M

mlwynn

Well Known Member
I am about to do fuel system testing. Can anyone tell me, at about what angle does an RV 8 climbs at Vx? I asked Van's. They were useless. There is another thread on the subject, but no one really ever said. An estimate of descent angle would be helpful, also.

Thanks,

Michael Wynn
RV 8 Finishing
San Ramon, CA
 
Um, you must realize the variables - empty weight, W&B condition, engine/prop combo, DA conditions.

And descent angle...what do you mean - steepest possible? Shallowest possible? Flight path, or aircraft pitch angle? Again, variables. Curious why the concern about fuel?

There is no answer to your question other than flight testing your specific airplane under varying conditions.
 
Last edited:
mlwynn said:
I am about to do fuel system testing. Can anyone tell me, at about what angle does an RV 8 climbs at Vx? I asked Van's. They were useless. There is another thread on the subject, but no one really ever said. An estimate of descent angle would be helpful, also.

Thanks,

Michael Wynn
RV 8 Finishing
San Ramon, CA
Click to expand...

According to FAA Advisory Circular 90-89A:

"First, place the aircraft?s nose at an
angle 5 degrees above the highest anticipated climb
angle. The easiest and safest way to do this with
a conventional gear aircraft is to dig a hole and place
the aircraft?s tail in it. For a nose gear aircraft, build
a ramp to raise the nose gear to the proper angle.

(2) Make sure the aircraft is tied-down and
chocked. With minimum fuel in the tanks, disconnect
the fuel line to carburetor. The fuel flow with a gravity
flow system should be 150 percent of the fuel
consumption of the engine at full throttle. With a
fuel system that is pressurized, the fuel flow should
be at least 125 percent. When the fuel stops flowing,
the remaining fuel is the ??unusable fuel?? quantity."

The anticipate climb angle is computed as the inverse sine of (Max climb rate/Vx) (all units in feet per second).

For example, if at best angle of climb your climb rate is 2000 ft/min and your air speed is 72 knots (7200 ft/min), the angle of climb is (sin)-1 (2000/7200) = 16 degrees. Add in the 5 degrees from the above reference, and you need a 21 degree deck angle.

This angle is difficult to achieve on the ground, and I have used a technique where I disconnected the fuel line fitting at the carb/injector, and elevated it to add enough height to make it equivalent to the deck angle required. Check with your DARling.
 
Vx angle

My best angle so far on a cold day, light weight was about 45* deck angle.

Just put the tail in a ditch and check at normal deck angles of 15* or so. Later on during testing at altitude go to the max.
 
Angle for Vx

Thanks for the replies. I understand the variables involved. This all started with the EAA webinare on fuel system testing. I was trying to get a feeling for how deep a hole to dig, how much to raise and lower the nose, etc. The EAA said to check with my kit manufacturer for the angles of departure and arrival. Vans really wouldn't say.

The place where this would be critical is on a missed approach with low fuel. As I understand it, I should know how much fuel I need to have in a tank to be able to depart at Vx and be assured of adequate fuel flow. I think the statistics bear that many first flight crashes are due to insufficent fuel flow. I wanted to test and document mine as I do not want to be part of that statistic. It's all about safety.

From the above, looks like 21-25 degrees up should be adequate for pre-flight testing. I'll report back how it goes.

Regards,

Michael Wynn
 
Here is the response I got from Van's:

[I]Bear in mind, this is an aerobtic airplane so the max angles are 90deg.
Sustained climb or descent deck angles are not something we have
measured - it depends on weight/hp/type of prop etc which can vary a lot.

The best guess would be to use about 30 degrees, that seems reasonable
for test purposes. Typically, we put the front wheels on a 6" block and leave
the tailwheel on the ground, but I don't know what angle that is. What have
other RV guys in Canada used for this test - it must have been done before?

Vans[/I]

Well the 6 inch block was a bit of a joke as I ended up pulling the plane up about a 5 foot hill we happen to have in our back yard. This plus the hole the tailwheel is in gave us 30 degrees. It was pretty steep and I'll sure be impressed if the plane will sustain this angle of climb. The fuel flow tests past easily.



 
When I see pictures like this, I get very concerned. It can be very dangerous to elevate heavy aircraft. Damage to the aircraft or people can result.

The test procedures were written in era where climb angles were 5 degrees, now we have aircraft that can climb at 20 degree angles. I have passed my concerns on to the authorities along with my recommendations for safe testing.

The purpose of the test is to ensure adequate fuel flow at a reasonably expected climb angle. The test results need to be 125% of the maximum engine requirement, yet we routinely get 200 to 300% from our electric boost pumps.

It is perfectly reasonable to put the aircraft in a nominal tail-down configuration and elevate the carb fuel fitting to the additional computed angle and run the test. This tests the fuel pickup, fuel lines, pumps, filters and fittings-- which is the whole purpose of the test.

To risk the aircraft or persons' health to follow obsolete prescriptive tests is absurd, in my opinion.
 
You must log in or register to reply here.