Cowling Sucking In

Question.

I’ve noticed in the last couple hours of flight time, at cruise speed my cowling in my -4 is being sucked in 1/2 an inch or more. I initially thought that my cowling may just need to be reinforced, but someone just recently mentioned airflow as a culprit.

I have no heating issues in my 0-320. Oil and CHT are always good (right where you want them). I have a carburetor. I would have thought that pressure/vacuum issues would cause some difficult situations with my carb, and that has never been an issue. I do not have a pentium.

Thoughts?

X

x,
Not sure I understand how the cowling could be buckling inward. The internal pressure should be trying to inflate the cowling like a balloon. My similarly equipped -4's cowling bulges outward very slightly at the cowling cheek joints.

Alan

Xkuzme1 said:

I’ve noticed in the last couple hours of flight time, at cruise speed my cowling in my -4 is being sucked in 1/2 an inch or more.

Click to expand...

Tell us more. Got pictures? Sucked in is unlikely...buckling perhaps?

RV-4 cowl deforming inward

When I first started flying my 4 with the early gel coat cowl I would get a buckle inward on top of the cowl between the firewall and rear baffles. Vans attributed the phenomenon to low pressure (Vacuum) behind the rear baffle created by the cowling exit. In my case I added a 1/4” of foam sandwiched between a couple of layers of fiberglass cloth to stiffen the rear portion of the top cowl and enjoyed the excellent engine cooling this set-up provided.
Looking back on it I wish I would have rigged up a manometer to measure the pressure differences. In the early 90’s we weren’t as analytical as today, we were just happy to have good cylinder cooling as measured by our rotary switched Westach CHT gauge.
Duff

No vacuum in the lower cowl volume, which of course includes the volume between the accessory case and firewall.

Even vacuum is a poor descriptive. If the upper cowl surface is bowing in due to air pressure, it would be more correct to think of it simply as "external pressure exceeds the sum of internal pressure and panel stiffness". Both pressures are likely to be above freestream ambient pressure in the aft cowl region. Typical lower cowl internal pressure for an RV is 2" to 4" H2O higher than freestream. Bill Lane has the lowest I've ever seen, near freestream.

What vintage cowling?

The early polyester cowlings were prone to deformation and the drawings actually show a little support T made from tubing for the nose area near the alternator. The later cowls with the honeycomb have almost no flex at all. Are you visually seeing it in flight, or do you have wear issues internally that are showing up? make sure your baffles aren't flapping over internally which would be about the only thing I could imagine if you hadn't changed any other configurations. The solid polyester cowlings were often altered or repaired, and a disbonding could be showing up as a possibility.

The one area where it is sucking in towards the motor is on the RH side just center from the oil door. It only happens when I am in a high cruise speed... (above 155-160 kts).

I have a fiberglass cowl.