Glide Ratio

[3-2-1-Now 126]

Hi, I raised this previously, and was told to ask later, so I'm asking later. :D What is the glide ratio of the real 737-800, and how close to this is the sim? I have found that all the FS aircraft I have tested have very excessive drag with all engines off. Has this been overcome in the 737, or does it suffer the same issue (understandable - as far as I'm aware this is some fixed value in FS and no separate parameter exists for it)? What is it like at idle power? FS seems easy enough to tune for this. Best regards,Robin.

[NGXfanatic 85]

Great question, and it speaks to the NGX's ability to construct a VNAV descent (on idle power) to the end of descent point. If they were able to figure out a way to get around FSX's despicable ground friction issue, I'm sure they could make some adjustments to the idle glide performance.

[emvaos 13]

Depends on gross weight and speed of course. -800 / -800WL ? Nightmare (you wont see more than an average of 2.6 to 2.8 flight path angle in descend... maybe 3.0-ish at high altitude descending in mach mode)-900 / -900WL Better around 3-ish FYI : All vertical speed model data are within 50 fpm of the flight with unreliable airspeed table. It is very accurate.

[Ian McPhail 55]

An idle power descent = an engine out glide (roughly, I know). It can glide some 90 odd miles from 30000 ft - and the space shuttle is/was a total glider once in the atmosphere on return. Also the story of the Gimli glider, the 767 that ran out of fuel, proved that all planes are gliders, its just the ratio that's significant. As emvaos says above about 2.6 to 3:1. A Cessna 172 if my memory still works, is about 7;1, and a modern glider ...... phenomenal!

[michal 44]

As emvaos says above about 2.6 to 3:1.

No, this may be an angle (degrees) but can't be the glide ratio. A typical modern jetliner has a glide ratio definitely higher than 15, and often 17-22. Only Concorde was really low - around 7.

[opherben 0]

Depends on gross weight and speed of course.

Hello,sorry, glide ratio is essentially not a function of aircraft weight. It is the ratio between the velocities of an aircraft down and forward relative to the air mass. It is determined by the aircraft lift to drag ratio, and varies with factors such as airspeed, configuration and compressibility. According to a rough calculation, the iFly B737NG-X value (at cruise airspeed where L/D is around maximum when engine effriciency is ignored) is around 12.5, which is realistic for transport category aircraft.

[michal 44]

sorry, glide ratio is essentially not a function of aircraft weight.

Agreed. Glide ratio is independent of weight, mainly depends on angle-of-attack. You get best L/D at certain A-o-A. But the best glide speed will of course vary with weight.

the iFly B737NG-X value is around 12.5, which is realistic for transport category aircraft.

It is a bit low. Important: When talking about glide ratio we always mean best-glide-ratio.

[opherben 0]

Michael,average figures used for the NG family:270 KTAS-2200 FPM (ROD) a. Higher TAS would increase the ratio.b. When I mean best glide ratio I say best glide ratio, otherwise I mean glide ratio.

[emvaos 13]

Was referring to idle descend which is basically what is of primary interest under normal circumstances. The second post talked about VNAV. Thought the post was inspired by the tendency of most add ons to descend like bricks that have little to do with modern jets. I then read in forums it "flies like a dream" instead of it flies like !@#%$#@!$% and I despair

[opherben 0]

The aircraft does fly nicely and is pretty close in both perfofmance and handling qualities, so despair doesn't belong here I think.

[3-2-1-Now 126]

Hi, Thanks for the replies! Great to hear it will actually perform something like accurate!

Thought the post was inspired by the tendency of most add ons to descend like bricks that have little to do with modern jets. I then read in forums it "flies like a dream" instead of it flies like !@#%$#@!$% and I despair

Yes it was, and +1 Best regards,Robin.

[znypan 0]

The total Cl/Cd for the B738 is 16. VBG0=216. Glide ratio varies mainly due to tailwind/headwind component. Are you referring to the zero wind glide ratio which should be ~16? Btw, What means realistic for a transport category aircraft? The Cl/Cd for the T7 is 19. Is this realistic for a transport category aircraft? Why not 10 since that's also a realistic number for transport category aircraft. The question is what the B737 glide ratio is. The important issue when it comes to hi fidelity FS sims like the NGX, is not if a value, that the majority of the people reading this thread probably don't understand or don't care about, is "realistic for transport category aircraft". The game is played on the accuracy of the numbers, not rough estimates and generalizations. It's about the accuracy in the VNAV modelling for example... Yes this demon that "real macho- - chuck norris- pilots don't trust/don't use" but probably they fly with the F/D off until cruise and then make a hammerhead maneuver because that's how the real pilots fly the 737. I'm exaggerating but if you look around in many other forums you'll end up with the idea that VNAV, is the system that Boeing installed to delude and kill the pilots.To use one of the AoA's videos example, I want to disengage the right hydraulic and see the yaw dumper going off. Because that's realistic for the 737NG. Not a general transport category aircraft thingy. This shows me that they actually simulated the B737 system and didn't simply put a flip/flop switch and say "we included FLT controls". The examples could go on and on btw. And one last thing regarding the flight model etc. I said it to another forum also. I have 5500hrs on the NG. I haven't flown it since 2003 though. In comparison to a pilot that is flying it daily I feel like the passenger of 34A. Yes, I can say if something feels like an NG but by no means I can compare my self to the line pilots that just came home from a 3 hrs flight on a 738 and have it all fresh when they launch the NGX. Thus during the Alpha and Beta I concentrated on things that work almost the same on the aircraft I fly today, FMC, AFDS etc plus reading tons of pages about an aircraft I thought I knew very well (the NG) in order to feel that I am on the same level with people like RSR, or my friend EV who knows this thing inside out plus he has a PhD in fluid dynamics (that includes aerodynamics btw so the Cl/Cd and other nonsense that we try to discuss here, for him is elementary school physics). This small intro is necessary to say the following. We should start thinking twice before we present ourselves like some kind of an expert on a specific aircraft type. I understand there are people out there who are professional engineers, pilots or whatever else with huge aviation experience. But let's also understand that PMDG doesn't simulate any airline transport plane. It's simulating the NG. So let's avoid general-isms and concentrate on the magnificent NGX. Like a hideous sportcaster in my home country (Mr. Touloubas) says "Alas, let's finally talk about soccer!"

[michal 44]

Pan, great comments, and thanks for providing the value 16, this is what some of us tried to come up with - the zero wind best glide ratio for the 737 (any).BTW, this number 16 means that an aircraft will be descending at an angle of roughly 3.6 deg.

[Jeff_B 8]

There are so many knowledgable people on this product its going to deliver what we all want.

[opherben 0]

a. I was referring to the actual glide ratio exhibited during idle descent without wind, which isn't at optimal glide speed, but rather factors in other cost components like crew pay and hourly aircraft operatig costs, therfore a higher airspeed and my slightly lower value. At the moment I have no official Boeing design parameters and calculated it during iFly B738NG-X flight on my system. For detailed B737 data I use Chris Brady's Tech Guide, and when necessary get it from my son currently flying NGs.The value range you mention for transport category aircraft is realistic, 10-20. The lower the value, the higher the fuel consumption for that type, e.g. Airbus aircraft.b. To respond to your detailed discussion on flight dynamics, first. I graduated the USN Test Pilot School with distinction in 1976 and worked as chief experimental test pilot in a world class air force and later as such in a similar class aircraft manufacturer. Second, this is relevant when I come to recommend a FS add-on to my son and others who fly professionally. Third, I don't need 5500 hours to know an aircraft. I flew as example for my dissertation a very complex USN aircraft for one flight only (it lost a generator which couldn't be fixed in time) following which (and one month of follow-on work) I issued a final report which included all performance and handling qualities charts for the entire flight envelope, which report was graded highest by the staff. As a result, NASA invited me at their initiative to fly the Enterprise Space Shuttle first reentries, and Princeton's prof. Curtiss, a world leader in flight mechanics invited me to become his research assistant.

The total Cl/Cd for the B738 is 16. VBG0=216. Glide ratio varies mainly due to tailwind/headwind component. Are you referring to the zero wind glide ratio which should be ~16? Btw, What means realistic for a transport category aircraft? The Cl/Cd for the T7 is 19. Is this realistic for a transport category aircraft? Why not 10 since that's also a realistic number for transport category aircraft. The question is what the B737 glide ratio is.The important issue when it comes to hi fidelity FS sims like the NGX, is not if a value, that the majority of the people reading this thread probably don't understand or don't care about, is "realistic for transport category aircraft". The game is played on the accuracy of the numbers, not rough estimates and generalizations. It's about the accuracy in the VNAV modelling for example... Yes this demon that "real macho- - chuck norris- pilots don't trust/don't use" but probably they fly with the F/D off until cruise and then make a hammerhead maneuver because that's how the real pilots fly the 737. I'm exaggerating but if you look around in many other forums you'll end up with the idea that VNAV, is the system that Boeing installed to delude and kill the pilots.To use one of the AoA's videos example, I want to disengage the right hydraulic and see the yaw dumper going off. Because that's realistic for the 737NG. Not a general transport category aircraft thingy. This shows me that they actually simulated the B737 system and didn't simply put a flip/flop switch and say "we included FLT controls". The examples could go on and on btw. And one last thing regarding the flight model etc. I said it to another forum also. I have 5500hrs on the NG. I haven't flown it since 2003 though. In comparison to a pilot that is flying it daily I feel like the passenger of 34A. Yes, I can say if something feels like an NG but by no means I can compare my self to the line pilots that just came home from a 3 hrs flight on a 738 and have it all fresh when they launch the NGX. Thus during the Alpha and Beta I concentrated on things that work almost the same on the aircraft I fly today, FMC, AFDS etc plus reading tons of pages about an aircraft I thought I knew very well (the NG) in order to feel that I am on the same level with people like RSR, or my friend EV who knows this thing inside out plus he has a PhD in fluid dynamics (that includes aerodynamics btw so the Cl/Cd and other nonsense that we try to discuss here, for him is elementary school physics). This small intro is necessary to say the following. We should start thinking twice before we present ourselves like some kind of an expert on a specific aircraft type. I understand there are people out there who are professional engineers, pilots or whatever else with huge aviation experience. But let's also understand that PMDG doesn't simulate any airline transport plane. It's simulating the NG. So let's avoid general-isms and concentrate on the magnificent NGX. Like a hideous sportcaster in my home country (Mr. Touloubas) says "Alas, let's finally talk about soccer!"