C172 Trainer Porpoising at higher RPM

[Tom_G_2010]

Something new has just started happening with my A2A 172. It is porpoising at higher RPMs! Actually it's more like riding a mechanical bull.

I was always taught when coming out of a climb to leave the throttle full until I reach cruise speed and then adjust throttle and trim for level cruise. That's how I've been flying the A2A 172 for quite some time now (and in the limited hours I have in a real 172 how I've done it there as well). During the past several flights with my A2A 172 it has started porpoising at higher RPMs. I have swapped back and forth between the A2A, another Add On and a stock aircraft and the only one porpoising is the A2A.

I read in some older forum posts that there was an AP bug that did that but, it was fixed and, I'm not using the AP. Any thoughts?

[Lewis - A2A]

Hello,

do you have your hands on the yoke or are you trying to fly just on the trim? Movement with power changes is how it should be requiring yoke input and then trimming for the change. Of course weather can then also play a part.

thanks,
Lewis

[Oracle427]

Thermals, windshear, turbulent air will all cause the aircraft to leave stable flight. If you are flying hands off this means that you need to correct the flight path using the yoke. This is perfectly normal behavior fora light aircraft.

I guess you are seeing the aircraft performing phugoid oscillations. These occur when the aircraft pitch changes either up or down and no control inputs are made from that point onwards.

If the aircraft is pitched up it will climb and slow down. As the lift generated by the wings diminishes at the lower airspeed, the aircraft will pitch down. The aircraft will now descend and gain airspeed and the wings will begin to generate more lift. The aircraft will then pitch up and the prices will repeat.

In certificated light aircraft that are designed to be dynamically stable, the phugoids will dampen and the aircraft will eventually level out after several oscillations. This assumes no other disturbances are made to the aircraft.

In real life, the atmosphere is rarely perfectly stable and it is not reasonable to expect the aircraft to fly straight and level without constant corrections by the pilot. True hands free flight, for more than several seconds, can only be accomplished with the use of an autopilot.

Lewis also raises a good point about not using the trim as a primary control. Maintain the aircraft flight path with the yoke and rudder and use the trim to relieve the control inputs. It is not good practice to release the controls and then try to trim the plane to maintain a flight path. Hold the controls while adjusting the trim until you can feel that you aren't applying force to the controls, then keep your hands on the controls unless the AP is engaged.

[Tom_G_2010]

Flying Hands on and doing my best to correct for the oscillations. I thought it might be weather as well but, would that not also affect the other two aircraft I tested with?

Being in a climb right after take off I'm hands on all the way. I'm adjusting my trim for a proper rate of climb and it's fairly stable as long as it's climbing at a decent rate. As I reach my cruise altitude I am lower the nose (with the yoke) to stay at my cruise altitude and waiting for air speed to build closer to cruise before touching the trim. When my speed is up (and by this point I'm wrestling the oscillations) I bring back the throttle and trim for cruise.

It's always a bit more work with the yoke to control any aircraft at full power but this seems way beyond normal. I'm having to pull back on the power quite a bit more than normal to get stable flight where I can trim and maintain cruise.

[Oracle427]

I can't speak for the other aircraft, but the A2A 172 is a very realistic representation of the RW behavior.

Flying Hands on and doing my best to correct for the oscillations.

You will need to keep practicing to address the above.

If you are positively controlling the aircraft using the yoke, you should have no oscillations. A pilot needs to make many early and small corrections. If you hesitate to correct a pitch, roll, heading, altitude excursion then you quickly allow the magnitude of the problem to increase and increase the difficulty in correcting the problem. If you notice the nose pitch up 2-3 degrees, put it right back where it was right away using the yoke. Don't let the aircraft's nose wander around. Place it where you need it and keep it there using the yoke and rudder.

It takes a fair amount of forward pressure to keep a 172's nose from pitching up when transitioning from the initial takeoff climb to cruise. As the airspeed builds you will need to apply ever increasing amounts of forward pressure to maintain level flight. You can roughly trim some of that pressure out rather than waiting until you reach cruise speed. Then you can fine tune the trim once you have dialed back the power to cruise. If you are letting go of the yoke to verify your trim and seeing the airplane abrubtly pitch up/down, I recommend against that. Try and get used to feeling the pressure changes on the yoke as you apply trim changes. You should be able to feel when you are no longer applying control force on the yoke. If you do it correctly, you can perfectly trim the aircraft without any pitch changes. Practice makes perfect!

[mallcott]

Flying Hands on and doing my best to correct for the oscillations. I thought it might be weather as well but, would that not also affect the other two aircraft I tested with?

Being in a climb right after take off I'm hands on all the way. I'm adjusting my trim for a proper rate of climb and it's fairly stable as long as it's climbing at a decent rate. As I reach my cruise altitude I am lower the nose (with the yoke) to stay at my cruise altitude and waiting for air speed to build closer to cruise before touching the trim. When my speed is up (and by this point I'm wrestling the oscillations) I bring back the throttle and trim for cruise.

It's always a bit more work with the yoke to control any aircraft at full power but this seems way beyond normal. I'm having to pull back on the power quite a bit more than normal to get stable flight where I can trim and maintain cruise.

First Question: What is your turbulence setting in AS2016 set to?
Secnd Question: What is your fps when the porpoising starts?
Third question: Which version of the sim are you using?

[Tom_G_2010]

I can't speak for the other aircraft, but the A2A 172 is a very realistic representation of the RW behavior.

If you are positively controlling the aircraft using the yoke, you should have no oscillations. A pilot needs to make many early and small corrections. If you hesitate to correct a pitch, roll, heading, altitude excursion then you quickly allow the magnitude of the problem to increase and increase the difficulty in correcting the problem...

Oracle427, Totally agree that the A2A 172 is very realistic and I have about 90 hours flying it so I know what it typically feels like and what is happening now is not typical. I appreciate the advice on staying ahead of the aircraft and making early small adjustment quickly in response to what the aircraft is doing. That's something that my CFI drilled me on when I was flying real world and it has stayed with me ever since. Staying ahead of they aircraft is a rudiments of flying.

I also agree with you when you said "If you are positively controlling the aircraft using the yoke, you should have no oscillations." Hence the reason I consider these oscillations atypical.

[Tom_G_2010]

First Question: What is your turbulence setting in AS2016 set to?
Secnd Question: What is your fps when the porpoising starts?
Third question: Which version of the sim are you using?

Mallcott,

On the third question, I'm running P3DV3 with the latest patches/updates and the A2A C172 has all but the most recent updates loaded.

I'll double check the Active Sky settings and look at my FPS after work today and let you know what I find. I typically leave Active Sky set to provide real world real time weather but I may have tinkered with it while letting people fly the sim at FlightSimCon last weekend so it's a good idea to go back and double check it.

Thanks!

[Lewis - A2A]

Let us know how it goes, ideally if you can fly as default as possible that'd help troubleshoot in case there is another factor at play here.

thanks,
Lewis

[Oracle427]

Let's see what happens when you remove the real world weather variable and replace with smooth air.

Just came back from flying a Super Cub at 12N with winds likely 7G15 based on the sock. I had to manhandle the stick to maintain airspeed and wings level during climb, base and final. Sometimes I needed to apply full aileron to stop abrupt rolls when we caught thermals. Perhaps AS weather is creating your conditions.

[mallcott]

First Question: What is your turbulence setting in AS2016 set to?
Secnd Question: What is your fps when the porpoising starts?
Third question: Which version of the sim are you using?

Mallcott,

On the third question, I'm running P3DV3 with the latest patches/updates and the A2A C172 has all but the most recent updates loaded.

I'll double check the Active Sky settings and look at my FPS after work today and let you know what I find. I typically leave Active Sky set to provide real world real time weather but I may have tinkered with it while letting people fly the sim at FlightSimCon last weekend so it's a good idea to go back and double check it.

Thanks!

You will find many, many posts about the excessive turbulence effect with Active Sky. If you haven't modified it, set it to about 0.30 and try the aircraft again.
FPS dropping below about 20 fps will also cause the issue you describe.

[Tom_G_2010]

. . . You will find many, many posts about the excessive turbulence effect with Active Sky. If you haven't modified it, set it to about 0.30 and try the aircraft again. . . .

Mallcott,

I can't get to the sim until after work but I did poke around online and I see several turbulence related settings in AS16 but 4 stick out as possibly impacting what I'm experiencing:

Maximum Wind Turbulence (0 to 100) default = 100
Turbulence Effect Scale (0 to 100) default = 70
Enhanced Turbulence (check box) default = Not Checked
Random Light Chop Turbulence (0 to 100) default =25

Your thoughts on where these should be set?

[Oracle427]

Try setting it to calm conditions first to eliminate all weather variables or just don't run AS16 to evaluate if it makes a difference.
Then work your way back in with AS16 and manually set some conditions to see what it does to handling. I found that the default updraft and downdraft settings can be very strong and cause issues with the autopilot, so I decreased those settings.
Do you know what the weather conditions were during your test flight with issues?

My AS settings:
Max Wind Turbulence: 100
Turbulence Effect Scale: 70
Max Wind Shear: 20
Enhanced Turbulence: Unchecked
Wake Turbulence Strength: 0
Max Downdraft: 125
Max Updraft: 125

[Tom_G_2010]

Try setting it to calm conditions first to eliminate all weather variables or just don't run AS16 to evaluate if it makes a difference.
Then work your way back in with AS16 and manually set some conditions to see what it does to handling. I found that the default updraft and downdraft settings can be very strong and cause issues with the autopilot, so I decreased those settings.
Do you know what the weather conditions were during your test flight with issues?

My AS settings:
Max Wind Turbulence: 100
Turbulence Effect Scale: 70
Max Wind Shear: 20
Enhanced Turbulence: Unchecked
Wake Turbulence Strength: 0
Max Downdraft: 125
Max Updraft: 125

Unfortunately I don't know what the weather was . I'll try to look up the historical weather this evening.

I'll also set everything to calm conditions and see what happens.

Thanks!

[Tom_G_2010]

It appears that the settings in AS16 may have been the issue. I Shut down AS16 and set P3D to calm weather and the plane responded much more as excpected. Next I'll bring AS16 back up and dial back the settings and see what happens.