Does Bypass Ratio Reliably Predict Thrust-to-Weight Ratio or Does Weight Class Override It?

Does Bypass Ratio Reliably Predict Thrust-to-Weight Ratio or Does Weight Class Override It?

A Look at Bypass Ratio and Thrust-to-Weight Ratio Across Four Generations of Commercial Turbofan Engines

By: Viraj Chakrabarti

1. Background and Research Question

Every turbofan engine on modern airliners has some air that passes through the engine core, and some that flows around the engine core (bypassed air). The ratio of air that flows around the core to the air that is bypassed is known as a bypass ratio. In this paper, I would like to analyse whether the bypass ratio of an aircraft can predict the thrust to weight ratio – that is, the total thrust of the engines divided by the max take-off weight of the aircraft. The thrust to weight ratio of an aircraft is an important metric, one that determines how powerful the aircraft is and affects acceleration, climb rates, and take off distances.

Since the 1970s, bypass ratio (BPR) has increased, from initially being 1:1 in earlier low-bypass turbofans, to now being almost 10:1 in newer engines like the LEAP engines, used in narrowbody aircraft family such as the A320, or the Rolls Royce Trent -1000 used on the widebody Boeing 787. Higher BPR is associated with better fuel efficiency and noise reduction.

One would assume that as BPR rises, TWR should rise too — bigger, more advanced engines should mean more power. But that assumption may not hold once you separate aircraft by weight class. This paper seeks to ask whether BPR is the only factor that influences TWR, or whether the answer actually depends on the weight class being looked at.

2. Data and Methods

The data set covers 10 commercial aircraft and their engines, spanning 1968 to 2018.

The aircraft chosen can be divided into the narrowbody and the widebody class, as shown in the table below.

Widebody Narrowbody

Boeing 777-300ER Boeing 737-200

Airbus A350-900 Mcougnell Douglas MD-80

Airbus A350-1000 Boeing 737-300 (classic)

        Boeing 737-800 (NG)

        Airbus A320 ceo

        Boeing 737-Max 8

        Airbus A320 neo

For each aircraft I recorded bypass ratio, thrust per engine, number of engines, and MTOW. TWR was calculated directly as:

TWR = (Thrust per engine × Number of engines) ÷ MTOW

Each aircraft was tagged with a Weight Class (narrowbody or widebody) and a Generation (Gen 1 through Gen 4), based on entry-into-service year, since engine technology tends to advance in breakthroughs in aerospace engineering increases in efficiency, rather than continuous improvement.

To test the research question, I calculated the correlation coefficient (r) and the regression slope between BPR and TWR three separate times: first using all 10 planes together, and then again separately for just the narrowbody planes and just the widebody planes.

Data by Generation:

Generation Avg. Bypass Ratio Avg. Thrust-to-Weight Ratio

Gen 1 (1968–1980) 1.37 0.269

Gen 2 (1984–1988) 5.35 0.303

Gen 3 (1998–2004) 7.05 0.306

Gen 4 (2015–2018) 9.80 0.311

Figure 1:

Figure 2:

3. Results

Across the pooled sample of all 10 aircraft, BPR and TWR are moderately correlated (r = 0.49), with a regression slope of about 0.005 , so each unit increase in BPR is associated with only a small average increase in TWR. Using only this, it is apparent that only BPR as a variable cannot be said to predict TWR.

Figure 3

However, once the sample is split by weight class, I noticed that there was a correlation for the category of Narrowbody aircraft, which show a strong positive correlation (r = 0.92) with a slope of about 0.0102 — roughly double the slope of the pool with both widebody and narrowbody. In comparison, widebody aircraft show a strong negative correlation instead (r = -0.99), with a slope of about -0.0442, meaning TWR actually falls as BPR rises when we look at widebody aircraft.

Figure 4

The whole aircraft category seems to be moderately correlated, however this is due to the strong negative and positive correlations observed in both the narrowbody and widebody groups. So weight class is influences which direction the relationship goes in, depending on whether the class of airplane is widebody or narrowbody.

4. Discussion and Conclusion

So, to answer the original question of the project, is bypass ratio a reliable predictor of thrust-to-weight ratio? This only holds true for narrowbody aircraft, not widebody aircraft.

For narrowbody aircraft, BPR is a strong, consistent predictor of TWR (r = 0.92), and the trend holds up across all four generations in the data set. For widebody aircraft, the opposite pattern (r = -0.99) can be observed, suggesting that BPR is inversely related to TWR. I would conclude that an analysis of BPR and TWR that doesn't account for weight class is incomplete in nature.

Sources

Aircraft and engine specs came from Wikipedia, manufacturer spec sheets (CFM, Pratt & Whitney, Rolls-Royce, GE), and aviation reference sites (AeroCorner, SimpleFlying, Aircraft Commerce, EntireFlight).

To access the full graphs, which display the aircraft types and the regression slopes, use this link: https://public.tableau.com/views/virajprojectdatascience/Dashboard1?:language=en-GB&:sid=&:redirect=auth&:display_count=n&:origin=viz_share_link