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  #81  
Old 03-05-2018, 10:10 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,572
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A few additional thoughts to consider......

The Rotax 912ULS fuel system is operating at a maximum pressure of ~7.5 PSI.

A simple rubber (vacuum style) hose pushed onto a hose barb with a couple of ribs would probably take at least 3-4 times that pressure before it would blow off. In the process of doing my own research on orifice sizes I attempted to pull the hoses from the dual barbed banjo fitting on one of the early versions of the VA-216 hose assemblies and couldn't. I had to cut them off.
Spec #'s always sound good (especially when part of an advertisement or infomercial) but if they are 400 X higher than what is needed for a specific application, what is the point (advertising hype is the only thing I can think of)?
My point is that in an application like this, there is no need for a hose assembly rated at 3000 PSI.
There is mention of uncertainty of the quality of the fire sleeve. Does uncertainty mean it is not good? No, it just means Steve and Tom don't know, so because of that think theirs is better, because they do know its specs. Fair enough.

Orifice size-
Van's has never specified the use of a particular size for the orifice.
The VA-216 production drawing specs the use of the dual barb banjo fitting, Rotax part # 956602, as supplied by Rotax with the engines.
The orifice size is what ever Rotax installs before delivering the part.

I confirmed that as best as we can tell, the .020/.021 orifice diameter is what has been supplied in the standard factory parts from Rotax since Van's first began development of the RV-12 more than 10 years ago.

It is now easy to understand why the Aircraft Specialty / Flightlines hose kit would help increase the fuel pressure in any given condition..... reducing the orifice diam. by 25% will significantly reduce the return flow. It wouldn't surprise my if it reduced it by 50%.
This could raise the potential for problems caused by operating in hot conditions. Especially when using auto fuel.

I can not explain why Rotax's documentation specifies a different diam. orifice than what they actually supply on their engines, but I will be looking into it.
It could be a documentation typo, or they could have purposely changed it to a larger diam. and didn't update the documentation. I intend to try and find out and I will report here what I find.

If you operate your engine with auto fuel and you are using one of the aftermarket hose kits, I would recommend caution as we head towards spring (here in North America anyway), and near the transition period of warmer temps and the dangers associated with winter blend fuel use in warm temperatures.
A large purpose of the return flow (which with the .020 orifice is about 1.5 GPH) is to mitigate fuel system vaporization problems. A secondary reason is to provide a flow path to bleed off fuel pressure in situations where the pressure builds because fuel expansion because of rising engine compartment temps after shut down in hot conditions (a build in fuel pressure could overcome the float valves and flood the carburetors).
__________________
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")

Last edited by rvbuilder2002 : 03-05-2018 at 10:17 AM.
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  #82  
Old 03-05-2018, 11:58 AM
Aircraft Specialty Aircraft Specialty is offline
 
Join Date: Jan 2012
Location: Oconomowoc, WI
Posts: 315
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Scott,

I can only assume that you didn't have time to read the document in its entirety as there are several items that need to be clarified.

1. First and foremost...... I am curious as to how you came so quickly to the conclusion that "It is now easy to understand why the Aircraft Specialty / Flightlines hose kit would help increase the fuel pressure in any given condition..... reducing the orifice diam. by 25% will significantly reduce the return flow. It wouldn't surprise my if it reduced it by 50%." -

As referenced in the document we published that was based on real world testing, we have a customer who has the new style VANS Stock Teflon fuel hose system. He is having low fuel pressure issues. We replaced ONLY the VA-216 hose assembly with a new VA-216 that we built with our external restrictor. It very minimally raised his fuel pressure and did NOT solve his problem. This is how we determined that the restrictor size and location does NOT appear to be the issue with those who are having low fuel pressure. In short, the smaller restrictor does NOT raise the fuel pressure appreciably. It is something else.

2. Our hose kit is not new and has been flying on a variety of aircraft worldwide for over 4 years in many different climates and different fuel blends with NO reported fuel pressure/vaporization issues. In addition, EVERY aircraft that was reporting fuel pressure issues with the stock kit that has made the replacement has had the fuel pressure issue appear to be rectified. In addition, those with our fuel system report more stable pressures than with the stock kit.

Months ago, you advocated testing our fuel system out on aircraft with low fuel pressure issues to see if they made a change. You stated that if there were positive results, that would be something to look into and perhaps be able to be used to solve the issues that some of your customers are having with their stock kits. Well, real world testing has shown that it makes a difference. If you are serious about trying to isolate the cause of the low fuel pressure issues that exist on some aircraft with the stock fuel system, it's in your hands now.


3. Regarding orifice size...... if Rotax decides to change the manual, and specifies a larger restrictor....it is VERY easy to drill our restrictor hole out to a larger diameter. If the information that has been published in the manual since we started building these kits is correct and Rotax decides to change their actual restrictor over to match what their manual shows, then that will be a more complicated process to fix the internal restrictor.

4. Firesleeve- Perhaps you can answer the question for us. Is the VA-216 utilizing AS1072 firesleeve? You are correct that we don't know if it is or not.

Here is an excerpt found online regarding that:

"SAE AS1072 Aviation Firesleeve. Our firesleeve meets the specifications of SAE AS1072E, allowing qualified hose assemblies to pass the fire resistance testing specification of AS1055D. It may be used for all fuel, oil, hydraulic, fire extinguisher and prop feathering lines."

5. You mention the 3,000 psi hoses and the fact that they are overkill. When we designed this kit, we were replacing a RUBBER fuel hose system. We saved over a pound of weight over the stock rubber system, put 3,000 psi hoses on it, had a much higher quality assembly and no 5 year replacement. In addition, the cost of our kit was very similar to stock rubber replacement hoses. That makes a lot of sense to me.

Now, the current RV-12 fuel system that Vans is using utilizes 101-4 hose (a 1,000 psi working rated pressure), and what appears to be an AFLEX hose assembly for the VA-216 hose (a 3,000 psi working rated pressure) There is no disputing that a TEFLON hose is much higher quality than the rubber hoses used originally. By nature Teflon braided hoses that are being used are going to be in a working rated pressure for these sizes of 1,000-3,000 psi. And you can make the statement that it is overkill, but the fact remains that Vans is doing the exact same thing. You are utilizing a multi thousand pound pressure hose on a 7.5 psi fuel system.

A few questions for you...

1. What are your honest thoughts on having a rubber hose with NO primary retention on the hose to hold the fitting in? I understand that you pulled on by hand and were unable to yank it out. There are specific proof tests for hydrostatic hose testing, and that is not one of them.

2. What are your thoughts on utilizing barbed fittings designed for rubber hoses in a Teflon hose assembly with a very thin wall versus a fitting with a smooth stem that is designed specifically for Teflon hoses? (As seen in the dissected va-216 hoses)

3. You state "In the process of doing my own research on orifice sizes I attempted to pull the hoses from the dual barbed banjo fitting on one of the early versions of the VA-216 hose assemblies and couldn't." - Why were you doing research on orifice sizes when Rotax has always provided the orifices?

4. Even though it is a minimal amount of fuel going through the return line, why is non conductive Teflon hose being utilized versus a conductive Teflon fuel line which is standard for aviation fuel and oil lines?

Scott,

This process has in NO way been intended to be an infomercial. Tom and I genuinely love doing this. We love designing products and working with testers to continuously improve them. We have personally spent thousands of dollars on this testing in order to get the information that people have asked us for.

Vans builds an incredible airplane. I love my RV-10. The RV-12 is an aircraft that has absolutely revolutionized the LSA market.

We offer options to those builders who want them. While we realize that a lot of customers want to build a completely stock airplane, others don't. We provide solutions for those that want to make changes that they feel improve their aircraft.

At the end of the day, this is about making a better product for everyone. You have said that work is ongoing at Vans to solve this fuel pressure issue. We didn't create an issue that didn't exist in order to try to sell something. We designed a product at the request of rv-12 owners, and a side effect happened to be that it appeared to rectify an ongoing problem.

You asked us to test our kit on aircraft with the low fuel pressure issue. Just because the results may not be what you expected doesn't mean that they should be dismissed or denied. As far as we are concerned, our testing is complete. We have a final version of our kit that works well and owners are very satisfied with. Should additional information come out in the future that warrants a change to the kit, we will do that.

Tom and I are open and available for discussion if you have any questions regarding our testing. If it can help Vans solve this elusive issue and also put it to bed, we are happy to be a part of the solution.

Happy Building and Flying
Tom and Steve
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Please email us at aircraftspecialty@gmail.com with any questions.

Your Source for Custom CNC parts, CNC cut instrument panels and labeling, Teflon fuel/oil hoses, Headsets, and many other items for Experimental Aircraft.

Last edited by Aircraft Specialty : 03-05-2018 at 12:18 PM. Reason: Punctuation change
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