Temporary antenna mounting on Blackhawk Helicopter

[JMac01]

Hello,
I've been asked by some folks in the US Military to look into temporarily mounting an array of various antennae on the bottom of a Blackhawk helicopter. My company's background is in mechanical design and electronic integration. So I'm trying to find out exactly how much I don't know about mounting antennae (or any appendage) to the bottom of a helicopter or other smaller rotary wing aircraft.
The design goal is to produce a modular antenna array that mounts through and to the "hell-hole" hatch in the floor of a Blackhawk helicopter. The mechanical design of the attachment to this through-hole is straight-forward. The design of the external antennae mounts is where I'm looking for pointers. Specifically:

1. Will a low-profile, tubular structure underneath the aircraft for the various antenna to mount to be sufficient?
2. Are there specific design shapes or configurations to avoid?
3. Does anyone have experience with a similar application that might have used a different approach?
4. At what point does aerodynamic drag become a major factor?
5. Where can I go for more info on Air Worthiness Certification and what that process might incur in dollar cost to a project?

The antennae to be mounted to the base structure underneath the aircraft will be standard omni-directional antennae in both low-profile radome configuration as well as short "stick" type models. Nothing over 6-8" in length.

My basic feeling is that they shouldn't affect the aerodynamics any more than the skids of the helicopter and or other appendages to the vehicle. Operating at relatively low speeds (compared to commercial fixed-wing aircraft) seems to make the design somewhat easier. Is this the case?

Any insight would be greatly appreciated!

Thanks,

JMac

 

[BrianR]

The hellhole hatch on a Blackhawk is very close to the ground, (Iv'e climbed in and out of enough of them doing mods. Looking at a small aircraft sketch it looks like the ground is at wl 186 and the hatch is at wl 203 so that gives 17" clearance before u/c takeup (perhaps 6") and uneven ground (??") is considered so you will need to kep the attaching structure low or risk losing it on a heavy landing.

Aerodynamic drag is unlikely to be an issue - look at the size of wheels etc.

You will need to re-stress the hatch, hinges and attachments to verify they can carry the new load in the airload, vibration and shock environment.

You say you are using omni antennas and they probably need a ground plane. I would think that the hatch surface would be evffective as a ground plane down to 600MHz and maybe 400Mhz at reduced performance. Electrical bonding the hatch for RF will be essential.

I cannot comment on attachment methods not knowing antenna needs.

and probably

 

[blueskiesup]

I read you question and I have a few comments for you. Sorry its a month after the fact!

The biggest aerodynamic issue you need to contend with is mast bending as a result of increased drag at a moment arm which can cause an effect.

What??!?!?

The rotor mast will last the predicted lifetime in configurations Sikorsky created. It could fail early if the pilot routinely flies around with the cyclic farther forward than any other aircraft as a result of the increased drag.

To determine the effect, fly to Vne clean and mark the cyclic position, you can use a tape measure to the windscreen or something. Then fly dirty to the same cyclic position, that speed is the new Vne.

Control and stability margins can be investigated too, but I would guess they aren't an issue.

Check out AC29-2C for guidance from the FAA.

 

[EDCCorp]

I also had a review of your question and agree with what everyone has said so far. In addition to what is said, above, do a check on the hellhole panel stress induced due to a side air load on the antenna. I never though this would have much of an effect either until working on a similar project for an antenna that was 8" tall. Straight forward flight had no effect however maneuvering did have an effect on the panel stress and the stress the doubler was able to take. As far as the structure itself that you are proposing to use, again like the above answers it is hard to answer without seeing what you have designed. For certification purposes you can use the FAR 25 requirements as a guideline, but being a military rotorcraft, you will probably have to talk to the US Military and obtain a copy of their requirements as they will have their own certification requirements, unless of course the US Military say that using FAR 25 and obtaining an FAA approval will be acceptable. As far as cost for certification, it depends on what you have to provide, but the generation of substantiation documents by yourself on average should cost the US Military about $3000 to 5000 or so depending on the complexity and your charge out rate. As far as other costs, it depends on what certification the US Military would like you to obtain.

 

[Higgler]

I recently asked similar questions for generically mounting antennas to aircraft, here are some of the short responses.

Two things for you to check out:

Thread2-67807 and Forum247

this guy knows his antenna mounting stuff based on responses to questions;
Joseph K. Mooney
Director, Airframe Structures - FAA DER
Delta Engineering Corporation

The requirement to mount antenna is spelled out in AC 43-13-2A Acceptable methods, techniques, and practices, Revised 1977, chapter 3. If you need a copy the AC is located on the FAA web site or contact me I will send you a copy.



Kevin

PS: As an antenna engineer, most wide beamwidth antennas like you are using are tricky and are supposed to be mounted to an official test bracket, usually a large ground plane. If you mount them just anywhere (say on something that's not a large flat surface), they may perform very poorly in some parts of the frequency band, not to mention have dropouts in the antenna coverage patterns. It's tough to VSWR test them after installation on the bottom of a helo, the ground can give false readings of poor VSWR. Good luck.