The abstract of the article is: Maximum loads acting on aircraft structures generally arise when the aircraft is undergoing some form of acceleration, such as during landing. Landing, especially when considering rotorcrafts, is thus crucial in determining the operational load spectrum, and accurate predictions on the actual health/load level of the rotorcraft structure cannot be achieved unless a database comprising the structural response in various landing conditions is available. An effective means to create a structural response database relies on the modeling and simulation of the items and phenomena of concern. The structural response to rotorcraft landing is an underrated topic in the open scientific literature, and tools for the landing event simulation are lacking. In the present work, a coupled sequential simulation strategy is proposed and experimentally verified. This approach divides the complex landing problem into two separate domains, namely a dynamic domain, which is ruled by a multibody model, and a structural domain, which relies on a finite element model (FEM). The dynamic analysis is performed first, calculating a set of intermediate parameters that are provided as input to the subsequent structural analysis. Two approaches are compared, using displacements and forces at specific airframe locations, respectively, as the link between the dynamic and structural domains.
As a rule, all experimental works have strong experiment and results description, but very poor mathematical background. However, overall merit and scientific approach of the work seems reasonable. My review with the authors' answers is below.
“Lines 104-105. In fact, there is no description of helicopter landing simulation methods. This requires several models to create a chassis, shock-absorbing suspension, and fuselage. It is likely that many of these works are classified. I believe that such studies were accurately carried out at the design bureau of the M.L. Mil MVZ. I propose to replace “In the literature on helicopter technology is missing ...” by “In open literature on helicopter technology is missing ...”.
Author Reply: The sentences have been accordingly modified in the manuscript.
“Section 4. Experimental activities. The influence of destructive effects on the structural elements of the helicopter from previous experiments can affect the results of the next experiments. How is the fatigue of helicopter structures taken into account? Will, there be a coincidence of results if you conduct an experiment with a new helicopter and the helicopter, that was already battered? I suggest you to give a short description of it”.
“Line 260, Figure 2. There is no fundamental difference between figures (a) and (b). Would the double dot “:” sign be correct after the “during” word?”Author Reply: We agree with the reviewer in that “The influence of destructive effects on the structural elements of the helicopter from previous experiments can affect to the results of next experiments”. However, in our experiments, we designed the test matrix in a way that only the last landing impact exceeded the Harsh Landing threshold with sufficient impact velocity. This apart, a check was made with proper NDT in order to assure the structure was not subject to major permanent damage with landings from increasing altitudes. Apparently only in the last landing, the only one that is defined as harsh from standards, a permanent damage was recorded near the landing gear attachments.
For sure, measures taken from different helicopters might be affected by different deviations from FEM/Multibody, and also the same helicopter during its life might experience deviation trends with respect to the initial condition (due to wear, due to fatigue, due to increasing plays/tolerances among components…). In fact, the main idea behind the usage of such model is to use it as a reference to identify deviation from a baseline condition during the operative life of the helicopter.
Author Reply: The caption has been modified as suggested. Please note that, on a large scale, the differences are not evident, but if you focus on the wheels (as suggested in the caption), the difference turns out to be quite significant, especially for the rebounded front wheel.
“Line 274, Table 1. Please change the short dash “-” to the minus “–” sign”.
Author Reply: The symbols in the Table have been accordingly modified.
“5. Line 349, “5.1.2. The shock absorbers characteristics” section. This helicopter model has shock absorbers, the technical characteristics of which are standardized and are indicated in its technical passport. Is it worth describing the simplest mathematical model of a shock absorber, if such data for the experiment can be taken directly from the shock absorbers passport? Moreover, in the section “5.1.3. The shock absorbers parameters identification”, the authors proceed to the unified data of shock absorbers. In my opinion, the article will not lose its value without this section at all. Otherwise, it is necessary to include all mathematic sentences for all the construction nodes”.
Author Reply: In reality, probably due to the fact that the helicopter was at the end of its life we were not able to capture the real dynamic behavior during landing by directly inputting the characteristics of the technical passport in our model. This would have created higher discrepancies which would have led to a higher unpredictability of the strain measurements. For this reason, we preferred to use “the simplest mathematical model of a shock absorber” for which we identified parameters to mimic the experimental behavior for one drop, then validating the model for other drops.
Section 5.1.3 has been removed from the manuscript.
The following paragraph was added to Section 5.1.2:
Since not every landing gear parameter was accurately known or easily identifiable and some simplifying assumptions were inevitably made in equations (1) to (6), a parameter identification procedure was carried out to adjust the multibody model shock absorbers characteristics in order to mimic the experimental dynamic behaviour. This adjustment process was performed on the 0.48 m drop, which was selected as the reference drop, since it exhibited the touch down critical velocity that identifies the onset of the harsh landing regime, thus obtaining the elastic and damping characteristics in Figure 5. Then, a comparison check was carried out for the whole experimental drop sequence, as shown in the next section.
“Line 447, Figure 9. The percentage error between the experimental and numerical results looks considerable”.
Author Reply: We agree this error is not negligible. However, please note that this is the percentage error on just the peak value. Figure 8 shows that, actually, there is a quite satisfactory global match between the multibody and the experimental forces, and Figures (6) (7) show even lower error for displacements. These errors are mainly due to a slightly different helicopter attitude at the instant of landing impact.
The following sentence was added to the manuscript:
It is finally worth noticing that further error reduction can be obtained by adopting more complex models for the landing gear strut load prediction and more advanced optimization strategies, which are however outside the scope of the present activity.
“Line 485, Figure 12. There is no explanation of the abbreviation FEA and EXP in the text above”.
Author Reply: The acronym FEA stands for finite element analysis and it is now defined in the keywords. The abbreviation EXP has been explained in the caption of Figures 12 and following.
“Line 485, Figure 12, (c). The numbering order of the tail boom sensors along the helicopter fuselage is not displayed anywhere”.
Author Reply: Figure 11.c has been added to the manuscript.
No comments:
Post a Comment