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Old 06-06-2011, 12:13 PM
flyboy1963's Avatar
flyboy1963 flyboy1963 is offline
Join Date: Oct 2008
Location: Lake Country, B.C. Canada
Posts: 2,433
Default Trim drag with CG changes - armchair quarterbacks feedback please!

Just had an 'aha' moment yesterday .....I was pretty happily cruising at 5500? and trimming and leaning to see what I could comfortably cruise at ?..for the :25 I was enroute from a gas run.
I was happy to see 150 kts on the GS window of my GPS, which is pretty good for my -9a, dirt all over it & all.
Then I looked in my ?check 6? mirror, and saw the elevator in a slight nose-down position. Hmmmm.
I was alone, with part tanks, and all my survival gear, gallon of water, tools etc. tucked behind the seats and in the baggage area. (about 50 lbs.) a slight aft CG position.
It made me wonder if an 0-320 FP FWF bird is inherently a bit slower than it could be, as I?m constantly cruising with the elevator horns exposed to the slipstream, as the tail is doing some lifting, rather than the usual downforce, in a ?neutral? position.

What do you W & B/efficiency gurus think about this?
I don?t see any weight I can put forward, so it looks like I?m stuck with this, unless I reset the elevator incidence?
Even so, is that just a cleaner way to achieve the same net force at the tail as trimming? ( i.e. no real gains to be had)
Perry Y.
RV-9a - SOLD!....
Lake Country, BC
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Old 06-06-2011, 12:42 PM
RPaul RPaul is offline
Join Date: Mar 2011
Location: Raleigh, NC
Posts: 4

Mr. Yaremchuk,

For a conventionally configured aircraft (tail behind wing) operating in the approved CG range, the aircraft should have positive static stability. This requires a net down force on the tail- so, I believe, that although you see a down deflection on the elevator the net force on the horizontal stabilizer is still working against lift (down). Keep in mind this is still a possibility given variables such as the incidence of the horizontal tail and the downwash effects behind the wing.

Reseting the horizontal tail incidence to achieve zero elevator deflection at this trim condition would result in the same net tail force as trimming had. And, I suspect it would still be a down force, albeit with zero "trim-drag", thus more efficient. Would it be worth the effort? I don't know, I haven't built an RV yet and have not thought about the effect this would have on control power at other operating conditions.

Mooney aircraft company did exploit the benefit of zero trim drag by having the trim mechanism change the incidence of the entire surface, and incorporating no trim tabs on the elevators.

One last thought- In general, an aft CG position is considered more efficient for most aircraft as you can reduce the downforce on the tail- more downforce = more lift required by the wing, and more wing lift means we have more induced drag.

Interesting to think about,
Ryan Paul
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Old 06-06-2011, 01:14 PM
Alan Carroll's Avatar
Alan Carroll Alan Carroll is offline
Join Date: Jul 2007
Location: Madison, Wisconsin
Posts: 778

Originally Posted by flyboy1963 View Post the tail is doing some lifting, rather than the usual downforce, in a ?neutral? position.
Actually the tail is still producing downforce. The confusing part is that the airflow in this area is not parallel to the longerons, its at a downward angle due to downwash from the wing. You can see this easily by dripping some dirty engine oil on the fuselage and watching where it goes.
Alan Carroll
RV-8 N12AC
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Old 06-06-2011, 02:02 PM
jbagley's Avatar
jbagley jbagley is offline
Join Date: Sep 2009
Location: Aloha, or
Posts: 282
Default elevator trim drag

I recently went through this exact same thing with my rv6a. The short-answer is don't change anything, you won't make a measurable difference and you could do more harm than good.

My RV ended up with 1/10th of a degree too much positive wing incidence. To compensate for this I figured I would need to increase the incidence of the horizontal stabilizer. After the first few test flights I noticed exactly what you are describing, the elevator in a nose down trim configuration instead of "in-trail" with the horizontal stabilizer. I added one washer to each of the bolts holding down the leading edge of the horizontal stabilizer. This decreased the so-called out-of-trim condition and got it to about a 1/4 - 1/2 inch in-trail (at the elevator horns) with just me and half tanks.

I was just about to add another set of washers to get it just perfect and decided to call Van's first. Can't remember the exact wording, but basically you want the elevator to be a little bit out of trail in the nose down direction at cruise in level flight. I was told that 1/4" is about right and to leave it alone. Something about positively loading the control surface that makes it more stable. The engineer at Van's explained in a way that made sense then and I am probably not remembering correctly.

Rv9's are a different airplane, your mileage may vary, and I am not an aeronautical engineer caveats apply.

Hope that helps!
James Bagley Jr
RV6A flying
RV6 #2 tail done and wings done
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Old 06-06-2011, 04:04 PM
scsmith scsmith is offline
Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,921
Default Trim Drag, Static Stability, Stick-free stability

Hi, Lets see if I can help here:

First, to put your mind at ease, there is very very little drag caused by trimming with small elevator deflection, except for the minor drag of the elevator horns sticking up in the breeze. Even that is pretty small. You can help that of course by putting nice round edges on the horns.

Trim drag, as a term, more classically applies to the additional "induced drag" caused by trimming a stable airplane, not the added profile drag --- although I suppose in a case where there was a really significant drag addition from balance horns or something, it might be included as well.

The extra induced drag is a double whammy - not only do you get induced drag from the horizontal tail as a lifting surface (lifting up or down) but when it is lifting down, the wing has to produce more lift to offset it, so the wing also has an increase in induced drag.

Static Stability: Probably the most widely held misconception in aviation is that the tail must have downward lift to make the airplane stable, and that zero lift on the tail would correspond to neutral stability. This is just simply not true. What IS true is that for a statically stable airplane, the tail can never lift upward at a higher lift coefficient (lift/unit area) than the wing, and including the effects of wing camber, a trimmed, stable airplane may only have an upward lift coefficient that is a fair bit less than the wing lift coefficient. BUT, it is often true that at low speeds, many airplanes have small amount of upward lift if the c.g. is toward the aft third of the c.g. envelope. This is especially true of sailplanes where performance is so important that we always balance the airplane in the aft third of the c.g. range, and because with a short wing chord, the effects of wing camber are lessened, allowing more tail upload. Anyone that doesn't believe this, I would invite you to get any good text on aircraft stability and control and work through the equations yourself.

As a stable airplane is trimmed at higher speeds, more download on the tail is needed. A stable airplane could easily trim with upload in slow-flight or best glide speed, but at cruise speed when the wing lift coefficient is very low (often 0.25 or less for GA airplanes) the tail is most always lifting downward. The faster you go, the more download will be required to trim. It may seem counter-intuitive that it takes a more forward stick position, and a trailing-edge-down elevator deflection, but this is overpowered by the nose-low angle of attack and the wing downwash angle on the tail. Considering only static stability for a moment, the incidence of the stabilizer has no effect on (stick-fixed) static stability. A change in stabilizer incidence will require a different elevator deflection angle to trim. And ignoring other factors, you might choose a stabilizer incidence that will lead to the elevator being in perfect trail at your typical cruise speed. That would eliminate any extra drag from elevator horns. But, as the engineer from Vans (Ken?) was quoted as saying, there may be good reasons not to do that, because:

Stick-force gradient/stick-free stability: For the case of RV's, where the elevator airfoil is symmetrical, if you have too much stabilizer incidence, you will find that the stick force gradients are very light, or even negative. It has often been commented that RV-8's, near the aft c.g. limit, have very very light stick forces at low speed. The CAFE flight test even shows a small negative stick force gradient at low speed with the c.g. on the aft limit. Those of us that fly high-performance sailplanes are accustomed to very light stick forces, and thats usually OK, but negative stick-force gradient is very undesirable. It means that as you slow down, you have to provide forward stick pressure to prevent it from slowing further. If you slow more, then more forward pressure is required. I've seen an extreme case of a badly rigged home-built glider where it also took more and more aft stick pressure the faster you went. Really unpleasant. This trait is often interpreted by pilots as being statically unstable, but it is not. It is, however, stick-free unstable.

A number of factors in the control system design and aerodynamic design influence the elevator hinge moment that feeds through to the stick, and how the hinge moment changes with elevator deflection and speed. But having too high a stabilizer incidence angle is one of the factors.

So my recommendation would be for most RV's, build it according to the plans. This is one of the things that the factory may have played with on prototypes to get to where they think its right, or they got lucky and hit it right the first time. But in general, part of the RV's great handling and feel is because they got the incidence right. If you are going to err, do it on the side of less elevator incidence (err in the direction of leading edge down). This will provide more desirable stick force gradient - what most pilots would associate with a more solid feel. The change in performance (trim drag) from adding some (L.E. up) stabilizer incidence would be really negligible, but the change in handling is noticable.

If you are building an RV-8 and you know you will often have a heavy person in the back seat, it wouldn't hurt to drop the incidence just a tad (something like 0.1 or 0.2 degrees L.E. down from the spec), but whatever you do, don't err on the side of more positive incidence.
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2020
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Old 06-06-2011, 04:12 PM
mike newall's Avatar
mike newall mike newall is offline
Join Date: Jan 2005
Location: Yorkshire, England
Posts: 2,177

Rear c of g's are beneficial if you can control and manage it.

I used to fly skydivers years ago and through about 4,000' in smooth air, I would get them all to move back and re trim. Worth nearly 100 fpm on a Cessna 206 !

Now that I work for a living......... On the A330, we have a trim tank in the tail that gets 5 tonnes of fuel pumped back into it. Starts through 25,500' to bring the c of g back - it is aiming for about 36 - 38% MAC. That saves a good chunk of fuel.

In context - Say I come out of Dominican Republic back to the UK with a full load and freight - we will be carrying up to 12 tonnes of fresh fruit and veg as well as 325 passengers.

Now, if I can get the load down the back including 100 bags in the rear hold, I will be spot on fuel burn.

If not and we have to run nose heavy for pallet size or configuration, I will burn an extra 750kgs (1650lbs) of fuel - just because of the poor c of g.

Mind you, we are fly by wire !
"I add a little excitement, a little spice to your lives, and all you do is complain!" - Q

Donated in 2021
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Old 06-06-2011, 05:12 PM
RPaul RPaul is offline
Join Date: Mar 2011
Location: Raleigh, NC
Posts: 4

Mr. Smith,
Thank you for your explanation. I learned something new today.

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Old 06-07-2011, 08:07 AM
SvingenB SvingenB is offline
Join Date: Apr 2007
Location: Norway, Stj?rdal
Posts: 598

Originally Posted by scsmith View Post
Static Stability: Probably the most widely held misconception in aviation is that the tail must have downward lift to make the airplane stable, and that zero lift on the tail would correspond to neutral stability. This is just simply not true. What IS true is that for a statically stable airplane, the tail can never lift upward at a higher lift coefficient (lift/unit area) than the wing, and including the effects of wing camber, a trimmed, stable airplane may only have an upward lift coefficient that is a fair bit less than the wing lift coefficient.
I have been of the (mis?)conception that as long as the centre of lift is aft of the cog, it will be stable. More like tail wheel vs nose wheel.
RV-4 #4520, Slow built
B Svingen
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Last edited by SvingenB : 06-07-2011 at 11:16 AM. Reason: aft vs fwd
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Old 02-11-2012, 01:36 PM
hqcook's Avatar
hqcook hqcook is offline
Join Date: Aug 2008
Location: Aurora, CO
Posts: 14
Default Thanks, Mr. Smith

Very helpful. I was out 0.2 degrees to the wrong direction, but couldn't imagine that it would create a problem with stability. Your discussion of stick force gradient helped me greatly. Thank you.

Hugh Cook
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Old 02-11-2012, 02:19 PM
az_gila's Avatar
az_gila az_gila is offline
Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011

Originally Posted by mike newall View Post
Mind you, we are fly by wire !
You mean the computer flies the plane, and you just make suggestions to the computer...
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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