Scaling Laws for Gas Turbine Engines

[Zipper730]

I'm curious if there's any formulas that help determine scaling for jet-engines? For example if the diameter were to double in length would the thrust increase by the square of this diameter (much like fluid going through a pipe)?

drgondog , X XBe02Drvr , W wuzak

 

[cherry blossom]

I am not one of the experts but a 0.855 scale RR Nene became the Derwent V. The Nene gave about 5,000 lb trust and the Derwent V gave 3,500 lb trust.

 

[Sergio]

I'm just a humble, forgetful, retired engineer and recall just how complex modelling factors were from my college days. I remember that you cannot always practically apply the same factor throughout to estimate a factor of a model's performance. For example, a yacht's maximum speed through the water does not change pro rata to its waterline length because of other speed-affecting rules though the 2^3 rule (2*2*2 = 8) provides a good estimate of the weight and catalogue price of similarly-specified yachts!

Assuming your factor of doubling the diameter also means you're doubling the length of the engine, I would start by expecting 8 times the thrust. It would also probably be about 8 times heavier.

You're right about the water pipe!

 

[thunderbird]

The answer is complicated.

As i understand it, the equations for predicting jet engine performance were available in the mid-fifties or early sixties. However, it takes quite a bit of explanation to describe what parameters are important. Engine diameter, for instance is tricky. Did one just upsize the whole engine or did one also change the bypass ratio? Can the two engines have the same engine pressure ratio or do materials limit one engine over another? Certainly jet engine design has seen the most change since WWII.

But to exactly address your question, other factors other than size, seem to be more important to determining engine performance. For instance, the F404 engines in F/A-18s is more powerful than J-79 engines in F-104 and F-4 among others, and more powerful than the J57 in F-101s and F-102, despite being for smaller and lighter. The F110 has about 50% more thrust than the TF30, despite being almost the same size. But some place i read that the TF30 had better supersonic performance, supposedly because of the digital controls on the F110.

But compare the J75 and J57 engines for performance change based in size. Supposedly, the J75 took the J57 and made it larger. The J75 was also a generation newer engine, so it might have benefited from internal material improvements in addition to the size increase.