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  #261  
Old 11-29-2017, 04:19 PM
Steve Melton's Avatar
Steve Melton Steve Melton is offline
 
Join Date: Apr 2011
Location: Cincinnati, OH
Posts: 3,536
Default whip it good

a concept idea for a fairing for a bottom mounted whip antenna to add an airfoil section to the vertical section. however, this addition will make it ripe for flutter. I will do some reduced velocity calculations to verify how bad it is. appreciate any feedback on flutter potential. this printed pieced is split and will clamp around the antenna. another option could be to let it swivel. I'm currently 50/50 on this one, not sure I'm even going to try it. The steel rod = 0.190 inch diameter but tapers towards the aft end.



proto1 print, I can print this much better but good enough for a fit check. I had an idea to print the fwd area solid and the aft hollow. This was my quick first run.



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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com

Last edited by Steve Melton : 11-29-2017 at 04:54 PM.
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  #262  
Old 11-29-2017, 05:01 PM
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Steve Melton Steve Melton is offline
 
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Location: Cincinnati, OH
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Default

Quote:
Originally Posted by RV8JD View Post
Experiment first by mounting it on the hood of a car, slowly increasing speed. Maybe use the runway for higher speed testing. At least you will know if you can get the antenna up to pattern speeds.

Keeping the antenna as far forward in the fairing as possible is beneficial for flutter (but no guarantee it won't flutter). Letting it swivel will alleviate side loads on the antenna attachment during slips and other non-coordinated maneuvering.
I was moving the antenna further aft to shift the fairing cg fwd and printing the fwd edge as solid, but in this case with the end fixed it don't think it will matter. letting it swivel may be the better approach.
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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com

Last edited by Steve Melton : 11-29-2017 at 05:06 PM.
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  #263  
Old 11-29-2017, 05:37 PM
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Steve Melton Steve Melton is offline
 
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my concern is with torsion and bending coupling of the antenna rod with the fairing. letting it swivel will eliminate the torsion component almost completely except for friction and ABS is slick so friction will be very low.
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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com

Last edited by Steve Melton : 11-29-2017 at 05:45 PM.
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  #264  
Old 11-29-2017, 08:44 PM
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Steve Melton Steve Melton is offline
 
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Default a good fit

there is .1" play the aft edge. I will recut that and give it a draft in flat at the horizontal fit (will actually be an arc) to allow at least .25" play at the aft edge. that should eliminate the torsional degree of freedom from interacting. plan to flight test this week. thanks for the comments.





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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com

Last edited by Steve Melton : 11-29-2017 at 08:53 PM.
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  #265  
Old 11-30-2017, 06:04 AM
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Steve Melton Steve Melton is offline
 
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Default coupling - don't want to do that

I don't believe we want to allow the fairing and the antenna to couple torsionally. Except there will always be some friction. ABS to steel coefficient of friction ~ 0.06. that's relatively low but still a risk.

step1. the antenna CG is far aft (long wire) and the fairing center of pressure (CP) is forward. it is better to have the CG forward of the CP so we have to decouple them in the lateral direction. that means the fairing has to be able to swivel somewhat.

step2. the fairing must not be allowed to flutter. the fairing is symmetrical so it's CP = 25% of cord. place the antenna pass thru at 25% cord. for a symmetrical airfoil CP does not change with AOA so if needed the pass thru wire could be moved slightly aft of 25% cord.

step3. move the the fairing CG forward of the CP by increasing the leading edge weight.

antenna freq response
17.2 Hz (hey, I guessed 20 Hz by twanging it with my finger, pretty close)
18 Hz
70.8 Hz
71.1 Hz
145.5 Hz
157.2 Hz



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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com

Last edited by Steve Melton : 11-30-2017 at 06:10 AM.
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  #266  
Old 11-30-2017, 09:27 AM
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F1Boss F1Boss is offline
 
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Default The attach fairing...

Looks like a whole lotta drag there too as the area where the wire comes out is round...so I would GUESS that the antenna wire cover is only a starting point?

Great idea IMHO!
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  #267  
Old 11-30-2017, 10:29 AM
BMC_Dave BMC_Dave is offline
 
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Location: Seattle, WA
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Default

How does the flow past this shape compare to an angled cylinder, which is oval?
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  #268  
Old 11-30-2017, 10:57 AM
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fl-mike fl-mike is offline
 
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Default

Steve,
Keep in mind that the angled rod Cd is not that bad really.
It is an oval cross section.

https://goo.gl/images/nVXjCo
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  #269  
Old 11-30-2017, 11:05 AM
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Steve Melton Steve Melton is offline
 
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Default

may get 0.3 lb drag reduction with a fairing for typical cruse speed. while not a lot, it's something.

The AV-17 is designed specifically for mounting to the underside of an aircraft, providing an excellent radiation pattern for air-to-ground communications. It has a 4-bolt mounting base and a low profile configuration that makes it ideal for helicopters and low-wing aircraft. The antenna is designed to operate at speeds up to 350 mph and altitudes up to 50,000 ft. It has a drag force of 0.66 lb @ 250 mph.
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Steve Melton
Cincinnati, OH
RV-9A, Tip-up, Superior O-320, roller lifters, 160HP, WW 200RV, dual impulse slick mags, oil pressure = 65 psi, EGT = 1300F, flight hours = 900+ for all

Simplicity is the art in design.
I was born an airplane nut. I have no explanation for it.
My Artwork is freely given and published and cannot be patented.
www.rvplasticparts.com
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  #270  
Old 11-30-2017, 12:20 PM
BMC_Dave BMC_Dave is offline
 
Join Date: Mar 2017
Location: Seattle, WA
Posts: 288
Default

Quote:
Originally Posted by Steve Melton View Post
may get 0.3 lb drag reduction with a fairing for typical cruse speed. while not a lot, it's something.

The AV-17 is designed specifically for mounting to the underside of an aircraft, providing an excellent radiation pattern for air-to-ground communications. It has a 4-bolt mounting base and a low profile configuration that makes it ideal for helicopters and low-wing aircraft. The antenna is designed to operate at speeds up to 350 mph and altitudes up to 50,000 ft. It has a drag force of 0.66 lb @ 250 mph.
So you're expecting a 45% reduction in drag from your fairing, well at 250mph? How'd you get there?
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