Design Evolution and Wind Tunnel Tests of a Three-Lifting Surface Regional Transport Aircraft

Round 1

Reviewer 1 Report

I enjoyed reading your manuscript. I have a few minor comments:

• At several points in the paper, you discuss the longitudinal/direcitonal stability of the aircraft, but then show a plot of C_M(C_L) with a referenced center of gravity location. From a wind tunnel testing perspective, it makes much more sense to plot the stability derivatives themselves. This removes the need to specify a center of gravity location, and removes a layer of interpretation for the reader. This would have been particularly helpful on Figure 6 (C_{M_{C_L}}) and Figure 9 (C_{N_\beta}}).
• It was not clear to me what the operation point for Table 6 was. Is this supposed to be the maximum L/D point? What is the angle of attack?
• Please add additional context to Fig. 14 if it is to remain in the paper. Additionally - the reference point doesn't matter for a flow visualization figure.

Author Response

I enjoyed reading your manuscript. I have a few minor comments:

Authors are grateful for the interest shown by the reviewer towards the paper.

• At several points in the paper, you discuss the longitudinal/directional stability of the aircraft, but then show a plot of C_M(C_L) with a referenced center of gravity location. From a wind tunnel testing perspective, it makes much more sense to plot the stability derivatives themselves. This removes the need to specify a center of gravity location and removes a layer of interpretation for the reader. This would have been particularly helpful on Figure 6 (C_{M_{C_L}}) and Figure 9 (C_{N_\beta}}).

We understand that showing the CM vs CL for a specific centre of gravity would require an interpretation for the reader. However, the aim of the paper was to derive information about the aerodynamic behaviour of the full-scale aircraft. In this perspective, authors focused on estimating the longitudinal and directional stability derivatives in the linear range of variation (both for CM and CN) which could be directly compared with a full-scale aircraft.

It was not clear to me what the operation point for Table 6 was. Is this supposed to be the maximum L/D point? What is the angle of attack?

In Table 6 has been updated to specify that the reported value is the maximum aerodynamic efficiency at about 6deg. Angle of attack.

Please add additional context to Fig. 14 if it is to remain in the paper. Additionally - the reference point doesn't matter for a flow visualization figure.

An additional context to Figure 14 has been added in the text to specify how the canard wake crosses the tail plane as the angle of attack increases, causing a non-linear behaviour of the pitching moment coefficient curve. The reference point info in Figure 14 label has been removed.

Reviewer 2 Report

The article “Design evolution and wind tunnel tests of a three-lifting surface regional transport aircraft” is interesting and relevant to the scope of the journal. Paper is well written and the structure is clear. However, there are two points that should be completed:

1. the state of the art - the authors cite several articles related to TSA configuration, but relatively old ones, do not take into account the most recent works, two examples:
   a) Three surface aircraft (TSA) configuration-flying qualities evaluation (2016) Aircraft Engineering and Aerospace Technology , Vol. 88, No. 2, p. 277-284, DOI: 10.1108/AEAT-02-2015-0055
   b) Three-Surface Model with Redundant Longitudinal Control: Modeling, Trim Optimization and Control in a Preliminary Design Perspective (2021) Aerospace , Vol. 8, No. 5,
   DOI: 10.3390/aerospace8050139
   It is recommended to refer to these two examples, especially since these articles deal with issues of stability and controllability, which is very important in the case of unconventional configurations.
2. The problem of stability of the presented configurations is considered only taking into account the so-called static stability.The issue of dynamic stability should also be mentioned, at least as a topic for future work.

Author Response

The article “Design evolution and wind tunnel tests of a three-lifting surface regional transport aircraft” is interesting and relevant to the scope of the journal. Paper is well written, and the structure is clear. However, there are two points that should be completed:

Authors are grateful for the interest shown by the reviewer towards the paper.

It is recommended to refer to these two examples, especially since these articles deal with issues of stability and controllability, which is very important in the case of unconventional configurations.

The state of the art - the authors cite several articles related to TSA configuration, but relatively old ones, do not consider the most recent works, two examples:

1. Three surface aircraft (TSA) configuration-flying qualities evaluation (2016) Aircraft Engineering and Aerospace Technology, Vol. 88, No. 2, p. 277-284, DOI: 10.1108/AEAT-02-2015-0055
2. Three-Surface Model with Redundant Longitudinal Control: Modeling, Trim Optimization and Control in a Preliminary Design Perspective (2021) Aerospace, Vol. 8, No. 5,
   DOI: 10.3390/aerospace8050139

The suggested references have been added in the introduction section.

The problem of stability of the presented configurations is considered only taking into account the so-called static stability. The issue of dynamic stability should also be mentioned, at least as a topic for future work.

This is a good point. The research activities dealing with this aircraft design were mainly focused on the feasibility of such an innovative layout. Thus, a first key point was to assess the static stability characteristics of the aircraft.