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Why Is The Boeing 720 Weaker And Lighter?  
User currently offlineDuke From Canada, joined Sep 1999, 1155 posts, RR: 2
Posted (7 years 9 months 4 days 2 hours ago) and read 5893 times:

This summer I posted a query about the internal differences between the different 707 models and the 720. One thing that is mentioned about this aircraft is that the structure was made lighter. In fact, in September I read in an old book that they went so far as to put lighter (thus probably weaker) parts at the joints of the fuselage (if I remember correctly). And of course there was less fuel capacity.

What I don't understand is, why did they do this? How does making an aircraft lighter make it better for shorter range flights (that was the purpose of the 720 - to be a shorter range version of the 707)? If anything, I would think that would not be a good thing, as the increased number of landings would be hard on the weaker structure of the aircraft. With the 747 (100-type)SR, I think they did the opposite - reinforce the structure for more landings.

13 replies: All unread, jump to last
 
User currently offlineTrex8 From United States of America, joined Nov 2002, 4745 posts, RR: 14
Reply 1, posted (7 years 9 months 4 days ago) and read 5816 times:
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you are right in thinking more landings may need strengthening but the main reason the 707 was "heavier" and "stronger" was to carry more fuel/payload which was not necessary in the 720. just because the 720 was going to be used for shorter sectors does not follow that it will be used for more cycles. in any case Boeing obviously thought there was adequate structural strength and lightening it would increase efficiency so it was not carrying deadweight needlessly.

User currently offlineSPREE34 From United States of America, joined Jun 2004, 2246 posts, RR: 9
Reply 2, posted (7 years 9 months 3 days 23 hours ago) and read 5751 times:

Quoting Duke (Thread starter):
put lighter (thus probably weaker)

Not "weaker." Appropriate for the need.
Too much can create other negatives. Same with engines.



I don't understand everything I don't know about this.
User currently offlineKhobar From United States of America, joined Mar 2006, 2379 posts, RR: 4
Reply 3, posted (7 years 9 months 3 days 21 hours ago) and read 5630 times:

It was designed to operate from shorter runways, hence the need for a lighter airframe.

User currently offlineFlyDreamliner From United States of America, joined Jan 2006, 2759 posts, RR: 15
Reply 4, posted (7 years 9 months 3 days 21 hours ago) and read 5613 times:

Why is a 772 lighter empty than a 772LR. Plane flies farther, needs to be stronger to support the greater weight of more fuel. Lighter planes take off and land in shorter distances, burn less fuel, and thus has to carry less fuel, and a lighter aircraft does not have to be as reinforced to withstand takeoffs and landings as a heavier one. there is no added benefit to a plane any heavier than you need.


"Let the world change you, and you can change the world"
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6420 posts, RR: 54
Reply 5, posted (7 years 9 months 3 days 20 hours ago) and read 5550 times:

There are dozens of examples of this. The longer range version needs to carry a lot more fuel and therefore needs a stronger an heavier structure.

But usually the lighter version makes it to the air first, like the DC-10-10 before the DC-10-30 etc. With the B707 and B720 it just happened in the opposite sequence.

The B720 was a rather hastily produced conversion of the B707 to blow the CV-880 out of the market at a time when the B727 was still years out in the future.

Boeing planners were probably surprised at what speed US airlines wanted to covert to jet comfort also at domestic flights at a time when loads of DC-6Bs were far from worn out yet.

And the B720 was quite successful against the CV-880.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineViscount724 From Switzerland, joined Oct 2006, 25117 posts, RR: 22
Reply 6, posted (7 years 9 months 2 days 17 hours ago) and read 5287 times:

The JT3D turbofan-powered 720B was always one of my favorite early jet aircraft, probably #2 after the DC-8. The 720B was the hotrod of the 707 family with the same engines as a 707-320B/C but a gross takeoff weight 100,000 lbs. less.

The non-fan engined 720 like those operated by UA and EA who never re-engined their 720s always seemed under-powered and sluggish. In contrast, the 720B was the opposite and gave the impression that it could easily take off on 3 engines. The extra thrust from the JT3Ds on a 720 vs. the non-fan JT3Cs was roughly the equivalent of adding a 5th engine.

I have good memories of several 720B flights on Western, NW and AA, and a couple of non-fan UA 720 flights.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6420 posts, RR: 54
Reply 7, posted (7 years 9 months 2 days 17 hours ago) and read 5237 times:

Quoting Viscount724 (Reply 6):
The extra thrust from the JT3Ds on a 720 vs. the non-fan JT3Cs was roughly the equivalent of adding a 5th engine.

In fact it was more than adding a 5th engine.

5 x JT3C (on 720 typically JT3C-6 - 13,500 lbs) = 67,500lbs.
4 x JT3D (on 720B JT3D-1 or -3 / 17,000 or 18,000lbs) = 68,000 or 72,000lbs.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 8, posted (7 years 9 months 2 days 15 hours ago) and read 5171 times:

The 720 was designed for short-range and short-runway operations. It had a shorter fuselage (reducing weight), a higher swept wing between the inboard pylons and the fuselage, Krueger flaps over the entire length of the leading edge, and a reduced fuel capacity. It was eventually replaced by Boeing's own 727, which was designed to be more efficient.

User currently offlineStirling From Italy, joined Jun 2004, 3943 posts, RR: 21
Reply 9, posted (7 years 9 months 2 days 12 hours ago) and read 5056 times:

Quoting N231YE (Reply 8):
It was eventually replaced by Boeing's own 727, which was designed to be more efficient.

More so on that....The B720 was a stopgap effort at filling an airline's need for a full-size, short-range jetliner that could do short hops like Denver-SFO, Los Angeles-El Paso, Dallas-San Antonio; routes where the existing 707s were way too much of an airplane.
The 727 was still a few years away, and Boeing was a customer driven company providing a melange of versions of the same frame. A practice that will effectively come to an end with the 787...."One (or two) sizes fit all.



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User currently offlineKC135TopBoom From United States of America, joined Jan 2005, 12134 posts, RR: 51
Reply 10, posted (7 years 9 months 2 days 10 hours ago) and read 4992 times:

Quoting N231YE (Reply 8):
The 720 was designed for short-range and short-runway operations. It had a shorter fuselage (reducing weight), a higher swept wing between the inboard pylons and the fuselage, Krueger flaps over the entire length of the leading edge, and a reduced fuel capacity.

Almost correct, except for the wing sweep. The wing sweep on the B-367-80, KC-135A/B, B-707-100/-200/-300/-400/-700, and B-720/B were all the same at 35 degrees. This is a much more radical sweep than any of today's airplanes, most being at 33 degrees, or less. It is also one of the reasons the B-367-80 series airplanes (which also included the B-727-100/-200) could cruise at a higher airspeed than just about everything (except the B-747 series and CV-880/-990).

In addition to the Krueger flaps on the B-720/B (later found on the B-707-300/-400/-700 series), it also had the fillet flaps the KC-135 had (fillet flaps were also added to the B-707-300/-400/-700). The KC-135 never had a full set of Krueger flaps, it does have a partial set of leading edge flaps just inbaord of the outboard engine stuts (the first large airplane ever equipped with these, due to the USAF contract requirement for slower approach speeds and shorter take off rolls). The B-707-100s/-200s did not have either Krueger or fillet flaps.


User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 11, posted (7 years 9 months 2 days 1 hour ago) and read 4841 times:

Quoting KC135TopBoom (Reply 10):
Almost correct, except for the wing sweep.

This information was derived from the Book, Boeing Aircraft since 1916, Written by Peter M. Bowers.

Quote:
The sweep of the leading edge between the inboard nacelles and the fuselage was increased to produce double taper and Krueger leading-edge flaps were installed for almost the full span (477).


User currently offlineAccess-Air From United States of America, joined Sep 2000, 1939 posts, RR: 13
Reply 12, posted (7 years 9 months 1 day 20 hours ago) and read 4743 times:

I have an OAG (Official Airline Guide) from February 1975 and is shows Western Airlines flights in their Boeing 720Bs to/from LAX to HNL.....So It did have at least that much range to do hops to Hawaii from the west coast.....
Western also used their 720Bs in a lot of as did Northwest on very short flights according to the guide.....

Access-Air



Remember, Wherever you go, there you are!!!!
User currently offlineDutchjet From Netherlands, joined Oct 2000, 7864 posts, RR: 57
Reply 13, posted (7 years 9 months 1 day 19 hours ago) and read 4711 times:

Quoting Access-Air (Reply 12):
I have an OAG (Official Airline Guide) from February 1975 and is shows Western Airlines flights in their Boeing 720Bs to/from LAX to HNL.....So It did have at least that much range to do hops to Hawaii from the west coast.....
Western also used their 720Bs in a lot of as did Northwest on very short flights according to the guide.....

Access-Air

Western did utilize its 720Bs on California-Hawaii and California-Mexico routes....the 720Bs did have more than enough range for transcon/hawaiian routes. NW flew the 720s on the JFK-SEA route for many years, until the 747s were delivered....another rather long service for the type. 720s also saw some transatlantic duty: AerLingus flew their 720s on the JFK/BOS-SNN/DUB route in the 1960s.

Although the 720s were designed with shorterhaul operations in mind, the airplanes still had a lot of capability, especially the 720Bs which were "overpowered"......like the muscle cars of the same era. Most of the missions flown by most 720s (actually the 707-020) were better handled by Boeing's own 727 family which was introduced just a few years later.

I had the pleasure of flying a Western 720B from ACA to LAX and flew a PA 720B from NAS to JFK....great memories.


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