Propeller-wing wake

[Beth]

Hi Emilio,

Sorry for another question! But I've been looking into the effects that a propeller-wing would have on the lift/drag performance of another propeller-wing further downstream. However, I've found little difference in the results making me think there might have been something I set up wrong - especially when evaluating at different angles of attack / distances between wings. (CL of second wing is marginally lower than CL of first wing)

I've attached the approach and basic vehicle image. In general the approach is:
- split vehicle into two: front and back -
1. prop.spin script to produce a prop slipstream for first row props
2. include this in wing evaluation VLM for front wing
3. compute_wing_wake at the x-position of second row props 
     ^ edited to take VD from already computed VLM
4. compute_propeller_nonuniform_freestream for second row props
5. prop.spin for next props slipstream
6. include this in next wing evaluation VLM

It could be that this approach isn't appropriate for Fidelity Zero wakes, but otherwise I'm not sure what might have gone wrong. I'm thinking that it might be to do with the wake calculated from the first prop-wing, and whether this is creating the correct trajectory ie. taking into account angle of attack and gravity? I'd love to hear your thoughts on my approach to this case study!

Many thanks,
Beth

[Racheal Erhard]

Hi Beth,

The process you laid out seems reasonable, however I think the major issue is in the way you have set up your vehicle. It looks like in your script you have split the aircraft into a "front" and a "back" vehicle rather than using the full aircraft in the analysis. This makes it so when analyzing the "back" vehicle, you aren't including any of the prop interactions from the front row. It is best to put all components into a single vehicle, and when you run the VLM it will analyze all interactions together, and you can extract the results from the individual wings as a post-processing step.

It also looks like from this script you are not up to date with the latest develop, so I would suggest updating.

Best,

Racheal

[Beth]

Hi Racheal,

Thank you so much for your reply, I'll retry this again using the whole vehicle - I was worried this may have been the reason! I think I chose the front/back method because I wanted to isolate which propellers would interact directly with which wings for the individual VLM but I suppose depending on the position of the wings this shouldn't be something I'd have to consider myself. I was under the impression that the VLM only considers the main wing, but I've just noticed that if a main wing isn't specified the VLM script considers all wings in geometry.wings. I'll try this again and compare the results. 

Also thanks for letting me know, I'll try updating!

Many thanks,

Beth 

[Beth]

Hi Racheal,

Thanks again for your help on this. I ran the VLM analysis with the whole vehicle and it worked a lot better! I just needed to edit the compute_RHS_matrix script called within the VLM to perform p.Wake.evaluate_slipstream on both wings (ie.wing_instance_idx=0 and wing_instance_idx = 1) since otherwise it just considers the velocities of the propellers for the last wing in the vehicle. 

Even though the results look a lot better, I don't think this approach considers the first wing wake's impact in the analysis - is this something I would also need to consider?

Many thanks,

Beth

[Racheal Erhard]

Hi Beth,

That's great to hear! As for the first wing wake's impact, the inviscid contributions are already accounted for in the VLM ( the VLM accounts for the vortex distribution from all vehicle components). The viscous wake correction is probably not necessary for your intended use, as the velocity deficit from the boundary layer decays with distance from the wing, and it looks like your wings are fairly spread out. This shouldn't effect the performance of the trailing wing.

Best,

Racheal