Bypass Ratio BPR - Effect on optimal altitude
 

Hi,

I have come across the question, if the bypass ratio of a turbofan does have direct consequences on its "best altitude" in regard to fuel economy.

Say, if a jet is re-engined with a turbofan of a larger bypass ratio (with most of the other factors staying the same)... will the larger bpr alter the optimal altitude?

Kind regards,
Peter

A good example would be when the DC-8-60 series (JT3D) was re-engined with CFM56. I don't have the numbers, but it should be easy to check.

Wheres 411A when we need him? LOL.
On the Tri-star, with the -200's,250's and -500's, the 524's would out climb the wing. The 747 is the same way. The wing can only do so much work at a given weight and altitude. The engines are way more fuel efficient, but the wing is the limiting factor at that point.
Thats my wine influenced point of view.

iwrbf,

Given that the lower the by-pass ratio, the higher the efficient cruise altitude I would guess the a higher by-pass ratio engine would lower the best economy altitude.

The old B707 100 series demonstrated this when they got the fan engines. It was well disguised by the desperately poor turbojet performance though - we performance engineers could see it but few others I venture to say.

Hi :-)

So you say a higher BPR should give a decreased optimal altitude? I read something years ago in an article about the CJ2 that the higher bpr is the main factor for its higher optimal altitude. I think I should try to find a book about turbofans :-)

Kind regards,
Peter

I agree.
High BPR- better for low alt performance
Low BPR- better for high alt performace
A compromise BPR is achieved based on specific aircraft role. e.g. in a two engined commercial jet you need-
a) Good high level cruise ability.
b) Good engine out climb gradient.
So....like everything else in this world.... compromise. :p

I got a question, is there less performance loss in a turbofan of a given bypass ratio when the pressure ratio is higher, or the core exhaust velocity is higher?

Jane, thinks for the trick question.

Most folks would talk about performance gain i.e. per lb of thrust per pound of fuel per hour. I have a tendancy to match this up with the mission.

I'll take the pressure ratio increase as my better performance improvement, that way I can do what I want with the inlet pressure/temp to the turbine, depending on how big a fan I've got and what I'm using the engine for like a fighter jet or a loirtoring sub chaser. Then its a matter of torque out of the turbinefor greater mass flow out of the fan or jet velocity out of the turbine exhaust for high speed manuevers