Learjet 35A N1/N2 Problem? Or not...

[cwburnett 1,809]

First, I love this airplane.  Recently bought it and loving the fact that FSW has been updating and improving this plane for years.  That's what sold me on it.  I have been flying it and the MU-2, which I've owned for years, almost exclusively for the last month.

I am trying to make sense of the engine performance in 4.2g vs the Flight Manual.  I am referencing a Learjet 35A/36A AFM with an approval date of 4-30-76 and a 2-23-81 reissue date.  It is possible it has different engines than what is modeled here?

I am using the Takeoff Power Setting chart (anti-ice off, standard nozzle) as well as the Maximum Continuous Thrust For Climb (N1) table.

N1 is the Fan Speed.  That is the third engine indicator from the top.  ITT is the middle gauge and N2 is the top gauge.  I've found that things perform pretty much as expected if I reverse the numbers - in other words, if I use the AFM N1 figures and apply them to N2.  It has led me to wonder if these have somehow been reversed, or if I am just being too picky trying to fly by the numbers and should just shut up, firewall the throttles and enjoy that I don't have to pay for engine maintenance on the sim. ;-)

I realize that the flight model was built/designed/tested/etc by a real 35A pilot, so the discrepancy doesn't make sense, but I can't reconcile it.  And it is always possible that I am not reading things right in the AFM, but as a real world commercial multi instrument pilot, I like to think I can parse an AFM.  On the other hand, maybe I have my hands on a version of the AFM that is either wrong, or is for a significantly different engine than was modeled.

I did a few tests to make sure I wasn't crazy:

KSBA: It is 61F and I'm at sea level.  96.4% N1 is called for by the AFM.  
When I advance the throttles to achieve 96.4% N1, my N2 is redlined at 102.5 and my ITT is 855.  I guess that's okay, I'm allowed to be up to 103% N1 or N2 for up to 1 minute, so as long as I reduce below 100% N2 within a minute of starting my takeoff roll, that's okay I guess.  ITT is 853, also very close to the 860C 5 minute limit, but under the limit, so okay.

After takeoff, I 'reduce' my power to MCT for climb, which at 20C RAT is 96.1% N1.  This yields a N2 of 101.3 and an ITT of 853.  These are both above the MCT allowable figures for N2 and ITT, so I reduce power to 832C ITT, which yeilds 99.2% N2 and 93.2% N1.  It seems unlikely that book N1 would give overspeed N2 and over-temp ITT, but I've seen crazier things in airplanes.

So, I went to KPWK.  Here it is 30F and 650' ASL.  Here takeoff N1 should be 93.6%
In this situation 93.6% N1 can't be reached at full throttle.  N2 is 99.8, N1 is 92.9 and ITT is 831.

So, I went to KBOI.  Here it is 32F and we are at 2,800' ASL.  Takeoff N1 should be 95.6%.
In this situation, N1 95.6% can't be reached, we max out at 93% and N2 is at 100.1%, with ITT at 831.

So, I went to KDEN.  Here it is 28F and 5,400' ASL.  Takeoff N1 should be 97.8%.
In this situation, again, N1 of 97.8% can't be reached.  We get 92.8% as the max, with N2 at 99.7% and ITT at 830.

I carried this one through climb just to see if I could ever attain target N1 for Climb:
10,000'@250kts 0C RAT N1 target 97.8% - at full throttle, I am at 95% N1, 99.2% N2, 846C ITT, climbing in IAS mode at 7,500 fpm
15,000'@250kts -5C RAT N1 target 97.8% - at full throttle, I am at 95.1% N1, 98.9% N2, 846C ITT, climbing in IAS mode at 9,000 fpm
20,000'@250kts -12C RAT N1 target 97.9% - at full throttle, I am at 93.6% N1, 97.4% N2, 836C ITT, climbing in IAS mode at 6,500 fpm
25,000'@250kts -20C RAT N1 target 98.2% - at full throttle, I am at 92.1% N1, 95.5% N2, 826C ITT, climbing in IAS mode at 4,500 fpm
30,000'@240kts -35C RAT N1 target 99.1% - at full throttle, I am at 90.4% N1, 93.4% N2, 814C ITT, climbing in M mode at 4,000 fpm

My impression of flying the book N1 targets on the Fan Speed (N1) indicator is that I get much better than book performance for takeoff and climb.  If I fly with the N1 targets on the Turbine Speed (N2) indicator, I get the performance I'd expect, which is still impressive when light, and middling when approaching MTOW - again, what I'd expect.

Interestingly, once we get into the flight levels, we can't apply the book target to either N1 or N2 in the sim.  And in every situation above, except for Sea Level operations, everything is done with firewalled throttles, which I find surprising, as that typically isn't the case.

So, again, my question is - is there a simple mistake here, or am I crazy and the AFM N1 targets simply can't be reached except at sea level?

For reference, according to the AFM, limits are:
N1/N2 100-103% for 1 minute
ITT 860 for 5 mins, then 832 for MCT

To be clear, I have been flying for a month using N1 AFM values for N2, and keeping under the limits in the flight levels and have been perfectly happy - and I can keep doing that.  Just thought I'd raise it and see what others/FSW thinks about it.

Thanks!

[capceo 117]

First of all, I totally and completely applaud your study centered approach to flying this airplane. It’s commendable, and quite frankly, impressive. 
 

Having said that, here are a few of my thoughts. 
 

You’re doing the best you can with the performance charts in the AFM, but unless you’ve got the right charts for the optional equipment installed on the FSW LR35, the numbers won’t make sense. For example, the MCT chart is changed by the installation of the -2C engines, which will be reflected not by modifying the existing chart in the AFM, but instead by adding a supplement to the AFM. Same goes for just about every power related chart in the book. The “standard nozzle” chart, is for airplanes that DO NOT have thrust reversers. If you’ve got Dee Howard TR4000’s, like the FSW, all of your power setting charts are going to be in the thrust reverser supplement, not in the AFM. Totally different set of charts if you’ve got Aeronca TR’s. The nozzle parameters change when TR’s are installed, and so do the charts. 
 

I also noticed at one point you said that your MCT was calculated to be 96.1%. Either the chart is totally off, or you’re referring to MCT at very high altitudes. Down low, below 10,000ft, the highest MCT I’ve ever gotten off the chart is 92.x%. 
 

Finally, I’ll say this. We spent a ton of time tweaking these engine settings in the last update. Would I say they’re totally reflective of the real airplane? No. Are they worlds closer than they were prior to the update? God yes. 
 

Unfortunately, it’s going to be extremely difficult to get them behaving exactly as the real airplane does. I’d say don’t crunch the numbers quite so hard, and enjoy the airplane. Heck, I fly the real one 600hrs per year, and I’m still excited every time I fly the FSW version at home. And considering how many hours of blood, sweat, and tears I’ve poured into this thing since Mark and I first started working on it years ago, I think thats saying something!

 

Blue skies my friend! And again, love your desire to be study level with this stuff. If you’re ever in the Teterboro, NJ area, let me know. I’d be happy to show you around the real thing. 

[cwburnett 1,809]

Excellent. Thanks for the speedy reply.  Again, so happy with this purchase.  I don't know where this plane has been all my life, lol.  It is light years more fun to fly than the modern tubeliners.

I will take your advice and not sweat it.  I'm just going to fly like I was before and probably try to heed the ITT limits, just to keep me on my toes.

And, yes, this table is showing MCT N1 limit all the way up at 97.1 with 10C RAT at 5k MSL.  Seemed like a dang high N1 at that altitude.  96.1 is 20C at the same altitude.

I used to fly into NYC for work every other week, would have loved to stop over at TEB to check out the 35!  If I'm ever back that way, I'll definitely take you up on the offer.  Thanks!

[capceo 117]

Just now, cwburnett said:

Excellent. Thanks for the speedy reply.  Again, so happy with this purchase.  I don't know where this plane has been all my life, lol.  It is light years more fun to fly than the modern tubeliners.

I will take your advice and not sweat it.  I'm just going to fly like I was before and probably try to heed the ITT limits, just to keep me on my toes.

And, yes, this table is showing MCT N1 limit all the way up at 97.1 with 10C RAT at 5k MSL.  Seemed like a dang high N1 at that altitude.  96.1 is 20C at the same altitude.

I used to fly into NYC for work every other week, would have loved to stop over at TEB to check out the 35!  If I'm ever back that way, I'll definitely take you up on the offer.  Thanks!

Interesting. Next time I fly the 35, I'll take a pic of our MCT chart with the -2C engines and post it here. I worked today, but flew the LR55, and our LR35 is currently down for an inspection.

If it makes you feel any better, our 35, which is the one we modeled the FSW off of, has N1 DEEC's, so every engine parameter is protected. When we apply takeoff power, we just push the power levers to the stops, and if N1, N2 or ITT limits are reached, the DEEC's will prevent them from being exceeded. This is also true at altitude. If you really need to, you can, at any time, firewall the thrust levers, and you'll never exceed a tolerance. (Provided the DEEC's are working correctly, and the fuel computers are ON and working normally.)

I'm serious. If you're ever in the area, reach out. Time with the jets and lunch are on me.

[ark 1,295]

cwburnett,

FYI, here's the the max continuous thrust chart for the -2C engines with Dee Howard TR4000’s. Joe asked that I post this for him.

Al

[cwburnett 1,809]

Sweet, thanks to you both.  I'll give this a go this weekend.  If the -2C TR4000 anti-ice off takeoff chart happens to find itself in front of a camera at some point, that'd be swell too - just to satisfy my curiosity! 

Thanks again, again!

[CaptKornDog 286]

On 12/19/2019 at 7:22 AM, cwburnett said:

Sweet, thanks to you both.  I'll give this a go this weekend.  If the -2C TR4000 anti-ice off takeoff chart happens to find itself in front of a camera at some point, that'd be swell too - just to satisfy my curiosity! 

Thanks again, again!

What were your results?

 

I seem to be seeing something similar, setting N1 per the book has my rocketing to no other with some very, very high temps and N2.  But if I set the N2 value to match, it seems much more under control.

[cwburnett 1,809]

It is better, but still calls for N1 that is higher than possible sometimes. What I've been doing is for takeoff, limiting ITT to 850 or N2 to 100.  Then abiding by the key limitations of not exceeding 832C for more than 3 minutes (and not exceeding 855 ever), so basically my procedure is:

Short Field Takeoff: N2 <= 100 and ITT <= 850

Long Field Takeoff: N2<=99 and ITT <= 832

Climb: at 1500' MCT defined by N2<= 100 and ITT = 832. I just maintain ITT 832, which basically involves slowly moving the thrust up as I climb until usually about 30-35k it is wide open and under 832c.

The model is OAT dependent so on a hotter day, you will have to live with lower thrust settings through the climb. It is also speed dependent from my observations. So if you climb at a faster rate/slower airspeed you can be temp limited higher up. 

I generally climb at 250/300/.73-ish which seems to work well for temps and also satisfies my desire to cover more ground faster at the expense of a little extra fuel. That is faster over the ground and a slightly slower climb rate than the book recommendation.

It isn't totally "by the book" but sticking to the 832C satisfies my desire to have some sort of limit to obey, keeps me busy during the climb and generally makes me a happy operator.

Edited January 9, 2020 by cwburnett