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low pressure area at the lower rudder fairing?

KayS

Well Known Member
Hi All,

hope everybody survived excess caloric intake and exhausting family members at christmas. here comes a question to the guys with some knowledge on aerodynamics:

is the tail end of the rudder's lower fiberglass fairing on an RV7 (where most RV's have their tail light) a low pressure area? where the pressure is lower than... let's say the cabin?

so if one runs a tube from the cabin to the tail that sticks out directly at the tail light, any fluid that enters the tube in the cabin gets sucked out in the air. without painting parts of the aircraft yellow for example.

Cheers
Kay
 
If you run the tube down the pilot side gear leg and exit out of the trailing edge of the wheel pant, the run is shorter and gravity helps :D
 
Terry: had that in mind but i'm afraid that the propellers slipstream is still effective in that area and the yellow stuff will be all over the place.
 
Most relief tubes I've seen have a Venturi at the exit to facilitate the suck. Also long runs of tubing would certainly be harder to maintain
 
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I would be afraid that a long run like that would allow fluid to pool in the tube and get smelly real quick
 
aerodynamics

Humm, a bottom mounted antenna that would serve that function could easily be designed. The gear leg is mentioned. Possible.

All of the trainer aircraft of WW2 that I am familiar with such as PT-19's, BT-13's, etc had that comfort feature. The exit was a small venturi device.

The cockpit of a low wing a/c typically has a quite low pressure, so the boys long ago made sure the flow would be in the right direction by the use of the little venturi. It might be necessary in an RV as well. There is no generally low pressure area below or behind an RV that comes to mind. Pressure on the fuselage surface more or less increases as you go aft from the wing.

I speculate that the exit would need be well away from any following surface and would need an augmenting feature to lower the pressure.
Ron
 
The trailing edge of the rudder should hopefully have some pressure recovery, so possibly comparable or slightly higher than cabin pressure. Even so, the 'slip stream' flow would still probably carry any liquid away efficiently. Probably want to test with water first.

In addition to the gravity pooling, another concern for such a long run of tubing is that it can freeze, plugging the tube. I had that happen in a glider once while descending out of wave. Was still at 20,000 ft or so.

Many glider guys now have a 12" tube that can be extended down through the belly through a rubber grommet. They retract the tube when not in use, and extend it for use. You are right that the propwash would make a mess of that idea.
 
thanks for the hints about freezing. a tube blocked by frozen liquid is probably not so cool when you wanna use it. solution could be to flush the system with a small amount of antifreeze, the stuff we use in our cars, prior actual use. pee entering the tube has around 98 °F, that temperature should do the job then.

as some mentioned, the trailing edge of the rudder or rudder fairing is not optimal. i'm going to try to exit the tube somewhere at the tail wheel. what could be an augmenting feature at the exit?
 
thanks for the hints about freezing. a tube blocked by frozen liquid is probably not so cool when you wanna use it. solution could be to flush the system with a small amount of antifreeze, the stuff we use in our cars, prior actual use. pee entering the tube has around 98 °F, that temperature should do the job then.

as some mentioned, the trailing edge of the rudder or rudder fairing is not optimal. i'm going to try to exit the tube somewhere at the tail wheel. what could be an augmenting feature at the exit?

NO, the freezing will happen in real time. My tube was only 2 ft long, with a funnel to pee into. About half way through, the funnel filled up because the tube stopped draining. I had to hold the funnel until I got to a lower altitude where it was warmer, and then it drained.

Do not exit the tube upstream of the rudder hinge. I have seen severe corrosion issues where liquid runs back on the underside of the fuselage and gets sucked up into the rudder cove, resulting in corrosion, all the way to the upper hinge. (again, this was on a glider).
 

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Decades ago, we took about 25 NASA flight controllers on a trip in the KC-135 Vomit Comet (not to fly parabolas, it was a transport trip). The airplane was configured with just that many seats in the very back of the cabin, and the the rest was basically gym mats. There was a very rudimentary Lavatory in the rear, with a urinal that went to a hose out the aft stinger of the airplane.

Probably due to the early morning nature of the flight, there was more use of the Lav than usual for the airplane, and the urinal started to back up - the crew determined that the heater on the stinger was failed, and the thing had frozen up. Yup - it happens!

Fortunately, the fact that we had 25 of NASA’s top “failure is not an option” flight controllers on board saved the day. The airplane had heater ducts running the length of the cabin, and there were bins of spare parts on board because anytime the airplane left the vicinity of Houston, they loaded to be self-sufficient. Before you know it, hoses had been rigged to route hot air from the ducts to the appropriate locations to begin a thaw. There might have been electrical drawings spread out on the floor in an attempt to recover the heater as well .... the memory fades after all that time.

What I do remember is that we recovered the Lav capability, and I personally decided that I’d always just carry gelling “piddle packs” in my airplanes - they work great with no trouble! And yes - adult diapers for an all-day exercise in a pressure suit.....

Paul
 
As a person upon whom TURP was committed nearly four months ago, I feel I am fully qualified to answer the question: "What's the easiest way to provide for 'creature comforts' while airborne?"

Well.... Depends...


:D:D:D
 
Something like this ?

here we go! if i would be an boeing/airbus engineer instructed to design a fuel dumping system i wouldn't want to rely on eletrical power (pump) to dump fuel overboard in case of an emergency. so i guess this is a nice low pressure area. and the relief tube could run inside wing conduit. super simple! and it should be far away enough from the prop wash.

will try and report back.
 
thought i give an update... after some further analysing and testing over the last year, i came to the conclusion that (at least in an RV7) it is basically impossible to route any body fluids from the cabin to the outside world without the risk to make a big mess.

things like freezing of tubes, higher air pressure at the tube outlet, dihedral of the wings and the high likelihood to paint the aircrfat yellow in flight, work against that otherwise great idea.

what's left is the option to leave the fluid in the cabin, in some sort of container intended for this purpose.

does anyone installed something similar in his RV? pictures maybe...?
 
Probably the best solution. The Air Force uses something similar on long-haul flights in fighter type aircraft.
 

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it is basically impossible to route any body fluids from the cabin to the outside world without the risk to make a big mess.
Thanks for your experimenting, sure helps the community ;)

left is the option to leave the fluid in the cabin, in some sort of container intended for this purpose

- Soda bottle (be sure to check the neck diameter, empty before 1st use), or
- Little John (add gasket to insure no leak, or refrain from negative Gs), or
- Travel John (unfortunately not reusable)
 

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I know of two RV-4's with relief tubes. Both routed the tubing down the tailspring, as far aft as possible, and work as expected. I've not heard of any flow issues or freezing events but they are in the warm southwest and probably don't fly in the temps with which you guys are concerned.

Building one has been on my list for a while because I really enjoy the big mess I make with the Gatorade bottle. Apparently an inability to loop up and then back down during use. A second option that I've heard being used is an 'external catheter' and bag (situated down near your ankle).
 
I would think if the discharge of said liquids were at the tail end of a main gear fairing, none would get on the aircraft when used in flight.

Steve, what say you?

Now, if you were taxiing propwash might be a problem...
 
...
- Travel John (unfortunately not reusable)

I can confirm that the travel john works great. As someone that likes to stay hydrated, anything more than an hour flight I'm probably going to use one of these. They say that you can use them multiple times until "full", but I have not tested this feature. Thanks for the reminder - I have a trip coming up, need to make sure I've got a good supply!

There's an RV-4 I know of that has a relief system with a hose to the tailwheel - it was used a lot when the owner and his wife flew a transatlantic trip. Not sure how they dealt with possible freezing...
 
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I can confirm that the travel john works great. As someone that likes to stay hydrated, anything more than an hour flight I'm probably going to use one of these. They say that you can use them multiple times until "full", but I have not tested this feature. Thanks for the reminder - I have a trip coming up, need to make sure I've got a good supply!

Multiple uses of a single bag (until full) works fine - I have utilized this technique many times as I criss-crossed this big continent. Beats descending out of a smooth tailwind ride at 14k+ into the bumps and swelter for a “natural break”, then having to claw your way back up there with oil temps at redline!
 
Had a conversation with a long distance ferry pilot (Trans Atlantic). His solution was to carry a few large sized Zip Lock bags. Freezer quality as they are stronger material & large because the Reservoir portion would hang lower reducing any potential for splash back.
 
Zip Lock bags. Freezer quality
Good one Ralph, thanks.
Gotta try this as: very light, extended capacity, reusable (conditional), and compact :)

My longest non-stop RV trip yet stands at 7:20, without any relief, but no 2 days are the same ;)
 
here comes a short update on the subject...

after a lot of research/testing and feedback from Vans, i finally came to the conclusion that it is basically impossible to route body fluid off airframe into the outside world during flight. at least that applies for an RV7.

several factors like high pressure areas around the aircraft structure, low pressure in the cabin, freezing tubes going uphill, wing dihedral, propeller slipstream etc. complicate the matter a lot.

meanwhile i figured that travel john works great.

thank you for the inputs.
 
Soil moist

I'm suprised no one uses this stuff.
Soil Moist JCD-05SM 8-Ounce Granules - 100047002 https://a.co/d/7cMRfUP
A couple teasooons in a baggie or Gatorade bottle would secure the waste from spillage. Cost almost nothing.
Amazing stuff. Makes an awesome hydrator for my humidor too.
 
I suddenly feel the need to lobby for creating a TFR over my immediate vicinity......does anyone have the number to the Secret Service?


OK...here's a follow-up scientific question....
on an average humid Florida day, how much altitude would be required for said discharge from this tube to become virga and not rain?
 
OK...here's a follow-up scientific question....
on an average humid Florida day, how much altitude would be required for said discharge from this tube to become virga and not rain?

Since, in my experience, the relative humidity in Florida is always 100%, it will never evaporate.
 
Piece o' cake!

You can buy the WW2 parts at ACS - it seems someone found the molds for the WW2 setup and those parts are new and ready to go. Bennair; Dunnatt. 3/8" tube - I used silicon tube and 3/8" aluminum tubing. In trying to figure about the flow rate, I checked flow using a std 1qt water bottle - I doubt any human will need more flow - there is plenty of vacuum. I put the venturi under the H Stab, attached to the L side of the fuselage.

I have not ported one to the aft seat - yet. I'll guess that would need a 3/8" plastic valve L /R to work correctly.

I have not tried it on any long trips yet, but I'll bet it will fit the need.
 
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