Increase in acceleration

[n505at]

When accelerating prior to take off I notice a abrupt increase in acceleration (we used to call this positive 'jerk'). It happens at about 40-50 knots and the engine has reached about 2500 rpm (though I am not watching the MP or RPM at the time). Can anyone explain this?

I am still flying Phase 1, RV7A, Lyc.O-360, LASAR ignition, Hartzell blended airfoil prop, Jihostroj governor.

Roy Samuelson
Oakland CA

[Kevin Horton]

I can think of a couple of possible explanations, but I don't know how to figure out whether either is correct:

1. At low speed a significant portion of the prop blade may be stalled, as the angle of attack between the airfoil and the local airflow could be above the stalling AOA. But, the AOA of each prop blade section decreases as you accelerate, and the thrust will increase. Maybe a large portion of the prop is unstalling at that speed, and thus the thrust is increasing. I suspect this would be progressive change though, rather than a sudden change, so I doubt this is the explanation.

2. Or, maybe the governor is seeing that the rpm is rapidly approaching the set point (2700), and it starts increasing the pitch to avoid an overspeed. The increase in pitch could cause a thrust increase, depending on how much of the blade is stalled vs unstalled.

[kentb]

But I have the same effect on my IO-320 CS RV9A.
I just enjoy it.

Kent

[frankh]

I notice a slight surge every time I take off...I assumed it was the prop governor that was trying to find the sweet spot.

Of course I'm never looking at the panel when I take off so don't know for sure.

Incidently...I find my Vy speed to be 110kts

What have others found to be the best Vx speed?...At first glance the 7a doesn't appear to be that good at getting off of short strips (at 2500fpm i seriously doubt that!)...But of course if you have a high forward speed then your climb angle...Or your climb per nautical mile number will reduce.

Solution...Go slower, i.e point the nose up....Iwas departing a 2500 ft runway the other day with trees..No real problem cus I just adjusted the angle to miss the trees by 50 feet...But I'm sure there is a better technique.

Any inputs?

Frank IO360 C/s

[John Clark]

It's the prop govenor sensing the rapidly increasing RPM. Sometime, when you have plenty of runway, try advancing the throttle rather slowly and the "surge" won't happen.

John Clark
RV8 N18U "Sunshine"
IO360 A1B6 Hartzell CS
KSBA

[Low Pass]

Originally Posted by John Clark

It's the prop govenor sensing the rapidly increasing RPM. Sometime, when you have plenty of runway, try advancing the throttle rather slowly and the "surge" won't happen.

John Clark
RV8 N18U "Sunshine"
IO360 A1B6 Hartzell CS
KSBA

Yep. As already mentioned, it's almost certainly the prop regaining control after exceeding the set point.

IMVHO, I wouldn't drop the hammer so fast that your prop overspeeds and has to increase pitch rapidly to control rpm. I personally increase power until reaching ~2700, let the governor gain control then feed in the remainder of power. Will even do this on shorter runways, but will hold the brakes until I get 2400-2500 rpms. Then as it begins to roll, add power progressively making sure it catches at 2700.

2 cents

[n505at]

As advertised, pushing the power slowly eliminates the acceleration surge.

Thanks for the suggestions.

Roy Samuelson

[ScottSchmidt]

I flew a Pitts S2-B that did the exact same thing but it was the prop govenor just starting to react to increase in speed like everyone has mentioned. Upon acceleration I would get a snap of the plane to the left when it happened. Slower acceleration fixed the problem, but I have flown another Pitts that didn't do it near as bad.

Don't really know mechanically how the govenor works but you may want to try another one just to see if it is different.

[osxuser]

Could it be depending on what prop the Pitts had? I could see the composite prop reacting differently than a aluminum.

[Sam Buchanan]

Of course I'm never looking at the panel when I take off so don't know for sure.

Might not be a bad idea to give the panel a scan during the takeoff run! Otherwise how will you know if the engine is developing proper rpm (ever take off on one mag??) or if the oil pressure is still in the green. Or, is the airspeed indicator working properly? I also crosscheck airspeed with the LRI to make sure nothing funky is going on with its probe.

I expect to see 2270 rpm (FP prop) as the plane is ready to rotate. If I don't see 2270, or the airspeed indicator isn't alive (plugged pitot) I'm looking around to find out why and instantly go into "we may abort this takeoff" mode. I rather find out about a problem on the runway instead of 200' AGL over the weeds.