Aircraft speed reduction in level flight.

[jcjimmy]

The ixeg B737 is a slick airplane - in more ways than one.

In level flight at 280 knots pulling the thrust levers back to idle, it seems (to me) that the a/c takes an inordinate amount of time to slow to 190kts. clean.

Further slowing with incremental flaps also seems slow. The speed brakes seem relatively ineffective as well.

I give myself an extra couple of minutes to slow down for the pattern or ILS and descent profile.

[Litjan]

Well, if you say so... Coincidentially this is an airwork that all fledgling airliner students have to go through - there is even a line in the scripts where they need to jot down the distance and time required to slow down. Because it is worlds apart from a propeller airplane or jet-fighter.

Jan

 

[jcjimmy]

Hi Jan,

Thanks. I've been retired now for 20 years. I flew BAC-11, B727, B757, B767 and MD11 and props before them.

I guess it is a feeling - "seat of the pants" - thing. Maybe, my memory is getting a bit foggy.

Ah, those were the good old days.

It is good to see the ixeg B737 in XP-10 in such good detail. Keep up the good work. Flight simulators have come a long way.

 

[Litjan]

Hi Jim,

if you flew the real things (especially 757s should be in the same league) then you are an authority on that issue to me!

I will double check our deceleration times and distances - just make sure (this has bitten me before) that your joystick throttle is calibrated to go all the way back to idle stop. If you are on autopilot and autothrottle then this should be no issue.

Thanks, Jan

 

[g650flyer]

Below are some numbers for descent rates attainable and decel timing for a 737NG BBJ.

 

                                  Clean         speed brakes           speed brakes modded wing

0.78M / 280 knots     2200 fpm       2600 fpm                3100 fpm

250 knots                 1700 fpm       1900 fpm                 2300 fpm

VREF 40 + 70          1100 fpm        1200 fpm                 1400 fpm

Losing airspeed can be difficult and may require a level flight segment. For
planning purposes, it requires approximately 25 seconds and 2 NM to decelerate
from 280 to 250 knots in level flight without speedbrakes. It requires an additional
35 seconds and 3 NM to decelerate to flaps up maneuvering speed at average gross
weights. Using speedbrakes to aid in deceleration reduces these times and
distances by approximately 50%.

So in the BBJ, it takes roughly 1 minute and 5NM to go from 280 to flaps up maneuver speed. Think its a close enough reference or guide of how a 737-300 may perform? its about the same length, but does have a longer wing(117 vs 94).

[Litjan]

On 9.5.2016 at 8:15 PM, jcjimmy said:

Hi Jan,

Thanks. I've been retired now for 20 years. I flew BAC-11, B727, B757, B767 and MD11 and props before them.

I guess it is a feeling - "seat of the pants" - thing. Maybe, my memory is getting a bit foggy.

Ah, those were the good old days.

It is good to see the ixeg B737 in XP-10 in such good detail. Keep up the good work. Flight simulators have come a long way.

 

 

11 hours ago, g650flyer said:

Below are some numbers for descent rates attainable and decel timing for a 737NG BBJ.

 

                                  Clean         speed brakes           speed brakes modded wing

0.78M / 280 knots     2200 fpm       2600 fpm                3100 fpm

250 knots                 1700 fpm       1900 fpm                 2300 fpm

VREF 40 + 70          1100 fpm        1200 fpm                 1400 fpm

Losing airspeed can be difficult and may require a level flight segment. For
planning purposes, it requires approximately 25 seconds and 2 NM to decelerate
from 280 to 250 knots in level flight without speedbrakes. It requires an additional
35 seconds and 3 NM to decelerate to flaps up maneuvering speed at average gross
weights. Using speedbrakes to aid in deceleration reduces these times and
distances by approximately 50%.

So in the BBJ, it takes roughly 1 minute and 5NM to go from 280 to flaps up maneuver speed. Think its a close enough reference or guide of how a 737-300 may perform? its about the same length, but does have a longer wing(117 vs 94).

I just checked on our model: 2000 feet of altitude, 48tons (105.600lbs) 280-250 takes 27s and 2.0NM. 250-210 takes 42s and 2.7NM.

I checked one of my movies from the real aircraft - and it takes 33s to go from 240 to 210. Thats roughly 11s per 10kts (less at higher speeds, more at slower speeds) - so another 10kts for the 250-240 and we are coming out at 43s (vs. the 44 I have in our model).

I am pretty happy with that!

Jan

 

[g650flyer]

5 hours ago, Litjan said:

 

I just checked on our model: 2000 feet of altitude, 48tons (105.600lbs) 280-250 takes 27s and 2.0NM. 250-210 takes 42s and 2.7NM.

I checked one of my movies from the real aircraft - and it takes 33s to go from 240 to 210. Thats roughly 11s per 10kts (less at higher speeds, more at slower speeds) - so another 10kts for the 250-240 and we are coming out at 43s (vs. the 44 I have in our model).

I am pretty happy with that!

Jan

 

There we have it, right on the money

[jcjimmy]

Hi Jan,

Thanks for taking the time and your input.

I checked again today and I am not disputing your numbers. My check is: I go from 280kts to 190kts at 3,000', 99,500 gw. and it takes 1:25 with the A/T engaged and 1:38 with the A/T off.

Like I said, this is a slick airplane. Boeing did a fine job. If it ain't a Boeing.....................  And ixeg likewise. Thanks again.

My next check is an emergency descent.

 

[Alpha Floor]

Very interesting thread! I always thought it was the NG's the ones that were slippery, not the Classics!

Jim: Those airliners you've flown, which ones where most slippery and which ones where less slippery? Is it true that the 727 could pretty much dive nose down and the speed would not come up? haha

On the Classics I've heard that the speedbrakes are not very effective.

[Morten]

11 hours ago, Alpha Floor said:

Very interesting thread! I always thought it was the NG's the ones that were slippery, not the Classics!

Jim: Those airliners you've flown, which ones where most slippery and which ones where less slippery? Is it true that the 727 could pretty much dive nose down and the speed would not come up? haha

On the Classics I've heard that the speedbrakes are not very effective.

Hi guy's,

Generally, we spent and insane amount of time and research into all these areas, and I will be very surprised if you find anything "off" on the flightmodel in terms of lift, drag, thrust that is not "nitpicking decimals".  We have all the real wing data, we have all the engine data (thrust & drag) we even have data on "speedbrake drag" and it's all been finetuned and reverse engineered.

As for what aircraft  are "slippery", what does this really mean? Basically it means that the higher Lift/Drag ratio an aircraft has the more slippery it is.  Typically GA aircraft have L/Dmax of about 10-12, transport aircraft around 15-22 and gliders maybe 30-70!  The 737CL has a L/D about 17 and a NG about 18-19 I think.  So the NG is a bit  more "slippery" (clean config) than the CL.  Also, one of our pilots that flies the CL says that those with retrofit winglets are a bit more slippery than those without.  You need another 2-3 extra nautical miles to slow down.  And why? Exactly - winglets increase L/D ratio 

What really surprised me working on this was how little effect the spreedbrakes have on the CL and only give you about 500 fpm extra decent rate.

[Alpha Floor]

18 minutes ago, Morten said:

Basically it means that the higher Lift/Drag ratio an aircraft has the more slippery it is

Exactly! I think that's the proper definition of "slippery", an aircraft with a high lift-to-drag ratio. Hence, "slippery" and "aerodynamically efficient" are synonyms.

Another fact is that the "slippery-ness" of an aircraft does not depend on its weight, at least directly. The L/D ratio of an aircraft is not a constant number, it depends on the aircraft's flight path vector (alpha and beta angles). So in level flight and for a constant speed, the weight will influence the angle of attack which will influence the L/D ratio. But if you keep the alpha constant with varying weight, the L/D ratio will not change. This is one of the hypothesis used to determine maximum range/endurance of aircraft, one of the control laws is called "constant alpha", and the alpha in question is the one of maximum L/D ratio.

Edited May 15, 2016 by Alpha Floor

[Litjan]

You will also see a reversal of the "winglet" effect at higher speeds. The most benefit is during slower speeds with a higher angle of attack. The induced drag is higher, and the winglets are more effective in reducing it.

At higher airspeeds the parasitic and form drag prevails, and the extra frontal and surface area of the winglets shows.

From a pilots perspective I find the effect most noticeable during slow-speed descents. Especially when you are a tad high anyway, and ATC asks you to reduce speed "minimum clean" to stay behind some slow-poke that is getting paid by the hour .

The extra "2 percent" efficiency gain during typical cruise speeds is something that only "really really great" pilots can "feel". I can´t.

Jan

 

[Morten]

22 minutes ago, Alpha Floor said:

Another fact is that the "slippery-ness" of an aircraft does not depend on its weight, at least directly. The L/D ratio of an aircraft is not a constant number, it depends on the aircraft's flight path vector (alpha and beta angles). So in level flight and for a constant speed, the weight will influence the angle of attack which will influence the L/D ratio. But if you keep the alpha constant with varying weight, the L/D ratio will not change. This is one of the hypothesis used to determine maximum range/endurance of aircraft, one of the control laws is called "constant alpha", and the alpha in question is the one of maximum L/D ratio.

Correct, this is why a heavier aircraft needs to either;

1. Fly lower to maintain optimum AoA (optimum altitude)

2. Fly faster to reduce AoA back to optimum.

The last option usually compromises the sweetspot for engine efficiency, so in reality you go for option #1

[Morten]

55 minutes ago, Litjan said:

The extra "2 percent" efficiency gain during typical cruise speeds is something that only "really really great" pilots can "feel". I can´t.

Right, but an aircraft with winglets will always be *lighter* than one without (identical trip/payload) because it needs to plan a little less trip fuel, so the total gain is aerodynamic + weight reduction gain, so the overall gain with winglets is bigger.  On a 737 the aerodynamic gain is about 2% but total gain is between 3-4% depending on sector type etc.

Edited May 15, 2016 by Morten

[g650flyer]

4 hours ago, Morten said:

Correct, this is why a heavier aircraft needs to either;

1. Fly lower to maintain optimum AoA (optimum altitude)

2. Fly faster to reduce AoA back to optimum.

The last option usually compromises the sweetspot for engine efficiency, so in reality you go for option #1

Now you are talking my language lol. This takes me back to the old days of flying when you really had to know the performance of the aircraft and use your performance manual to squeeze every ounce of efficiency out of the aircraft. When I first started flying, my first jet was the heavy C-141B. The performance manual was thick and had a slew of pages of spec range charts. The flight engineer would work the charts and pass you climb and cruise data. You would climb 2000ft above optimum and stay there until 2000ft below optimum and step climb to stay in the sweet spot. Some times climbing wasn't always the answer. There were times when I have climbed, and decided to go back down due to winds and positive temperature deviations. At times you would work the HF radios and airline common freqs to get an idea of what the winds were at various altitudes. I've been in some fuel situations where the weather brief and wind charts were way off, and you had to make quick decisions. You had to consider climb capability, fuel burned during climbs, winds and maybe slowing to fly the top of the curve(100% max range in the spec range charts). We did fuel calculations once per hour and compared the numbers against the flightplan to see your fuel trend. If negative trends are caught early, you can change things and make a difference. In the KC10, you did the same thing. We had extra benefits with extra fuel tanks. The FWD, CNTR and AFT tanks made it possible to adjust CG inflight. Once at cruise, you move the CG to the aft limit of 29%. The tail heavy CG made the horizontal stab streamline, reducing drag. We called this making fuel because the projected landing fuel on the flightplan would increase by 5,000 pounds or more. In rare cases in certain parts of the world, we would file for a block altitude, set cruise power at optimum and cruise climb.

Now days its all computerized and dispatch. In the Gulfstreams, I have a FMS what if page in the perf pages. I can set cruise speed or altitudes and it will display actual perf against what if perf for fuel cost. The plane uses a mixture of downlink winds/temp and actual winds/temp for the calculation. Most airline guys have flight following dispatch that monitors weather and performance, and will refile updating more efficient routes.

As you stated earlier, winglets also depend on the operation. Winglets and a aux fuel system was added to the C-32s(757). Unfortunately, the small fleet and marginal gains led to the winglets not being cost effective.

For some nostalgia for all, I attached a DC10 spec range chart example. We used to work these charts to death lol. Amazingly, .825 was our standard long range cruise speed.