002 000857 000070 209700 000000 000007 000002 000983 000080 209700 000000 000007 000002 001080 000090 209700 000000 000008 000002 001171 000100 209700 000000 000009 000003 001264 000110 209700 000000 000010 000003 001360 000120 209700 000000 000011 000003 001470 000130 209700 000000 000012 000003 001582 000140 209700 000000 000014 000004 001690 000150 209700 000000 000014 000004 001784 000160 209700 000000 000016 000004 001881 000170 209700 000000 000017 000004 001978 000180 209700 000000 000018 000005 002067 000190 209700 000000 000019 000005 002153 000200 209700 000000 000020 000005 002240 000210 209700 000000 000022 000005 002327 000220 209700 000000 000024 000006 002411 000230 209700 000000 000025 000006 002500 000240 209700 000000 000027 000006 002588 000250 209700 000000 000030 000007 002687 000260 209700 000000 000032 000007 002771 000270 209700 000000 000034 000007 002862 000280 209700 000000 000037 000008 002945 000290 209700 000000 000039 000008 003030 000300 209700 000000 000043 000008 003139 000310 209700 000000 000048 000009 003288 000320 209700 000000 000053 000010 003429 000330 209700 000000 000059 000011 003590 000340 209700 000000 000065 000012 003729 000350 209700 000000 000070 000012 003865 000360 209700 000000 000078 000013 004021 000370 209700 000000 000084 000014 004159 000380 209700 000000 000092 000015 004334 000381 209700 000000 000094 000016 004364So some basic analysis, 4 minutes from take off to fl140, with a derate to 60 degrees and climb 2 engaged, distance taken was 14 nm. That equates to an average climb rate of 3500 fpm, which is rather good, at times it was pegged at 4000fpm, (FSpassengers did not command a seatbelt off command until passing through FL245, by which time the climb rate had dropped to below 2000fpm). I did notice that there was no appreciable attitude change at FL100 as the plane accelerated to 305 ish knts, and this is different to the 767 addon.To fl380 with no step climbs pauses, was 94nm, in 16 minutes which is 2375fpm, a saturn rocket or an F15 will do better but not by much. by comparison an other similar addon gives this performance up to fl240, I must admit that I stopped the climb here to gather cruise data for a profile I am playing with but here goes:000000 361100 000000 000000 000000 000000 000010 361100 000000 000001 000000 000316 000020 361100 000000 000003 000001 000839 000030 361100 000000 000003 000002 001385 000040 361100 000000 000002 000003 001854 000050 361100 000000 000002 000004 002213 000060 361100 000000 000004 000004 002450 000070 361100 000000 000005 000005 002652 000080 361100 000000 000008 000005 002879 000090 361100 000000 000010 000006 003083 000100 361100 000000 000012 000006 003306 000110 361100 000000 000019 000007 003902 000120 361100 000000 000024 000008 004221 000130 361100 000000 000028 000009 004446 000140 361100 000000 000032 000010 004680 000150 361100 000000 000036 000010 004924 000160 361100 000000 000042 000011 005243 000170 361100 000000 000047 000012 005486 000180 361100 000000 000052 000013 005740 000190 361100 000000 000057 000013 006002 000200 361100 000000 000062 000014 006252 000210 361100 000000 000068 000015 006498 000220 361100 000000 000074 000016 006765 000230 361100 000000 000078 000016 006964 000240 361100 000000 000084 000017 007185 000240 361100 000000 000086 000017 007239 000241 361100 000000 000086 000017 007239 As you can see the overall climb to fl140 takes 10 minutes so is on average 1000fpm, with a total climb time to fl240 of 17 minutes which equals a rate of 1411fpmThe equivalent for the 757 is fl240 in six minutes which is spot on 4000fpm. also the average pitch on the 757 is greater than the 767 The 757 climbed to FL240 in 27nm which is an average pitch of 8.4 degreesThe 767 climbed to FL240 in 86nm which is an average pitch of 2.63 degrees.oh and the GE's on the 767 are burning about 1.8 times as much at FL240 than the RB211's on the 757.More raw data to follow on climb rates.

[ghiom 2]

ChrisI am happy to see that some of us have no problem with the 757 VNAV climb. You are saying that the 757 is climbing at the required speed (mine is always 2-5 knots below) and the required power (mine is always below the EPR max setting).can you tell me what is your trick?I would be very glad to close this issue.Thank you for your helpRegards,ghiom