N

n505at

Member
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
 
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.
 
Don't know why.

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

Kent
 
Yeah

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
 
Its the govenor

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
 
John Clark said:
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
Click to expand...
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
 
Last edited:
Update

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

Thanks for the suggestions.

Roy Samuelson
 
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.
 
Could it be depending on what prop the Pitts had? I could see the composite prop reacting differently than a aluminum.
 
Of course I'm never looking at the panel when I take off so don't know for sure.
Click to expand...

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.
 
Last edited:
True

But if the thing is not pulling like a freight train then its pretty clear something is wrong, besides the audible warning from the engine management system would also get your attention if you lost oil pressure or a CHT went high or something.

The only thing I look at is the ASI as its coming (or should be) up.

Just the way I do it.

Frank
 
Sam and others feeling the surge!

Sam, I am not flying yet so anything I say should be suspect...plus....on many occassions where I have had a tough build question you have always come through with the best advice......so now that sufficient accolade has been passed your way.....here comes the slam!!!! From all I have heard from all you flying/experienced RVators....Throttle in.. in the AIR...so how would one have time to scan the instruments.

Now that the dumb statement by yours truely is out of the way I must admit as I am in take off mode in the Cessna land-o-matic I fly I pay close attention to RPM/Oil Temp-pressure/air speed indicator. Luckly I have not had to turn off the burner and shut down as all indictaors have been the the green.

Frank @ SGU RV7A "NDY" $$$$ no longer a problem....the big money items are about to be ordered so any last minute advice about engine/EI/FI/Instruments/prop is welcome until wednesday when my check book will be about 60K down!!!!!!!!!!!!!! :eek:
 
Last edited:
But if the thing is not pulling like a freight train then its pretty clear something is wrong,
Click to expand...

I have departed the runway with one mag off (don't ask...it can happen if you get distracted during the runup...) and the RV-6 was still pulling like a freight train, but a train with 40 less rpm than normal. I rather get that resolved immediately instead of climbing out on one mag. In this case a quick glance showed the left mag toggle OFF and that was quickly resolved as rotation occurred and I saw rpm instantly jump up to the normal 2270. Without the pre-rotation check, how long would I have flown around on one mag before I figured out what was going on? Would I have been sharp enough to notice climb or cruise rpm was down 40??

From all I have heard from all you flying/experienced RVators....Throttle in.. in the AIR...so how would one have time to scan the instruments.
Click to expand...


Frank, in spite of the excellent acceleration of an RV there is still plenty of time to take a quick glance at a couple of critical instruments just prior to liftoff. I admit this might be difficult for the very low time RVer who is still chasing the plane, but once you accumulate some hours the takeoff "slows down", or your mental processes "catch up with the plane" to the point where you can definitely check things out prior to rotation.

I think it is important for us to train ourselves to make that final instrument check so we can make a good go-no go decision. This is just another aspect of getting our brains up to the same speed as the plane. :)

Thank you for your kind comments!
 
Last edited:
Sam Buchanan said:
Frank, in spite of the excellent acceleration of an RV there is still plenty of time to take a quick glance at a couple of critical instruments just prior to liftoff. I admit this might be difficult for the very low time RVer who is still chasing the plane, but once you accumulate some hours the takeoff "slows down", or your mental processes "catch up with the plane" to the point where you can definitely check things out prior to rotation.

I think it is important for us to train ourselves to make that final instrument check so we can make a good go-no go decision. This is just another aspect of getting our brains up to the same speed as the plane. :)

Thank you for your kind comments!
Click to expand...

Sam

True...so true.....which really makes that transition training to the RV important. When I made that first flight in the Cessna 182 from the Land-o-matic 150 I was way behind the plane. I can only imagine the problems I will have in my transition to the RV I am building. :confused:

Thanks to you Sam and to all who lend so much valuable info to our community. I consider all LIFE SAVING INFO!!!!!

Frank @ SGU RV7A "NDY"
 
n505at said:
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.
Click to expand...

It's probably a combination of factors...lots of things are changing at the same time. Here's a half-baked theory:

- The engine is accelerating.
- The airplane is accelerating, so the prop is unloading.
- Once the prop reaches 2700 RPM, the governor is ramping up the pitch to try to maintain that value.

In a powerful, lightweight airplane like an RV, the first two happen more or less on the same timescale. In a heavy airplane (say, a 172RG), the engine is done accelerating before the airplane has picked up much speed.

I suspect the governor is having trouble "keeping up" with all the changes. Or, to use more engineering-like terms, you've exceeded the bandwidth of the governor's control loop.
 
osxuser said:
Could it be depending on what prop the Pitts had? I could see the composite prop reacting differently than a aluminum.
Click to expand...

It most likely was. The S2-B had an aluminum Hartzell and the S2-C had a 3 blade composite. Alot less mass on the composite.
It did seem like there was a surge still.
 
You must log in or register to reply here.