Hole Reinforcement?

[bux]

I would like to get an STC for my Piper Cherokee. I want to hang a pod from the same location as the step. However, I want to run wire into the plane which requires that I drill a hole.

How do I calculate if reinforcement is necessary without analyzing the entire airplane for a hole that will have an insignificant effect on the structure (this has to be proven to the satisfaction of the FAA)? If it requires reinforcement, how do I calculate that? References on the subject?

This does not seem like a "Major Repair or Alteration" to me.

Thanks,
Rich

 

[Sparweb]

Hanging the pod on the aircraft is the real question. Removing the step and installing some kind of pod is a change to the aircraft's basic configuration. If the pod is even slightly bigger than the step, then you need an approval for the job.
How big is the pod? What's it for? Are there electronics inside? Is the wire connected to the aircraft electrical system? What happens if if faults?
You are required to evaluate these things, even if it's a "minor" modification, so as not to invalidate your certificate of airworthiness. Also keep in mind that unapproved mods could also invalidate your insurance.
For the hole issue, you need AC43.13, and a book on aircraft structural design, like Bruhn's Analysis and Design of Flight Vehicle Structures.
Changes to personal aircraft is a big can-of-worms that many owners avoid or just hide as long as they can.


STF

 

[bux]

Basically I am planning on housing a small camera inside the pod. The pod would be somewhat tear drop shaped with dimensions of 6"x5"x18". Given a weight of only a couple pounds, the estimated aerodynamic and inertial forces would be much smaller than the forces encountered by the structure when someone stands on the step. Also, the additional forces needed by the vertical stabilizer to counteract the pod would be insignificant (by my engineering judgement).

The hole would be to run control cables to the cockpit to be connected to a laptop. Therefore, there is no additional load or modification needed to the electrical system.

On another note, I have thought about simply removing the step because it is extra drag and weight. Is this considered a major repair or alteration?

Thanks,
Rich

 

[Sparweb]

I think you can consider the step as a piece of "equipment", hence you should look up the aircraft "minimum equipment list". I don't know if a Cherokee has a MEL. If the step is nothing more than a "J" bolted onto the side, it doesn't have much impact on emergency egress, either. (Sorry, haven't given a Cherokee a close look before, I work on bigger stuff). If the step is part of the door, then that's another consideration. The pod seems much larger than the step would be, so you have to consider aerodynamic loads in normal and icing conditions, at the aircraft's highest speed, or greatest yaw, the pod's total mass, etc., etc., etc...

A camera implies you want to do some business out of your airplane. Have you considered the possible ramifications in licensing/registration/insurance you need if that's so? I'm in Canada, so I'll be of limited help there if your plane's registration starts with "N".

Most officials I know, if they heard the phrase "bolt on a camera pod", would instantly expect you to have the installation approved, say, by a DER. Have you explored this route?

There are a dozen more questions that come up, even on a simple modification like this.
By the way, yes, the hole will probably need reinforcement. How? That's a question for an engineer who can look at the aircraft to answer.



STF

 

[MintJulep]

Ignoring the FAR's for the moment, you would want to drill the hole in a "low stress area". If the stress in that area after the hole is made doesn't exceed the fatigue limit for the material it is probably not a real problem.

Of course you would need an analysis of the entire structure to find the low stress area in the first place.

Perhaps you could find a previously approved mod that allows the drilling of holes. The installation of an anetnna or some such. You could then argue that a hole is a hole, and if the FAA allowed a hole in a certain location for some other reason running a wire through a hole in the same location wouldn't be a problem.

 

[Nigel]

This actually sounds like a simple modification that could be installed on your aircraft using a 337 field approval. You will need some DER approved data to do this.

Come on people we are talking about a PA28. We don't need to be concerning ourselves with fatigue issues here. A simple static strenght analysis and some weight and balance considerations is all we need here.

Rich, email me and we can discuss this, I will be happy to help get this fitted to your aircraft.

Nigel Waterhouse & Associates
Aeronautical Consulting Engineers

Transport Canada and F.A.A approval & certification of fixed and rotor wing aircraft alterations: Structures, Systems, Powerplants and electrical. FAA PMA, TC PDA.
n_a_waterhouse@hotmail.com