Hello to all

Been almost 6 years since I have made any new build. I came back home 2 weeks back after finishing my aerospace masters and had a gap of few months until I start another journey. So what was better than to revisit my love for building planes. I have worked almost 5-6 days on this till now, doing the necessary research and CAD design of formers since no RC plans of RQ-4B are available anywhere. I can share the files if someone wants to build their own. I will get the CAD parts laser cut in Aeroply probably this week or early next week since I am waiting for my building supply shipment.

All the electronics was salvaged from my old builds with just the need to buy the battery since all my old ones are puffed (If any senior people know a way out I can try it too)

My motivation behind this build is the scale metal replica with Luftwaffe scheme I bought last year. It sits right on my working table and somehow I am very excited to hold it every time and just admire the beauty. This is going to be slow build since the work will depend on time availability and priorities. I hope to apply most time in planning so the execution part is smooth.

Past few days have been spent on modeling, getting 3 view drawing printed in A0 and salvaging parts from old planes which I have shown below. It resembles a glider configuration with very high aspect ratio wing, So I am aiming for around 0.5-0.7 Thrust to Weight ratio. Would be glad to get other's opinion on this since it is my first glider type build.

Model Details:

Length - 968mm (3 feet 2 inch)
Width - 96.7mm (3.93 inch)
Height - 311mm (12.25 inch)
Wingspan - 2660mm (8 feet 8 inch)
One 64mm EDF 4000KV (1.05 kg thrust)
60A ESC
4 turnigy servos

Hope you all enjoy the process as I will.

Regards
Yash

Images -

Nice project.
Possibly the first 25 AR wing attempt in India...

How did you estimate the AUW?
What airfoil is proposed?
Chord approx 6" at root, 2" at tip?
Re 30k or less at the tips?

Please post updates as you progress
Lots of interesting issues, aero as well as engineering...

Best wishes

Thank you Iyer Sir for showing interest in this project  , and yes it is around 25ish AR.

I am aiming optimistically for 1.1-1.3kg AUW, but it could go off since it will change during build. I took all electronics and they weigh around 750-800g ( battery+esc+edf+6servos+landing gear set), other than that I hope to have wood and foam to weigh around 200-300g. But certainly from experience it can go up so I took 0.7 Thrust to Weight as a realistic guess and 0.5 as pessimistic guess.

Airfoil is NASA LRN 1015 which is available on airfoiltools.com.

Great guess sir, it is 6'' at root and 2 1/3'' at tip.

Sir, for Re I am not sure yet. I have been reading papers on it but couldn't find anything solid. Seems like an interesting problem, let me try to do one CFD analysis and I will get back to you.

Regards
Yash

Quote from: yash10896 on June 26, 2021, 01:13:07 PM

1. Airfoil is NASA LRN 1015 which is available on airfoiltools.com.

2. Sir, for Re I am not sure yet. I have been reading papers on it but couldn't find anything solid. Seems like an interesting problem, let me try to do one CFD analysis and I will get back to you.

Whenever I come across an interesting project like this (or, for example, an SAE task), I wish I was a trained aerodynamicist. Its too late for that, so I take what pleasure I can, from amateur investigations.

Hope you won't mind a few occasional rambling thoughts.

So here goes...

I had asked what airfoil is proposed. You said LRN 1015.
This is the airfoil on the full size.
Perhaps you plan on using it for the model too.

I looked into it at 10m/s, as a starting point.
A 2" wingtip chord at 10m/s will be at about 30k Re.
So I looked at the LRN 1015 polars at 50k Re on airfoiltools

SHOCKINGLY BAD!

Something didn't seem ok.
The LRN in the airfoil name, I guessed, meant Low Reynolds Number, no?

A little more research showed that the polars smoothed out a lot even at Re 50k,
at Ncrit 5 instead of 9!

(Though the L/D is still pretty bad!)

This is where I'm out of my depth.
I don't know how Ncrit 5 smoothens the polars compared to Ncrit 9.
Does it mean your 1015 wing will fly better if it is not too smooth?

I feel really frustrated at my lack of knowledge.
Anything you tell me would be a great help.
Regards

Quote from: K K Iyer on June 27, 2021, 07:56:31 PM

I looked into it at 10m/s, as a starting point.
A 2" wingtip chord at 10m/s will be at about 30k Re.
So I looked at the LRN 1015 polars at 50k Re on airfoiltools

SHOCKINGLY BAD!

Something didn't seem ok.
The LRN in the airfoil name, I guessed, meant Low Reynolds Number, no?

A little more research showed that the polars smoothed out a lot even at Re 50k,
at Ncrit 5 instead of 9!

(Though the L/D is still pretty bad!)

This is where I'm out of my depth.
I don't know how Ncrit 5 smoothens the polars compared to Ncrit 9.
Does it mean your 1015 wing will fly better if it is not too smooth?

I feel really frustrated at my lack of knowledge.
Anything you tell me would be a great help.
Regards

Those were some interesting observations which made me look more in depth. I took 2 days to go through everything and here are some of my ideas.
I still need to validate my CFD setup with experimental for which I would need 2-3 more days. In meantime I am showing all the things done so you can also take some time to find anything weird that I may have done.
Firstly, airfoiltools.com uses XFOIL to predict those polar curves, and XFOIL has "e^n method" to implement turbulence factors. The Ncrit is used to define disturbances in wind tunnel originally, Ncrit 9 means smooth flow and Ncrit 1 means very turbulent flow, just because ncrit 5 shows smoother curves doesn't directly signify roughness is better. This can be read more in XFOIL original documentation section "Viscous Formulation".
http://web.mit.edu/aeroutil_v1.0/xfoil_doc.txt
It is also based on Potential Flow theory and uses Vortex Lattice Method(VLM) for inviscid flow and RANS eq for viscid cases so it is limited in terms of analyzing low reynolds number flow. More info on this is available here: http://www.xflr5.tech/docs/Part%20IV:%20Limitations.pdf
Also, more basic info about XFOIL and XFLR5( just XFOIL but based on c++ ) can be found here : http://www.xflr5.tech/docs/Part%20III:%20The%20viscous%20flow_rev1.2.pdf

Some analysis done on XLFR5 which is similar to what you saw on airfoiltools.com but has viscous effects included:


This shows difference of pressure coefficient from inviscid to viscid.


Now, there is a reason XFOIL is only used for initial guess of fluid flow. It is not universally correct for all cases and this is true for all aerodynamics. Something might work for one but not for others. My prof at Duke used to say "All models are wrong, some models are useful" and this is kinda like a fact. I found one paper which does this comparison for low reynolds number flow, but still this is valid for just that airfoil which was analyzed. Here is one snapshot from it. Link of paper : https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0153755


So, I tried doing my own setup to check on different models. Two in particular- k ω SST and Spallart Allmars.
Mesh and result of k ω SST. I couldn't get y+ to be within 1 since I have 500k limit on ANSYS(this mesh is 479k), I will take sometime to optimize my mesh and get y+ under 1 then maybe better results follow,




NASA did a study on LRN1015 and has experimental data, but the parameters are different. So I will do one simulation at which experimental results are available and verify my setup. Then I will try to simulate my case which might take some days. This is the case I will work on - [0.2M,Re 500k, at alpha=0 cl=0.4]:



Here is that doc:
https://core.ac.uk/download/pdf/42819982.pdf

More documents to read if you have interest in it:
https://ijret.org/volumes/2017v06/i05/IJRET20170605005.pdf
https://www.researchgate.net/publication/292869470_COMPUTATIONAL_STUDIES_ON_NASA_1015_LRN-IL_AIR_FOIL_FOR_LAMINAR_AND_TURBULENT_CONDITIONS_AT_DIFFERENT_ANGLES_OF_ATTACK
https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=15934&context=rtd


Again, I would always love to have those rambling thoughts. Makes me work more on things I like to do.

Regards
Yash

Started assembling the fuselage formers. Will put epoxy adhesive tonight and let it cure meanwhile. I am also putting the laser cut plans I created in pictures below. If somebody want corel draw files, I can share that too.

I am thinking of working on fuselage and also in parallel figure out the wing problems. One idea I had was to use vortex generators as I saw in one thread on rcgroup.com, that people have seen improvement. But if anybody has more ideas or things to experiment I am up for that too.

Regards
Yash

Excellent CAD work and laser cutting.
And the jig is great - so simple yet effective!

Amazing work, I'll be following your progress closely! Keep us updated!
One question, hope you don't mind, why did you choose to build the RQ 4b, which has a very bulky fuselage, compared to the MQ9 Reaper, which has a much more streamlined fuselage?

Thanks Karthick for your interest!

Surely, I am happy to answer any questions. The main reason I chose RQ4B was one I bought a diecast model of rq4b last year which fascinates me still and other reason is I had EDF lying around from previous builds so I wanted to build something which can utilize it since I don't want to spend on electronics at all. This is very cheap build for me in terms of expense just for RQ4B.

I have spent this week trying to figure out few problems and will update by early next week on the progress.

@yash10896,

At Re 50k (esp at ncrit 5), can you take a look at S4061 vs lrn 1015?

Regards

Interesting build..  it will be a build to watch

Consider twist/ washout on the tips to counter lre effects.. have a look at the Prandtl proposition (AI Bowers)
you may want to revalidate the estimates of the AUW.. For an estimated AUW of 1.3/1.5 kg, and with 700gms of electronics, very less is left for the airframe ..
Also with a 1.3/1.15kg auw and the wing area .. the wing loading and wing cubic loading will be low, more like a sailplane  ..

happy building. Looking forward to the build and the flights

Regards
Rahul

@KK Iyer Sir

I have been working on changing the airfoil since we last talked, I chose S3021 for main wing and SD8020 for V-Tail. That is why there was a big gap in update as I rethought everything and changed the whole wing and v-tail skeletons. The fuselage is same as before, just needed few changes in wing mount since max thickness got reduced from 15% to 9.5%. I will get the ribs laser cut in 2mm balsa and it will have 1mm balsa sheeting, this might be done in one week.

@rahulroy31

Thank you for taking interest. I have implemented 3 degree of washout in the cad design since the last update. Also, the AUW will change and I can provide more accurate assumption in coming weeks.

Regards
Yash

So a major update on build. Finally figured out everything in CAD stage and got parts other than fuse laser cut two days back which included ribs for main wings and vtail as well as some misc parts. Below is the images of build progress. The hatches are cut out and with sheeting they will be joined together. Let me know if anyone has any build or design related questions.

Total weight of structure till now is - Fuse(342g) + Wings(166g) + V-Tail(21g) = 529g

Regards
Yash

  nice build.

@sahilkit Thanks 

Nicely coming up. 

Have you modified the design or is it 100 scale?

@manu Thanks

@sharlock The design is exactly same as full scale, only the airfoils were changed to account for low reynold number at wing tip.

Any updates on this build. Been following it keenly.

Update

Had a bit of progress since last time. I know this build is very slow but I try to do when I get space and time. The wings are done with all the wiring, same for v-tail. The hatches for fuselage took most of my time to get it fit properly, had to redo several times to get a sturdy rigid fit. Also, the navigation lights got installed and everything is connected with a single MPX connector for each wing to make detachment streamlined. Next I will start sheeting the wings and make wing connection box. If any questions, do ask.

Regards
yash

  Truly Awesome