I am building a Onex and am currently installing Dynon avionics. The Onex is a very small airplane and there is not a lot of room for stuff, but Dynon allows for remote installation of the transponder and the com radio so that helps because not everything has to go behind the panel, where room is limited. I ended up installing the transponder behind the pilot seat and put the antenna close to the transponder. After I installed all of this, it occurred to me that the transponder antenna is about 6 inches from my rear end, and transponders put out a pretty good shot of electromagnetic energy. Does anyone know if this could be a potential health hazard? The com radio antenna is about 20 inches from my head as well, but I don’t think it puts out as much energy as the transponder.

While you are here, I have a couple of other random building questions. The diagram below shows the fuel system installation for a Rotax 912 engine. Item 7 is an inline fuel pump. It is suggested that a fuel line is to be run around the fuel pump with a check valve (item 8). I am assuming that this is to bypass the fuel pump in case if fails and blocks fuel flow. I have a facet inline fuel pump which will not block fuel flow if it fails, so I am assuming I do not need this bypass line with the check valve. Is there any other possible reason why this bypass line might be needed?

One other question. This Onex is a trigear and I increased the length of the nose gear strut by 3 inches to accommodate a bigger diameter prop which is needed for a Rotax engine. This changed the deck angle from zero to about 3-4 degrees nose up. Is this going to create any unforeseen problems? The alternative would have been to have new main gear fabricated that was taller to keep the deck angle at zero, which would be quite a bit of rigamarole. I am wondering if this will make it fly off the runway a bit early or if this effect will be negligible.

Personally, I am not keen about having a 200-250 watt microwave transmitter that close to my torso or head. Too much conflicting info in this regard ...

On your transponder antenna, it's mounted on a ground plane which will block the EM radiation from your jewels, I think. On the fuel line, those Facet pumps are "free flow" so the bypass line wouldn't be needed. Before flying, though, I would definitely perform a flow test with the pump off.

I'd move the TXP antenna for similar reasons as listed above. Now's the time to make any changes, and I'd be having similar doubts as you.

I would remove the fuel pump bypass line and it's associated hardware and verify engine max power fuel flows can be achieved through your system and the facet pump before flying.

I would ensure my trim tab is set neutral or a touch down on first flight. Definitely double check this with other Onex flyers. The last thing you want is a plane that is trying to jump into the air on it's maiden flight before the wing is really flying. This sort of issue is best handled by being aware of the potential issues and being ready for it.

OK, I'll step up to providing the dissenting opinion.

Facet fuel pumps have a failure mode which WILL block fuel flow. Spend a little time and do some in-depth research and you will find this information. The commonly-held opinion that Facet pumps are "fail safe" is correct for the majority of failures, however it is drastically incorrect for that one failure mode that blocks flow.

In my Glasair Sportsman I have a Facet pump installed downstream of the 4-way selector valve but upstream of the gascolator and engine-driven fuel pump. Having originally held the opinion that Facet fuel pumps were fail-safe, I proceeded down this merry path until a person with direct experience gently took me aside and set me straight. His engine quit on takeoff, thankfully with lots of runway left. Root cause was failure of the in-line Facet pump.

The check valve bypassing around the Facet pump is a necessity. I've used an Andair check valve featuring AN-6 nipples on each end. JEGS was a good source for some fancy AN-style fittings which allowed the pump and check valve to be plumbed in a minimum of space (Tees with a female on one end and a pair of males, as well as swept 90 degree elbows with a female on each end). I don't know what size fuel line you are using so this selection of hardware may not apply to your case.

Vis a vis the transponder, getting its antenna and coax away from your body is a good idea. Keep in mind that even RG400 coax leaks some energy at 1GHz. The official minimum safe distance for the transponder is actually quite small, however I don't think we truly understand the long-term effects of microwave exposure, even at low levels. If you can easily move the antenna, I would suggest you do so. Be aware that mechanical obstruction of the radiated signal path is the most common cause of ATC losing transponder replies. Gear legs, exhaust pipes as well as primary aircraft structure can cause a "blind spot". As a result I would suggest you give some thought to antenna location. The Onex doesn't have a lot of real estate available so this is one of those "think about it long and hard before boring holes" kinds of exercises.

As for your landing gear question, I suspect you will want to talk to people flying tri-gear Onex and Sonex aircraft. On first principles it would seem elevating the nose will lead to the wing always being closer to a takeoff angle of attack. This will undoubtedly cause an increase in takeoff distance since the wing will be producing excess drag from the moment of brake release. Keeping the angle of incidence nearer to zero for the initial takeoff roll helps reduce drag, thus shorten takeoff roll. You see this with taildraggers - as soon as the tail will fly, the tail comes up to produce a zero-lift angle of incidence on the wing - even the Valdez STOL folks do this!

Similarly, having the wing closer to a "flying" angle of incidence while the airplane is on its gear will likely make the airplane much more susceptible to upset from wind gusts etc. Most tri-gear designs get the wing down to a low- or no-lift angle of incidence as soon as the weight comes on the nose gear in order to "kill" the wing's ability to produce destabilizing lift. If the wing is producing lift, the airplane will be squirrely on the ground. Given the light weight of the Onex, one wants, as much as possible, to optimize for stable ground handling.

PS: I think the Onex looks severely cool sitting on its gear with the wings folded. Visions of an F4 Corsair come to mind!

Sorry - I should have paused before making my previous post.

I just took a second look at your photo. That's RG58 coax on the transponder antenna. Do yourself a major favor and replace it with RG400. RG58 has far poorer shielding than RG400. Installing RG400 will be better for you from a health perspective, and will bring the double-bonus of channeling more RF energy to the antenna rather than having it splatter around inside the airplane causing interference with other systems, particularly your headset audio.

"Tune" the antenna length using a proper (forward & reflected power measuring) watt meter or a grid dip meter.

Using a SWR meter to tune antenna length is kind'a like building fine cabinetry using dull axe.

These responses have been very helpful!

I am going to put install the bypass with the check valve just to cover the possibility that the Facet pump might block fuel flow, better safe than sorry.

I think I will move the transponder antenna further aft. I also am going to upgrade the transponder cable as Mark suggests. I was going to buy an EMF meter and test the EMF levels around the “jewels” in flight before moving it, but I might as well move it now. One advantage to the current location is the antenna is very close to the transponder which I believe is a good thing (minimize dB loss). One problem with moving it back is the transponder antenna is not supposed to be within 3 feet of the com antenna so I may have to keep moving the transponder antenna further back until I am out of the 3 foot range.

I did find this article about aviation transponder EMF hazards, but I am a bit too dumb to understand it. It talks about 30 and 80 watt transponders but I think mine is 250 watts. However the safe distances are measured in a few centimeters if I am understanding it correctly.

https://assets.publishing.service.gov.u ... Rpd031.pdf

The deck angle thing is an unknown (puts the "experiment" in "experimental"). As I get closer to flying, I am going to try to get some time in somebody’s trigear Sonex to prep myself for the Onex first flight. I could try to simulate the 3 degree deck angle by holding back pressure in the Sonex during the takeoff roll and see how it handles.

You guys worry too much. Having the antenna on the bottom of a metal fuselage is way better than having it just about anywhere in a plastic or fabric airplane with a limited ground plane. I agree with the comment about using better cable to reduce loss. LMR-240 is about the same diameter ad RG-58 and has about 0.08 dB loss per foot as opposed to RG-58 which is about 0.2 dB per foot.

You guys worry too much.

I would be very leery of lengthening the nose gear strut. Not only will you have an angle-of-incidence problem, but you would be changing the dynamics of the nose gear, possibly creating a shimmy problem. I don't understand why a Rotax engine needs a longer prop. I believe that a Rotax-powered RV-12 uses a longer, ground-adjustable (70"?) Sensenich prop, but I don't think it's mandatory. My engine is a Lycoming but my prop is only 62" in diameter. Yet, somehow, that 62x75 prop will easily pull me through the air at 200 mph.

There are plenty of tailwheel airplanes that takeoff and land just fine in a 3-point attitude. For a nosedragger, as long as the critical AOA minus the wing angle of incidence is more than the deck angle you should be fine. Have you ever seen a Piper Seneca loaded with checks?

I was thinking something similar...Once in a while I used to takeoff with an aft loaded J35 Bonanza where the nose strut would extend and it would be nose high and I don't think I even noticed any difference.

Which has a greater radiation impact, XPDR EMF around the jewels or sleeping next to someone who had a banana for dinner? My guess is that they are two completely different types of radiation.