1. Title of the invention

Tare and interference corrections of offset mounting including the influence of model interference on the offset fitting in low speed wind tunnels

2. Field of invention
Testing methodology for estimating the offset fitting interference for low speed wind tunnel facilities.

3. Use of invention
This method of offset fitting interference corrections are an improvement over the normal practice of interference corrections method, and this could be deployed for all types of models at low speed wind tunnel facilities.

4. Prior art
The offset interference corrections estimation used to be carried out in different ways in different tunnels. Most often, the model is to be suspended from the roof top and the forces on the offset fitting were measured in the presence of the model. Sometimes, only the offset fitting was tested at various side slip angles with angle of attack set to 0 degrees. In improved version, the offset fitting forces are measured at the required attitudes, both Alpha and Betas. To get the better offset interference data, additional stings were fabricated and were used as shadow stings.

5. Draw backs of prior art
The first method i.e., suspension of the model from rooftop is cumbersome and more over this methodology gives off set fitting data at discrete alphas and betas. Testing offset fitting alone is void of the effect of model's influence on the sting, and hence may not be perfect. Though the influence of model may be small, it is always better to include the same for better results. To overcome all these, principle of shadow sting is deployed which calls for additional sting support and time. This also results in additional blockage and hence results in in accurate values.

6. Comparison between prior art and present invention

7. Aim of the invention
Aim of the invention is to find out a simple and economical test setup to get the interference corrections for all types of models for all attitudes.

8. Summary of the present invention
The influence of the offset on the basic aerodynamic loads of the aircraft model was measured using shadow analogy. A simple floor mounted sting with provision for changing alphas was fabricated, and the model under test was mounted in inverted position. The sting with offset was also mounted in the reverse direction and the forces on the offset in the presence of the model were measured at all attitudes. The effect of model influence on the offset fitting was found to be substantial.

9. Brief description of drawings
Fig -1 shows the Experimental setup, while Fig -2 shows the model in the test section with offset mounting.

Figures 3-8 show the effect of offset fitting correction with earlier methods.

10. Statement of invention
The offset fitting interference corrections in the presence of the model for all attitudes could be achieved with a simple test setup. The offset fitting corrections arrived by this method is one order better than earlier methods.

11. Detailed description of invention
The aerodynamic loads measured by offset mounting scheme in low speed wind tunnels will also include the fitting loads along with the interference effect of the aircraft model on the fitting. These offset loads along with the interference have to be deducted from the overall loads measured. Classical method of measuring of this offset loads with a novel sting set up was deployed to take care of this.

A simple strut sting with provision to deflect the model for various required alphas was fixed on the floor. The Offset fitting was mounted to the metered sting in inverted position. The loads on the offset fitting under the influence of the strut fitting were measured at various attitudes. The model under test was mounted in inverted position on the strut and the loads on the offset fitting were measured for various attitudes. From both these measured values the effect of model interference on the offset fitting was obtained. The model was then mounted on the offset fitting and the aerodynamic coefficients were arrived after subtracting the interference effects.

The scaled model is fixed to the offset fitting, after carrying out the tare test as per the scheme depicted in Figure-1. Both Alpha and Beta could be varied to the required attitudes and the aerodynamic co efficient after corrections could be obtained.

The effect of the interference corrections are shown in the following figures. During the exercise it was found that the influence of model interference on the offset is not negligible as it was presumed during earlier methods.

The present method of offset corrections caters to all attitudes and corrections are incorporated for all coefficients, unlike the other methods which are at discrete attitudes.

Claims

1. The influence of model interference on the tare values of the fitting at all attitudes of model are included in arriving at the offset interference corrections.

2. The earlier techniques are devoid of model influence as the fitting with an assumption that the effect is insignificant. However, it was found that the effect of model influence is not negligible.

3. The mounting scheme is unique, simple, economical and environmentally friendly.

4. This novel method adopted for offset fitting interference corrections are an improvement over the normal practice of interference corrections method, and this could be deployed for all types of models at low speed wind tunnel facilities.