pcm

My Feedback: (13)

After your plane takes off and your engine unloads what do you guys figure for additional RPM`s.

As a rule of thumb I always figured an extra 1000 but I have no idea if this is correct or even close.

edge_fanatic

My Feedback: (4)

But I'd have figured a few hundred, maybe three hundred....If 1000, then a YS 140L at 8000 on the ground would be in danger in the air, if I recall their specs correctly.

I'd also expect that the difference will vary a bit based on temp and altitude (unless the relationship between tem, altitude and RPM is linear...which I doubt).

But I am guessing...heh

rsieminski

Based on prop and airframe too.

pcm

My Feedback: (13)

Hmmmm. 300 sounds low to me I guess. I know my big gas engines for example my 5.8. When on the ground I can tach 6,900 rpm with a 26x10 and you can hear the engine pulling.

When I get in the air it sounds much crisper and the prop really starts to bark. That is why I was wondering if a 1000 was close. Seems like if it was within 300 or so it would almost sound the same on the ground.

But maybe a few hundred would make the difference?

Reason I am interested in this is because between all the fun flys I have been too I have heard from no difference to 1,500.

i4c

pcm

We are currently testing our pre-production onboard tachs. One of the test planes is the new Hanger 9 Alfa trainer. This plane has a 45 size engine. For a more realistic test, we removed the three blade prop and installed a APC 11-6. Turns 11,510rpm static. Unloads to 12,900 in the air with a slight 10 degree dive. This tach holds peak rpm in ram memory untill the receiver is turned on/off again. Also shows your receiver battery voltage to the 1/100 of a volt. Weighs approx.1 1/2 oz. Info and pics will be posted on our website in a couple of weeks. Ask any questions.

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Rod Johnson

pcm

My Feedback: (13)

Thanks. I will keep an eye on your site. 1,390 pretty good gain in RPM.

seanychen

My Feedback: (4)

The unload amount depends on the engine size and its rpm range. If it's a .40 peaking at 14000 on ground, it can probably unload 1000. If it's 1.60 peaking at 9000 on ground, it will unload at most 300~400. I cannot hear that much difference in pitch on my 1.60 in the air at full throttle. Doppler's effect will throw off my sense of pitch.

OhD

It will unload if you are in a dive or flying at full throttle on the level but it will load down to something less than you get on the ground when you load it in a vertical climb or large loop.

Getting the peak rpm is interesting but not useful for selecting props. You need a data logger to continuously sample the rpm through your maneuvers. You'd also like to have airspeed too to select the best prop.

FlooredCOBRA

I know on my FIA GRRT .40 i can tell a huge difference when it unloads in the air.

On ground it tac's at 24,100 and in air i have no idea but it literally sounds like it is about to rip apart or like a power-band kicking in.

I would be curious as to how much it does unload in air compared to ground.

i4c

FC
One model of our new tach reads to 40,000rpm at 100rpm resolution. It is currently being tested by Sid Clement (two hundred mph club on props) and we will have his unloaded results any day now as the weather lets him test. He will post his results here on RCU probably on the 200mph thread. We have tested the 20,000rpm tach on planes and it works great. The IMS show is in Pasadina, Ca. next week (17-19) and we have a new display showing these tachs. If you are there, please drop by and check it out.

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Rod