Why are Boeing 737 engine intakes flattened along the bottom edge?

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Winston

Lorenzo von Matterhorn
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Yes, I've actually wondered about this. Now, I know.

[video=youtube;iKtvAWxjF4k]https://www.youtube.com/watch?v=iKtvAWxjF4k[/video]
 
Because they all landed hard and hit the engine cowling on the runway. They just left the dent, as it didn't interfere with continued operation.

Just kidding. It is for ground clearance. With two blown tires and a deflated strut, they don't want the engine hitting the ground
 
The video gets it mostly correct.

The flattened nacelle with the flat-bottomed inlet lip was done on the 737-300/400/500 (AKA "Classic" now). The nose gear was actually lengthened but the main gear was unchanged save for beefing up (the Classics being heavier than the original -100/-200 family and most versions longer).

With the Next Generation 737s (-600, -700, -700C, -800, -900/-900ER, P-8) the nacelle was redone so that the inlet lip is round though the nacelle itself is was still flattened on the bottom for the same reasons - to make room for the IDG (integrated drive generator - the electrical generator), and the EDP (the engine driven hydraulic pump) while maintaining ground clearance. The fuel pumps are not on the engines as the video stated - they are in the wings. The gear lengths were not changed going from Classic to Next Generation but the main gear was actually all new as the wing itself was new and larger.

On the MAX the nose gear was lengthened again (which made repackaging a bunch of stuff that is in the lower nose of the airplane in order to make room "fun") and the even larger diameter engine was pushed even more forward and up to allow enough ground clearance and they were able to get the engine-driven accessories on in such a way that the nacelle did go back to being nice and round as was shown in the inset picture of one of the flight test MAX airplanes.
 
You are right! Even as I was typing that something felt funny. The -600 is the Next Gen version of the -500 "classic".

Have fixed my post.
 
The Classics are flat too. That's for sweeping the runways!!!:cool:
 
The video gets it mostly correct.

The flattened nacelle with the flat-bottomed inlet lip was done on the 737-300/400/500 (AKA "Classic" now). The nose gear was actually lengthened but the main gear was unchanged save for beefing up (the Classics being heavier than the original -100/-200 family and most versions longer).

With the Next Generation 737s (-600, -700, -700C, -800, -900/-900ER, P-8) the nacelle was redone so that the inlet lip is round though the nacelle itself is was still flattened on the bottom for the same reasons - to make room for the IDG (integrated drive generator - the electrical generator), and the EDP (the engine driven hydraulic pump) while maintaining ground clearance. The fuel pumps are not on the engines as the video stated - they are in the wings. The gear lengths were not changed going from Classic to Next Generation but the main gear was actually all new as the wing itself was new and larger.

On the MAX the nose gear was lengthened again (which made repackaging a bunch of stuff that is in the lower nose of the airplane in order to make room "fun") and the even larger diameter engine was pushed even more forward and up to allow enough ground clearance and they were able to get the engine-driven accessories on in such a way that the nacelle did go back to being nice and round as was shown in the inset picture of one of the flight test MAX airplanes.

Methinks you're a Boeing guy. The -700C was my first program as a shiny new Boeing engineer back in '97. Later was a Lead Engineer on Section 46 of -900ER and Lead on Section 44 for Max 8 and 9. Never did work 737 propulsion, but dabbled in Struts for 777 and 747 briefly.
 
There's a design criteria typically used for wing engine nacelles where the bottom of the nacelle should be (from memory) at least one nacelle diameter above the runway, to minimize ingesting FOD. For the 737 with the higher bypass ratio engines, this would have involved lengthening the landing gear, so they went with the bent bottom cowl. There was a lot of CFD involved to make sure flow quality requirements into the engine were met.

There's the other requirement that with two flat tires on one side you don't drag the engine nacelle, but the ingesting FOD requirement was the driver. Even with bent bottom cowl, the bottom of the engine proper is still closer to the runway.


Charles E. "Chuck" Rogers
 
There's a design criteria typically used for wing engine nacelles where the bottom of the nacelle should be (from memory) at least one nacelle diameter above the runway, to minimize ingesting FOD. For the 737 with the higher bypass ratio engines, this would have involved lengthening the landing gear, so they went with the bent bottom cowl. There was a lot of CFD involved to make sure flow quality requirements into the engine were met.

There's the other requirement that with two flat tires on one side you don't drag the engine nacelle, but the ingesting FOD requirement was the driver. Even with bent bottom cowl, the bottom of the engine proper is still closer to the runway.


Charles E. "Chuck" Rogers

Even flattened, those don't even have a 1/2 diameter clearance from the ground. Besides a generic caliber type distance criteria, is there any CFD to show FOD suction? For example in fuselage and wing structures we have criteria loads for tool drops that are a weight x height energy...is there something analogous for suction forces?
 
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Methinks you're a Boeing guy. The -700C was my first program as a shiny new Boeing engineer back in '97. Later was a Lead Engineer on Section 46 of -900ER and Lead on Section 44 for Max 8 and 9. Never did work 737 propulsion, but dabbled in Struts for 777 and 747 briefly.

I was. I've been away since the end of October 2016 when I took a voluntary layoff after a nearly 38-year career. The last 1/3 of that time or so was in the 737 Configurations and Engineering Analysis group. The last airplane I worked on was the 737 MAX-10. I had the spatial integration task for the 700C (so I ran the FlyThru meetings).

If you had Section 46 of the 900ER then you had the fun of dealing with the mid-exit door. I flew home from NARCON in Houston on an Alaska 900ER last night....


As for ground clearance....What seemed to drive the normal attitude ground clearance requirement for the MAX was clearing runway and taxiway lights....as well as adverse attitude situations others have mentioned. With a 64+ inch diameter fan, the ground clearance is far less than that from the bottom of the nacelle is a fraction of that, even back as far as what we now call "Classics". I don't know what the number is for JT8D-powered 737s. I was working 757 as a new hire back in those days.
 
Yep, I had the mid exit door but I don't remember the surround structure being all that challenging. Section 44 is far more interesting and challenging with loads from every direction. I left the MAX program in early '16 after wrapping up the -9 analysis. Never got any -10 work, though they were starting to talk about the -7 and its variations, some of which I guess never made it off your desktop.
 
The -7 is built and was rolled out a couple of weeks ago so I expect a first flight real soon now. I'm really interested to see how the -10 main gear works....which takes us back on topic as the length of the airplane and how much can rotate for takeoff without whacking the aft end of the fuselage on the ground is a real performance constraint. The -10 has a main gear with some interesting mechanical features to make the airplane taller only when it really needs to be and not when we don't want it to be (resting on the ground normally).
 
No kidding? I haven't visited with them much since leaving the group, so I didnt realize the 10 did that. The MLG wheel well is a pretty cramped place.
 
As what you Boeing guys call a "meat servo" i can say the -10 is going to be VERY interesting.

With the 900 now tail clearance is a big issue, the trailing link gear of the -10 will be interesting.
I still haven't seen how they will retract, the gearwell does not have much room as it is now.

Until the split scimitar winglets were put on we had to worry about dragging an engine nacelle
in certain wind conditions, now we worry about the lower winglet. With the trailing link main gear
the engine out characteristics in a cross wind should be interesting.
I'm guessing the -10 won't have anymore thrust because the vertical stab is the same size as the
other MAX's?

Yup the -10 is going to be interesting to fly...
 
“Meat servo”...lol. 20 years in the industry and a private pilot myself, and I’ve never heard that one before. [emoji23]


TRA #16895 L2
 
As what you Boeing guys call a "meat servo" i can say the -10 is going to be VERY interesting.

With the 900 now tail clearance is a big issue, the trailing link gear of the -10 will be interesting.
I still haven't seen how they will retract, the gearwell does not have much room as it is now.

Until the split scimitar winglets were put on we had to worry about dragging an engine nacelle
in certain wind conditions, now we worry about the lower winglet. With the trailing link main gear
the engine out characteristics in a cross wind should be interesting.
I'm guessing the -10 won't have anymore thrust because the vertical stab is the same size as the
other MAX's?

Yup the -10 is going to be interesting to fly...

As of when I left, the top thrust ratings of the LEAP-1B will be the same on the -10 as on the -9/-8. Save for the landing gear the -10 is a "simple stretch" (and not very much of one, really) though, of course, it's never that simple. The final configuration of the -10 main gear (and all that it takes to get it into that PACKED main wheel well) were finalized after I left so I, too, am wondering how it all will turn out.

I'm also interested to see the execution of the plan to get the fuselages from Wichita to Renton when they are that long as well as how much complication they will cause in the first final assembly position in the Renton factory where all the insulation and most of the wiring and plumbing get installed.

I wonder how much difference there is in crosswind margin before you scrape the lower blade on the APB split scimitar winglets vs. the MAX winglets' lower blade. That's another detail that is sort of out of my realm of knowledge (I was mainly in systems and spatial integration).

I'll have to run that term "meat servo" by my soon-to-retire Alaska captain friend.....
 
Even flattened, those don't even have a 1/2 diameter clearance from the ground. Besides a generic caliber type distance criteria, is there any CFD to show FOD suction? For example in fuselage and wing structures we have criteria loads for tool drops that are a weight x height energy...is there something analogous for suction forces?

From an article on the "Hamster Mouth Inlet" for the 737 engines from Air and Space:

"A big inlet so close to the ground might act as a vacuum cleaner for rain, slush, and runway debris. Gregoire’s team found that the critical measurement was not the size of the inlet but the ratio between the inlet’s vertical opening and the distance between the lower lip and the ground. Flattening the lower lip reduced one and increased the other, and kept the new 737 out of the danger zone."

Full article here:

https://www.airspacemag.com/history-of-flight/how-737-got-its-hamster-mouth-180952391/


Charles E. (Chuck) Rogers
 
It was my understanding that the inlet design was one of the first "large scale" uses of CFD.
Of course, I heard this from a company selling CFD software......
 
I don't know, but it is plausible. At the time there was a Cray-1 at the Bellevue data center as well as some big IBM and CDC hardware, so there were the best resources on the market at the time (early 1980s) available for CFD.

It has certainly been used to help refine the nacelle shapes on subsequent generations - the Next Gens and now the MAX.
 
Because they all landed hard and hit the engine cowling on the runway. They just left the dent, as it didn't interfere with continued operation.

Just kidding. It is for ground clearance. With two blown tires and a deflated strut, they don't want the engine hitting the ground
Yes, I figured it was for ground clearance, but I wondered why design the aircraft to be so low to the ground that that was necessary while others weren't doing that. The video made the reason clear.
 
As the fellow in the video noted, two objectives of the initial 737 design were ease of loading and ease of servicing. Another is the ability to use over wing escape hatches for emergency egress without having to have off-wing escape slides. This saves space and weight. As the 737 has grown, this has been quite the constraint as there is no good place to put such slides in the airplane. The places where they live in other airliners are already quite full of stuff on the 737.
 
As the fellow in the video noted, two objectives of the initial 737 design were ease of loading and ease of servicing. Another is the ability to use over wing escape hatches for emergency egress without having to have off-wing escape slides. This saves space and weight.
Yep, and that was the answer to my wondering about why the plane was designed to be such a low-rider that the flat portion was necessary on the intakes. Also, I should have realised, but didn't, that the low model number (737) might mean that the design was so old that it was designed for turbojet engines and any upgrade to the much larger OD turbofan engines would present ground clearance problems.
 
I don't know, but it is plausible. At the time there was a Cray-1 at the Bellevue data center as well as some big IBM and CDC hardware, so there were the best resources on the market at the time (early 1980s) available for CFD.

It has certainly been used to help refine the nacelle shapes on subsequent generations - the Next Gens and now the MAX.

Here's an excellent paper on the history of the application of CFD at Boeing. A very interesting read.

https://calmarresearch.com/NF/STG/AGPS/media/aiaa-2003-3439.pdf


Charles E. "Chuck" Rogers
 
Yes - the basics of the airplane have been around for 50+ years now. It has been re-engined three times (Classic, Next Generation, MAX) and rewinged once (Next Generation) over that time as well as being stretched and repackaged many times. While the basic structural and systems architecture remains, it has been refined and improved with each generation. Capacity and range are up with each generation while fuel consumption, maintenance costs and noise are down. Quite a remarkable run for a product, really. I'm glad to have had a small hand in bringing the Next Gens and the MAX into existence.
 
It was rewinged for the first time between the -200 and -300. -300 was the first time 737 used the supercritical airfoil.

Guy
 
Ah....that certainly is plausible. I was absorbed in 757 and then B-2 systems work during the time the 737-300 was being done so was unaware of major aero changes like that. I only got involved on 737 at the beginning of the Next Generation after I made it back to Renton following a stint on 777.

I will revise my mental "history of the 737 configuration". Thanks.

Added later: I remember seeing pictures of the -300 (in model form) with flutter booms on the wingtips but I figured that was due to the rather major effect of the larger, heavier and differently attached engines. I didn’t realize that the wing had been relofted as well. But they figured out how to do without that form of flutter ballast before the airplanes actually flew.

This makes me wish I still had access to the configuration document for the -300 as I’d want to read about this wing change. But of course that’s now not possible since I don’t work there anymore.....
 
D6-38604 ?

Ah....that certainly is plausible. I was absorbed in 757 and then B-2 systems work during the time the 737-300 was being done so was unaware of major aero changes like that. I only got involved on 737 at the beginning of the Next Generation after I made it back to Renton following a stint on 777.

I will revise my mental "history of the 737 configuration". Thanks.

Added later: I remember seeing pictures of the -300 (in model form) with flutter booms on the wingtips but I figured that was due to the rather major effect of the larger, heavier and differently attached engines. I didn’t realize that the wing had been relofted as well. But they figured out how to do without that form of flutter ballast before the airplanes actually flew.

This makes me wish I still had access to the configuration document for the -300 as I’d want to read about this wing change. But of course that’s now not possible since I don’t work there anymore.....
 
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