There has been some discussion on AMRT start height errors going on in the US F5J Team Select thread, which isn't the right place for it. I'd like to continue this discussion in it's own thread. Some of this was in the other thread:

https://www.rcgroups.com/forums/show...lection/page19

One effect I have tested is the spinner yoke acts as a centrifugal pump in front of the firewall, and lowers the pressure in the fuselage. I have tested this on a Radian, which is the only glider I have with holes for air flow in the firewall.

My Radian is reasonably well sealed up with all the cooling inlets and the exhaust area on the bottom taped up, but of course it is foam which is somewhat porous, has pushrods, and a big canopy held on with two magnets. The stock Radian spinner is quite smooth on the back side, so I didn't expect it would do much. Running the motor increased the altitude reading by about 1.0m, so even it was trying to pump a little air out of the fuse.

I then substituted a spinner that is open on the back, with a more standard kind of yoke that runs in front of the firewall. This is similar to the Riesenauer, HyperSpinner, GM, etc. spinners. With the canopy unsealed, it increased the altitude reading by 5.8m. If I taped the canopy up, the reading increased by 7.8m. The yoke works as a centrifugal (I like centripetal better!) pump, pumping air out of the fuse. I couldn't run the yoke super close to the firewall because of the collet I had to use, and I suspect the closer the yoke is to the firewall, the larger the pressure drop in the fuse might be.

The results in the log below are without a prop on the yoke, so it is definitely a pumping effect, and not caused by airflow from the prop over the fuselage (thanks Joe, forgot to mention that).

If someone is running a firewall with vent holes, and a yoke style spinner, their altimeter could be reading 5m to 8m too high on every start altitude. Not a huge points difference perhaps, but it adds up. The points spread between first and second at the TS was 74 points after 18 rounds. After normalizing, that 3 to 4 points a round in increased start height due to pumping, might become 5 to 7 points a round.

F5J gliders do not need cooling flow in the fuse. The drag of any air going in through the front of the spinner and through the fuse will be very high - the insides of spinners and the fuse are not very aerodynamic, why do you want air flowing over that mess inside for the gliding portion of the flight? Dirt and dust can enter the fuselage from landings. I just don't see an upside to a ventilated firewall on an F5J glider with a 30 second max motor run.

An additional drawback of airflow through the firewall is that the warm air from the motor, ESC and battery flows aft, usually over the rest of the electronics. It was mentioned in the other thread that the temperature can increase of 20C above ambient in the electronics due to this heating. Without any airflow through the firewall, I do not see any heating of my electronics after motor shut down because there is no movement of the air aft. The motor still cools fine through the thin fuselage walls.

The easiest way to avoid all these issues is with a solid firewall.I think solid firewalls should be an F5J requirement, which would also prevent people trying to use the ram air effect to lower their start altitudes.

I've 3D printed a little button to fit in the "turbo" hole of my HyperSpinners, to prevent air from flowing in the front. Even with a solid firewall, in gliding flight any ram air entering the front of the spinner will be jetting out around the gap between the spinner and fuse and around the propeller blades, creating more turbulence and drag. A small thing, but easy to do.

Kevin

Interesting........possibly a 'tempest in a teapot' when all is said and done. I use a solid firewall in my F5J planes and the turbo spinner from Esprit with its single, allen screw retainer for the collet. The button over the screw seems worthwhile. My ESC and batts. are at about ambient temp. after a typical 20-25 sec. motor run. A little warmth can be felt through the fuse. over the motor but with 20-30 min. between flights in a typical contest there is no reason for elaborate cooling schemes.

I can imagine a spinner tweaked to push air in instead of pull it out. If the fuselage was tightly sealed. Even if the firewall is solid, it still has wires going through it. If you caulk the hole, I'm sure there are still pressure effects depending on where the holes to the outside are in the fuselage. I suppose this affect will be less, though. On the order of how far up you can go if you pull to straight up.

There are no wires through most "firewalls". The "firewall" is just behind the spinner in most cases. The motors are usually behind the "firewall", with only the motor/gearbox shaft protruding through to the spinner yoke. That hole is sealed by the gearbox or the front of the motor screwed tightly to the "firewall", unless there are additional holes cut through the "firewall" for cooling air.

Not really a firewall, but that is what they are commonly called. It is a motor mount mostly.

Yes, the "turbo" spinners with an opening in the front are always sold with the idea that they provide ram effect cooling air at higher pressure into the fuselage. When the motor is running, I highly doubt that is true. When the motor is off, it is just added drag you really don't need. There will definitely be pressure changes.

There may be some other pressure effects from leakage or holes in low pressure areas, but I think those are much smaller than the pumping of the spinner yoke.

Kevin

The F5J logger retains the highest reading from launch until 10 seconds after the motor is turned off. I suspect that air leakage would quickly neutralise any impact from the prop presurising the fuselage, certainly within 10 seconds and so the reading would not be seriously affected. To retain the pressurised readng you would need to lose height after turnoff at a rate at least equivalent to the presure equalization, in which case your final height will still be an accurate measurement of where you start the gliding phase.
The Europeans have been running these events almost every summer weekend for about 6 years, hundreds of flyers, many venues in most Euro countries. They are very competitive and also very suspicious of each other, if there were easy ways to cheat they would have been detected and addressed well before now.
This is a non issue.

I'm not sure how you figure that.

Imagine the prop yoke depressurizes the fuselage through a ventilated firewall, so the height at the top of the powered climb is 8m higher than actual height. The motor shuts off, and the glider goes into a normal descending glide for the next 10 seconds. All the readings after motor shutdown will be lower than artificially high peak at the top of the climb.

The AMRT will store the value of the top of the climb +8m. That isn't cheating anyone but yourself. If you want to record a start altitude that is too high, I'm sure your competitors would be OK with that.

Kevin

Kevin, I believe your test data and figure it's worth some time to eliminate 8m (or more with more powerful motors) of height penalty on every flight. Next plane I'll use a G10 motor mount. Am thinking some E6000 (Shoe Goo) may work to plug the vents in the aluminum mounts many of us are using...

I think it is an experiment that is worth replicating to validate empirically. I don't have any ventilated firewall; could somebody try to :

• with a ventilated firewall, monitor the ARMT altitude at stationnary with full motor

and report results here.

I am not convinced until the tests are made in flight and not static.
Only problem with that is how? Possibly put a fan in front of the fuselage while using the static test method? I wonder if the effect will be magnified or nullified?

Charlie

I am still mystified about the concept of the prop yoke "pumping" air out of the fuse. F3A guys use cutout spinners/backplates/firewalls to increase cooling airflow into the fuse.

Originally Posted by Silent-AV8R

I am still mystified about the concept of the prop yoke "pumping" air out of the fuse. F3A guys use cutout spinners/backplates/firewalls to increase cooling airflow into the fuse.

A centripetal (centrifugal) pump moves air by radially accelerating it with rotating vanes. This creates a low pressure at the centre of the vanes, which draws air in, and it expels the air radially at the tip of the vanes.

A motor with a yoke is an inefficient centrifugal pump. The yoke acts as the vanes, which radially accelerates the air, and expels it off the tips. There will be a low pressure area in the centre, which happens to be where the openings in the firewall are. It will create a low pressure area at the centre.

Kevin

OK, I guess what I learned about pumps operating and repairing them on nuclear submarines operated on a different theory than how you think a prop yoke "pump" works.

One issue with your model is there is no connection between the input point and the output. On most of the centrifugal pumps I have seen the flow enter at the center and is accelerated out the radius of the impeller which normally has slanted, and more efficient vanes.

I might believe that very little air actually gets into the fuse, but I am not willing to throw everything I know about how pumps work to believe that somehow the prop yoke is pulling air out of the fuse.

Originally Posted by cptsnoopy

I am not convinced until the tests are made in flight and not static.
Only problem with that is how? Possibly put a fan in front of the fuselage while using the static test method? I wonder if the effect will be magnified or nullified?

Charlie

Hand launch into a thermal with no props and then start the motor while flying at a relevant speed while maintaining altitude.

I've stayed quiet on this but can no longer. I simply do not see, or believe that in the same relative air that there will be an altitude reading difference affected by speed or how sealed the fuselage is. First, the speed is relatively minor. When the altitude logs after 10 seconds the speed will be dissipated considerably. Second, it would be difficult to seal a fuselage sufficiently to make a difference. Most have openings in various areas to prevent adequate sealing. If we were climbing to higher altitudes, going real fast or could really seal the fuselages maybe.

I think where the real issue occurs is the difference between brands of devices. Until independent evaluation occurs in real conditions of all the brands that is when we'll know. Otherwise all we can do is fly on.

For those of us who have been flying 5J for several years, there has been no obvious efforts at cheating, no chance taking because of the ability to restart. We take our altitudes and fly on.

There, I feel better.

Darwin N. Barrie
Chandler, AZ
Team Futaba

Well, an hypothesis has been laid, so experiments should be conducted to validate/invalidate it. Ideology doesn't belong to science, only evidence matters.

This is a nice little science project after all.