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Engine placement for 6 engine airplanes

Posted: Sun Sep 12, 2021 2:23 am
by kitplane01
In all of history there have been 5 jet aircraft with six jet engines (not counting VTOL aircraft).

The B-47 had six engines, each wing with a double-pod and a single pod. The An-225 has six engines, each wing with three single pods. The Martin XB-48 also had six jets, each wing with three pods.
The XB-70 had six engines in two pods under the delta wing. It was highly supersonic, so different rules apply. The Scaled Composites Stratolaunch was the result of mating two 747s, so again different rules apply.

Given the two reasonable choices of each wing having a double-pod plus a single pod, or having just three single pods, which is better? Presumably both the B-47 and An-225 design teams were making the best decision they could, so why the different outcomes? A double pod comes with less wetted area than two single pods so less drag, but does it have any disadvantages?

(Yes, I know the right answer is "use fewer bigger engines", but that's not always possible).

Re: Engine placement for 6 engine airplanes

Posted: Sun Sep 12, 2021 4:20 pm
by LH707330
The 6 individual pods give you more safety from contagious failures at the expense of more wetted surface. Engines in side-by-side pods have experienced contagious failures on Il-62s:

https://en.wikipedia.org/wiki/LOT_Polis ... s_Flight_7
https://en.wikipedia.org/wiki/LOT_Polis ... light_5055

My guess is on the B-47 they figured that the presence of ejection seats and the outer engines on their own pods gave them enough margin in case an inner pair went bust, while the An-225 designers had the two LOT flights fresh in their memories and also wanted more safety margin for cargo and pax operations. There's an early render of the 707 with the double pods as seen on the B-47, evidently Boeing figured the wetted drag penalty was worth the safety gain. Pan Am 843 appears to have vindicated that design decision:

https://en.wikipedia.org/wiki/Pan_Am_Flight_843

Re: Engine placement for 6 engine airplanes

Posted: Mon Sep 13, 2021 1:09 am
by kitplane01
LH707330 wrote:
The 6 individual pods give you more safety from contagious failures at the expense of more wetted surface. Engines in side-by-side pods have experienced contagious failures on Il-62s:

https://en.wikipedia.org/wiki/LOT_Polis ... s_Flight_7
https://en.wikipedia.org/wiki/LOT_Polis ... light_5055

My guess is on the B-47 they figured that the presence of ejection seats and the outer engines on their own pods gave them enough margin in case an inner pair went bust, while the An-225 designers had the two LOT flights fresh in their memories and also wanted more safety margin for cargo and pax operations. There's an early render of the 707 with the double pods as seen on the B-47, evidently Boeing figured the wetted drag penalty was worth the safety gain. Pan Am 843 appears to have vindicated that design decision:

https://en.wikipedia.org/wiki/Pan_Am_Flight_843


On the LOT-7 flight, the one engine blowing up took out TWO other engines. That cannot be good. (and the controls to the elevator, which was quite bad.)
On the LOT-5055, the one engine blowing up ALSO took out the controls to the elevator.
Makes me wonder about the Il-62 and it's elevator controls!

Can a 707 maintain altitude with 2 engines one one side and nothing on the other?

Although in the six engine scenario, even losing a pod gives you 4/6 of the total engine, which is enough for any reasonable plane to maintain altitude.

Re: Engine placement for 6 engine airplanes

Posted: Mon Sep 13, 2021 1:19 am
by kalvado
An-225 is a stretch of an-124, so it started with 4 individual pods. Designing separate double pod would be too much, I assume. A wing root plug containing an extra engine in a same pod is easier.

Re: Engine placement for 6 engine airplanes

Posted: Mon Sep 13, 2021 3:01 am
by GalaxyFlyer
kitplane01 wrote:
LH707330 wrote:
The 6 individual pods give you more safety from contagious failures at the expense of more wetted surface. Engines in side-by-side pods have experienced contagious failures on Il-62s:

https://en.wikipedia.org/wiki/LOT_Polis ... s_Flight_7
https://en.wikipedia.org/wiki/LOT_Polis ... light_5055

My guess is on the B-47 they figured that the presence of ejection seats and the outer engines on their own pods gave them enough margin in case an inner pair went bust, while the An-225 designers had the two LOT flights fresh in their memories and also wanted more safety margin for cargo and pax operations. There's an early render of the 707 with the double pods as seen on the B-47, evidently Boeing figured the wetted drag penalty was worth the safety gain. Pan Am 843 appears to have vindicated that design decision:

https://en.wikipedia.org/wiki/Pan_Am_Flight_843


On the LOT-7 flight, the one engine blowing up took out TWO other engines. That cannot be good. (and the controls to the elevator, which was quite bad.)
On the LOT-5055, the one engine blowing up ALSO took out the controls to the elevator.
Makes me wonder about the Il-62 and it's elevator controls!

Can a 707 maintain altitude with 2 engines one one side and nothing on the other?

Although in the six engine scenario, even losing a pod gives you 4/6 of the total engine, which is enough for any reasonable plane to maintain altitude.


Yes, two engine performance and controllability are certification requirements. Pilots are trained and checked on two-engine out drills. Heck, tri-motors have to show two-engine out work.