RT vs Owen Bell Cargo Pod

[aldoradave]

There is a good chance that I will be putting a pod on whatever 337 I buy (getting close). First off, does anyone have a really good idea as to the cruise speed penalty? Then, can anyone compare the pods offered by Owen Bell and RT Aerospace. Finally, is there any good bad or ugly in dealing with either of these companies?

Thanks


Dave Dillehay

PS On my way to my second pre buy at W29 in Maryland, should know Friday.

[Ernie Martin]

Dave, sorry if this sounds rude, but you asked the speed-penalty question 3 weeks ago and Herb answered, with links to several threads where the question has been answered before. Newer users should remember that the "Search" feature above should be used before posting questions -- it can get you quicker/instant answers and often provides a broader range of responses.

Ernie

[aldoradave]

Dear Ernie,

While I did mention the airspeed penalty (for which I have searched and found no definitive answer), the most important question, as the title suggests is a comparison of buying the mod from RT Aerospace or Owen Bell. I would still very much appreciate input on that question.

Dave Dillehay

[Hank Biesbroek]

Check Section 7 of your POH. 7-14 of the '69 POH states a loss of 3 MPH.

[aldoradave]

Dear Hank,

Thanks for bringing that up. Of course I thought that Cessna should have had some data on that, but since I don't have a Manual yet (should have tomorrow) I just wanted something in "concrete". That said, it looks like the pod is a $10,000 no brainer. I just want to know whether to buy it from Owen Bell or RT Aerospace.

Thanks


Dave Dillehay

[rhurt]

My POH for a T337G says the pod penalty is 3 kt airspeed, 50 fpm climb, and 1500' service ceiling. The service ceiling is a question in my mind though, since the plane is certified to 20,000' but I haven't found a service ceiling listed. I expect it would still do 20k with a pod, and probably 25k.

Does anyone know what the rules are about taking a plane past its certified ceiling? Does that make you a test pilot with void insurance?

I would buy the pod from Owen. I am biased though. He built the pod on my plane (337a), and one difference may be that he has an access door in the front of the pod for access to the front gear service panel. He's been doing fiberglass work for more than 30 years and has enough molds to make an O2 from scratch. You could even stop in Nashville and have him install it during your trip back.

[edasmus]

Dear rhurt,

I happen to have in my possession an old “Air Facts” magazine from September 1972 (equivalent to today’s “Flying Magazine”) that has a review of the 1973 T337G that was written by Leighton Collins (father of Richard Collins). In this extensive review, Leighton answers your question specifically. I will quote it verbatim. Before I do however, a humble opinion for your thought. Anytime an airplane is operated outside it’s certified parameters, the insurance company would most likely cry foul and the FAA would certainly cry foul. Ya all be careful out there. Nuff said!

Here is the answer to your question according to Leighton Collins for the 1973 T337G:

Why 20,000 feet?

“This 20,000 foot operational limitation is quite interesting. Limitation is not a good word because this suggests that maybe there is some weakness in the pressure vessel which determines the 20,000 foot figure. That isn’t the case at all. If you went above 20,000 feet the cabin would not overpressurize because the regular and back up valves would take care of that just like they do at the lower altitudes, maintaining and guarding the 3.35 psi differential. But the cabin altitude would go up and at an indicated altitude of about 12,000 feet cabin altitude a red light would come on indicating the need for supplemental oxygen. So the 20,000 foot limitation, as far as the pressurization system is concerned safety-wise really had nothing to do with pressurization structure. But when a manufacturer puts on a placard take him at his word, because if you don’t something is going to happen that you don’t like. In this case it is probably that the engines would overheat.

They settled on the 20,000 foot figure at Cessna for two reasons. The first is economic and the other will surprise you.

Turbocharged piston engines will usually deliver 75% power up to around 25,000 feet and in most of the pressurized piston engine aircraft up to now that was settled on as the limiting altitude mainly because it meant maximum cruise speed and above that altitude would start slowing down. But Cessna was shooting for pressurization at a price. They could have selected 25,000 for the 337G like they have for their larger pressurized airplanes, but to get a 10,000 foot cabin at that altitude would have meant a thicker skin cabin and heavier windshield and windows and more weight. Additionally engine cooling would have raised the weight and cost because even though it is sometimes unbelievably cold thin air just doesn’t carry away the heat from the cylinder head fins. All in all, if the 337G’s operational altitude were 25,000 feet, which it could have been as well as not, the price might easily have been another $25,000.


A Policy Decision

The other consideration in settling on the 20,000 feet was a very laudable one and rather unique. At Cessna they reasoned that this airplane would introduce high altitude operations to an entirely new group of pilots, one that might not be as aware of the operational facts of life at 25,000 feet as they might need to be. For instance, at 25,000 feet if cabin pressure is lost there is barely time to get an oxygen mask. Additionally, getting a clean airplane from 25,000 to 15,000 quickly requires some doing. As they figure it, anyone inadvertently depressurizing the cabin at 20,000 can make it to 15,000 in time. Or at least have gotten on the masks by then. They also gave a thought to when the airplane would get old and the door seals might not be maintained properly. In short, there’s got to be something between the bottom and the top rung of a ladder. We think the decision was a wise one.”


The End



Hope this helps. The article is 21 pages. This was one portion of it that relates directly to your question.

Ed Asmus

[skymstr02]

Originally Posted by rhurt

The service ceiling is a question in my mind though, since the plane is certified to 20,000' but I haven't found a service ceiling listed.

The definition, according to aircraft certification is:

Service ceiling. This is the altitude at which the rate of climb drops below 100 feet per minute. It marks the maximum practical altitude at which the aircraft can operate.

This is not a limitation imposed by the FAA.

[edasmus]

I agree with your definition of "service ceiling." I do believe though (however not 100% certain because I do not operate a P337) that the P337's (pressurized models) have a maximum "certified" ceiling of 20,000 feet. It would be a limitation imposed by Cessna and enforced by the FAA. This number would be lower than the "service ceiling" or "absolute ceiling." The certified ceiling and service or absolute ceilings have nothing to do with each other, I think....

[skymstr02]

The FAA does not have an altitude limitation on the airplane, per the type certificate data sheet.
The only limitation is 15,000 hours life limit on the windshield and side windows, and a 12,000 hour life limit on the ice light lens.
The information manual is not FAA approved, so any limitation that is not covered by a required placard is advisory in nature and not binding by regulation.

[rhurt]

Gentlemen,

Thank you for answering my certified ceiling question. It was a real issue for me last October when I wanted to cross an area of rain associated with a front and tops were up to 20k. I suppose I can get the entire 21 page article at the library.

I think your insurance money would be wasted above 20k and Leighton Collins would be right in chiding you for cutting the margin too thin - unless you were already wearing oxygen.

Honey Bunny is out at a scrapbooking party tonight, so I am able to get my T337G POH out of the closet and up by the computer without making her feel like I am ignoring her to spend time with my mistress.

I mis-quoted the book last night. The handbook says the pod reduces the Single Engine Service Ceiling by 1,500' and in a separate section lists the single engine service ceiling for varying OAT and weight. For example, SE service ceiling at 4600 lb and 0C is 16,700'. Any place in the table where the single engine service ceiling would be higher than 20k is marked with an asterisk denoting "altitudes that exceed the maximum operating altitude".

The pod could offer a dilemma for mountain flying if you wanted a Skymaster so you could maintain minimum IFR enroute altitudes on one engine, but the 1,500' penalty from the pod put you below them or even into the peaks.

Randy Hurt