Trim / Speed Gain on Seat Position?

[1001001]

I recently took a long trip in my 201, which lasted just under 5 hours due to ridiculous headwinds.  I am not a tall person, at 5' 8", and weigh about 185 (clothed).  My legs are kind of short, so I usually fly with my seat all the way forward so I can manipulate the rudder pedals and brakes fully.  About two hours into the flight, I was getting a little restless and decided to stretch out, so I moved the seat back to where I could touch the rudder pedals (no yaw damper).

After I did that, I noticed the airplane started to climb (single axis autopilot) so I corrected altitude and adjusted the trim downward.  After I did that, the airplane seemed to pick up several knots of ground speed (like 3-5, if I recall correctly).  I thought maybe that was due to a headwind change, but found that my TAS had increased as well.  

I guess in theory, a nose-down trim change should result in a small speed increase, but I was surprised at the magnitude of the gain.  Does this make sense?  I was under the impression that the pilot seat was so close to the CG that it wouldn't make much of a difference, but I probably moved the seat back around 4"-6" or so, so my moment shifted a bit.  I haven't done a W&B calc on what that change would do, but I thought the observed effect was interesting.  I usually fly with about 30 lb of ballast in the luggage compartment (it seems to help with the flare in landing) when I don't have rear passengers.  Would increasing the ballast cause me to have to trim more nose-down and imprive cruise speed?  It seems like it should, to a point.

 

Does anyone else experience this?

[N201MKTurbo]

The horizontal stabilizer makes down lift. Lift causes induced drag. Moving the CG aft reduces the required lift of the tail therefore reducing the induced drag. Your airplane is the most efficient at the rearmost CG limit. Adding ballast will increase the lift needed by the wing and reduce the lift needed by the tail. I'm not sure if it is a good tradeoff or not.

We should ask @Blue on Top

Edited October 29, 2021 by N201MKTurbo

[1001001]

1 hour ago, N201MKTurbo said:

The horizontal stabilizer makes down lift. Lift causes induced drag. Moving the CG aft reduces the required lift of the tail therefore reducing the induced drag. Your airplane is the most efficient at the rearmost CG limit. Adding ballast will increase the lift needed by the wing and reduce the lift needed by the tail. I'm not sure if it is a good tradeoff or not.

We should ask @Blue on Top

Right, see that extra induced drag is what made me think that the tradeoff would be nil, however, moving mass around inside the airplane should allow favorable trim changes to get best cruise attitude, to a point. 

There has to be some optimum point at which adding ballast doesn't make an improvement anymore, because any gain in performance of the wing will be countered in drag induced by the tail.  But up to that point, you'd think it could be beneficial.

[PeteMc]

That's why I (and I'mguessing some others) always leave stuff in the baggage area.  Staying more on the aft side of the envelope will always help a bit. 

[N201MKTurbo]

https://aviation.stackexchange.com/questions/606/why-would-a-glider-have-water-ballast-if-it-is-trying-to-stay-aloft-without-an

 

[1001001]

5 minutes ago, N201MKTurbo said:

https://aviation.stackexchange.com/questions/606/why-would-a-glider-have-water-ballast-if-it-is-trying-to-stay-aloft-without-an

 

Interesting stuff, thanks!

[TTaylor]

There is an optimum point, not just farther back is better.  Too far forward CG and you are pushing the tail down, too far back and you are pushing the tail up, ideal is where the elevator does not have to do work.  I was racing gliders this summer in France.  One of the teams had a sensor on their elevators to detect where neutral was on the elevator position.  They would use this to adjust the GC for optimum flight.  Gliders have water ballast in the wings and tail.  They would start out near aft CG and dump tail ballast until they got to optimum.  Might be an opportunity to add a sensor to our planes.

[Austintatious]

I slide the seat back as well and note an increase in speed.  I may pick up around 2 or 3 knots by doing so... But I am in a rocket So it really makes little difference other than comfort.

Edited October 29, 2021 by Austintatious

[A64Pilot]

One of the things that drives aft CG limit is called the stick force neutral point, aft CG  is actually in front of that point.

It amounts to the point where zero effort is required to change pitch up or down, it’s a very unstable aircraft at that point, release the yoke and the aircraft will most likely go to full travel either up or down, the stick force neutral point will I believe correspond to the point where the horizontal is producing no lift. Not sure of that though.

‘Anyway the further aft the CG goes, the less stable the aircraft becomes in pitch, so approach this aft CG ballasting with caution.

‘It’s pretty normal if racing an aircraft to ballast to the aft CG limit, the greater the arm, the less ballast required of course.

[philip_g]

As expected the further aft my check is the faster and better the plane handles. To a point, as expected from the aerodynamics. 

[David Lloyd]

I have wondered about CG having a meaningful effect on speed and have planned to test.  Two 200 pounders in the front seats, full forward and minimal junk in the baggage would put my airplane at the forward edge of the envelope.  Moving my seat back as far as it would go and sending the other meat lump to the rear seat would move the CG back quite a bit, although not to the aft limit.  First test at altitude, level flight in smooth air for several minutes then every 10 seconds for maybe two minutes, write down either the indicated airspeed from the GI-275 or calculated TAS from the GNX375.  Then move the seat and meat lump and repeat.  Post results.

Will it be more or less than two knots difference?

[A64Pilot]

If they are big ole boys, it may be 2 kts, but 2 kts on a cross country race may be the difference in winning and losing.

Many older Mooney people used to swear that a Mooney would “get on step”, that is if you climbed a couple of hundred ft above assigned altitude, the leaving climb power in descend to assigned and reduce to cruise power. They were certain that your stabilized cruise was faster than if you climbed to assigned, reduced to cruise and just held altitude .

I don’t think so, but so many were so certain, that maybe?

The big boys, as in airliners will cruise right at the stick force neutral point with stability being computer augmented because they burn less fuel there. How much I have no idea, but even tiny savings add up.

 The negative lift from the tail is very similar performance wise to the aircraft being heavier by the same amount

Edited October 29, 2021 by A64Pilot

[MikeOH]

There's this regarding "on the step":http://www.nar-associates.com/technical-flying/step/step_wide_screen.pdf

The author's credentials are legit.

[PT20J]

Moving the CG aft without changing the weight reduces induced drag in both the tail and the wing since the tail is generating less downward lift and the wing is generating less upward lift to carry the tail load. If you move the CG aft by adding weight, it gets more complicated because the wing now has to generate additional lift to carry the added weight. Recall that induced drag is a product of lift.

Skip

[Pasturepilot]

I routinely fly with full aft seat. The wing leveler tracks well enough that I don’t need a hand on the yoke often,  and I pick up some amount of speed. Not a lot, but every bit helps. 
 

Also, with the CG pushed aft like that, I can make minute pitch changes by moving one foot forward or aft. It keeps my knees from getting stiff over the course of a long flight, too. 

[Blue on Top]

On 10/29/2021 at 9:45 AM, N201MKTurbo said:

The horizontal stabilizer makes down lift. Lift causes induced drag. Moving the CG aft reduces the required lift of the tail therefore reducing the induced drag. Your airplane is the most efficient at the rearmost CG limit. Adding ballast will increase the lift needed by the wing and reduce the lift needed by the tail. I'm not sure if it is a good tradeoff or not.

We should ask @Blue on Top

@N201MKTurbo  This is a perfect introduction to my November "The Mooney Flyer" article on a trip around the weight and balance envelope.  The upper left corner of the W&B envelope is cutoff with a diagonal line that runs from the lower left side (starting at the top of forward regardless vertical line) and going up and to the right to the horizontal maximum gross weigh line.  This diagonal line is very close to a line for equal tail power.  In other words, the tail is producing the same down load along that entire line.  As the CG is moved aft or the airplane gross weight is lightened, less tail power will be required.  So, to answer the question more directly (but getting somewhat technical), if the ratio of the added weight divided by the aft movement of the CG (slope of the diagonal forward limit line), the airplane will require less tail down load.  This is hard to explain, but very easy to see if one plots it on a weight and balance chart.  A picture is worth a thousand words    Bottom line:  yes, it will be more efficient ... better if the weight is already in the airplane.  In other words, store oil, tools, etc. in the aft baggage compartment.  But there is more to consider ...

I am NOT a fan of "Charlie" weights.  CG is not just CG for flight characteristics.  Mass distribution plays a big role in dynamics.  Here's a quick example (also in the article).  If one has 400 lbs. of useful load to get to gross weight, adding 400 lbs. at the current CG won't change the airplane dynamics a lot (performance will decrease some, though).  Now, if we put that same 400 lbs. in the airplane but distribute it to 300 lbs. in the nose (5' forward of the current CG) and 100 lbs. in the tail (15' aft of the current CG), the CG remains in the same position.  BUT, the dynamics of the airplane are completely different!  Ask figure a skater.  If the airplane were to depart controlled flight, recovery could be in question.  Ask the IAC (International Aerobatic Club) guys.  Tail weights are highly frowned upon.

Last comment (for now  ).  In cruise, the airplane will loose about 1 knot per 100 lbs. of additional gross weight.  

Edited October 30, 2021 by Blue on Top

[jaylw314]

I recall reading somewhere the the Spirit of St Louis when fully loaded was likely so far aft CG that it had negative stability, so it could not be successfully trimmed initially and would have been a bear to fly.  I don't know how you'd go about proving that, but the EAA people who've made replica's have attested to their poor stability, even when nowhere near as fully loaded as the historic flight.

If true, it makes Lindbergh's feat all that more impressive than it already is

[1001001]

Thanks, everyone, for the replies.  There is some really useful information here.  

[Blue on Top]

20 hours ago, jaylw314 said:

If true, it makes Lindbergh's feat all that more impressive than it already is

The Wright airplanes were also very aft CG.  In fact the 1911 glider went to Kill Devil Hill with two (2) sections aft of the wing, and Orville (along with Alex Ogilvie) added a third aft section.  Orville wrote to Wilbur that the glider flew better with the fixed vertical surface out the furthest.  Although this surface would have made the glider less directionally stable, the additional weight out front would have helped the longitudinal stability.  The bag on a pole out front was merely weight (not the autopilot as told to the reporters).  My estimation is the CG was somewhere near the trailing edge of the wing.

[A64Pilot]

20 hours ago, jaylw314 said:

I recall reading somewhere the the Spirit of St Louis when fully loaded was likely so far aft CG that it had negative stability, so it could not be successfully trimmed initially and would have been a bear to fly.  I don't know how you'd go about proving that, but the EAA people who've made replica's have attested to their poor stability, even when nowhere near as fully loaded as the historic flight.

If true, it makes Lindbergh's feat all that more impressive than it already is

I believe that wasn’t all that uncommon, the Wright Flier I’ve heard was pretty unstable in pretty much all axis.

Even the P-51 with its fuselage tank filled was prohibited in maneuvers due to aft CG.

Aft CG, doesn’t usually become much of an issue for an experienced pilot. unless they are IMC, then it can become a more serious issue.

[jaylw314]

2 hours ago, Blue on Top said:

The Wright airplanes were also very aft CG.  In fact the 1911 glider went to Kill Devil Hill with two (2) sections aft of the wing, and Orville (along with Alex Ogilvie) added a third aft section.  Orville wrote to Wilbur that the glider flew better with the fixed vertical surface out the furthest.  Although this surface would have made the glider less directionally stable, the additional weight out front would have helped the longitudinal stability.  The bag on a pole out front was merely weight (not the autopilot as told to the reporters).  My estimation is the CG was somewhere near the trailing edge of the wing.

Of course, their longest flight was only 38 minutes as opposed to Lindbergh's 30 hour transatlantic flight!  That makes my head hurt just thinking about it

[carusoam]

Great question 1001001…

You have found The driving force for MSers to break out their WnB calculator apps…

Is easily supported by easy speed gains…

With GPS on board… it’s not too hard to capture the facts while you are flying your own plane…

Of course… it also helps to know about how your plane works… the more the merrier…

 

Then take a pic out the window, aft…

note where the elevator bob weights (?) are hanging… are they down, or way down?

 

As TT pointed out above….  Moving the Cg back can have its limits….

You certainly don’t want to fall out the back of the envelope…  the plane can stop flying and not recover…

 

So… if you saw the speed increase just by sliding the pilot seat back….  Welcome to MooneySpace!  There are a few other cool things you can learn around here…. But none are as good as that nugget… 

 

Got a WnB app? Want a recommendation?

 

PP thoughts only, not a CFI…

Best regards,

-a-

[JeffMirs]

Dr. Norm Smith is rolling in his grave laughing. This was well discussed 20 years ago. Always slide the seat back with one/two people aboard. Reduces the drag of trim.