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Nose down trim in cruise

caapt

Well Known Member
I finally put two plus two together and realized that my near full nose down trim in cruise was the cause of my slow cruise speed. Cg does not appear to be the problem. Low speed, below say 120 ias, seems fine. But 130-145 results in nose down trim near the limit. It seems to me my horiz stab is not producing enough lift at high speed cruise.
Any one else have this problem?
 
SNIP….It seems to me my horiz stab is not producing enough lift at high speed cruise.

This does sound like a rigging issue. I don’t remember for the RV-14. Is there a spacer under the HS forward spar? From you post it seems the HS is providing too much down force. A spacer under the HS forward spar will mitigate that.

But - before you experiment with spacers and changing the HS angle of attack, verify you have it per plans.

Carl
 
Holy moly this sounds scary to me, as a non-expert only 20% through my build. Are you saying you cannot do, say, 75% power no hands without climbing? Or are you saying the fastest level cruise speed is 130-145?
 
The HS of the 14 comes with pre-drilled holes and there isn't much room, if any, for adjustments of incident setting like other RV models such as RV6 or 7. So, it is hard to imagine that the incident is not set correctly.

Have you checked to see how many degree of deflection you get from your trim tab? The older kit had a shorter connecting rod for the servo and VANS has updated that part with slightly longer part which gives more nose down deflection.
Also, it would be good to double check your W&B. Some simple items such as if you don't have the canopy fully closed or seat belts where they need to be, all those could affect your W&B, though probably not to the point that gives you such nose up attitude.

The suggestion to double check the rigging is a very good one as some thing seem to be very off.
 
Those are good ideas, but……

I am using the longer arm on the trim tab. I can trim it hands off at 150 rich of peak. But the trim tab is very close to the limit. The real issue is the drag from a deflected elevator and trim tab. This doesn’t seem to be a cg issue. Also, I checked, the HS is right side up
Thanks for the help
 
I finally put two plus two together and realized that my near full nose down trim in cruise was the cause of my slow cruise speed. Cg does not appear to be the problem. Low speed, below say 120 ias, seems fine. But 130-145 results in nose down trim near the limit. It seems to me my horiz stab is not producing enough lift at high speed cruise.
Any one else have this problem?

I suggest that you have no trim problem. Your horizontal stabilizer flies ‘upside down’, eg, it develops downward lift. The faster you go, the more downward lift, the more nose down trim you need. Look at the tab. It moves down below the stabilizer more than above it. On the ground, move the trim to the previous cruise position. Go back and look. The tab should be pretty close to level with the stabilizer (depending on cg). As to cruise: more info, please. kIAS, KTAS? What density altitude? Try to compare to Vans’ numbers?
 
Also not sure about the 14, but this could be a side effect of wing angle to the fuselage. Is that pre-drilled on the newer planes, or something you have to measure, jig and drill like on the 6 and earlier?
 
Also not sure about the 14, but this could be a side effect of wing angle to the fuselage. Is that pre-drilled on the newer planes, or something you have to measure, jig and drill like on the 6 and earlier?

I believe this is in the plans as 25-02, and appears to come predrilled (referred to as the “F-01405B Bulkhead Bar Assemblies“), on both the spar the bulkhead bar sides.

Screenshot 2022-08-17 at 11.39.40 PM.jpg
 
These are typical cruise conditions for me. On this flight, my gross weight was 1871 and CG was 84.96.
 

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Nose down

These are typical cruise conditions for me. On this flight, my gross weight was 1871 and CG was 84.96.

Another -7 owner here, commenting on observations…

At 8 gallons per hour in cruise, assuming you have a 390 engine, is definitely LOP.

Pitch attitude is definitely negative, interesting. Did you calibrate your pitch by leveling the canopy rail and setting pitch to zero there?

Your nose down trim is quite deflected toward the full nose down position, yet CG is well forward of the aft limit.

Is the -14 designed for cruise with reflexed flaps? Reflex could get the pitch more near zero and neutralize some of that extra lift, reduce drag by getting rid of that excess lift, unload the horizontal stabilizer and remedy nose down cruise.
 
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These are typical cruise conditions for me. On this flight, my gross weight was 1871 and CG was 84.96.

I would argue that pitch trim indication is not necessarily reflective of actual trim tab position. Have you done some testing in the hanger to see exactly what the trim tab looks like when the EFIS shows what it is showing in this pic?

Larry
 
My pitch at start of take-off roll is typically around +3 deg. That makes sense. When I did my W&B I had to remove all the air from the nose wheel to reach 0 deg on the main longeron. In level flight, my pitch is generally +2 deg.

My CG is frequently aft when I fly. Empty CG is 82.02, right at the forward limit, but get in the left seat, add a passenger, load 100 lbs of stuff in the back and top of the tanks, and I'm at the far aft end of the envelope (88.24). In that case, I need a lot of down pitch trim. The picture below shows the max elevator deflection at nearly full down pitch, which is what is needed under these fully loaded conditions. My RV-14 flies great fully loaded with nearly max down trim although perhaps at a loss of a few knots in cruise airspeed.
 

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Aft CG, faster aircraft

As a matter of observation that is consistent with the science of aerodynamics, when my CG moves aft my aircraft is incrementally faster in cruise as the tail lift drag is unloaded.

Has anyone seen anything different? This -14 wing must be quite different than the -7. To make lift my -7 needs a positive angle of attack, at least 2 degrees nose up during level cruise flight at any altitude.

LOP operation in every case in these pictures. My manual trim is only a couple turns in the nose down direction. Trim tab is slightly above the elevator. Elevator is in trail or slightly trailing edge down when near the aft CG.

My wheel pants are off for repair in these two photos flying. You’ll see a 100 fpm climb on the VSI, hand flying with no autopilot, there are 1 to 2 degree pitch changes are normal. Nothing nose down in level flight though.
 

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Pitch trim

I’m going to go out on the limb and say it’s your connecting rod for the servo is the older style and shorter than the one Vans uses now.

I need to be at my hangar later today and will measure mine upper and lower limits. I think it’s almost impossible the incidence settings are incorrect on the 14.
 
This is the second thread I have read on this, and am concerned with my 14 build starting.

I dont know why folks post about the pushrod length.
Needing full now trim in cruise flight is a RIGGING problem!
It is not a pushrod, LOP, or weight and balance problem (side by side seating over CG is easy).

The pic of the elevator is compelling. Properly trimmed cruise flight should have zero deflection in the elevator. With my -7 build, there were shims put under the HS forward spar to help get zero trim. That was one of the typical adjustments during Phase 1. Im not far enough along reading the -14 build to know if this is also included.

Seems there is something happening between old and new -14 builds. Older builds appear to have no trim issues. More recent ones have a "Full down trim in cruise" problem. (and again NO, its not the servo pushrod length)
Wonder if Van's is aware of this and has had any response.
 
Is there really a problem here?

I finally put two plus two together and realized that my near full nose down trim in cruise was the cause of my slow cruise speed. Cg does not appear to be the problem. Low speed, below say 120 ias, seems fine. But 130-145 results in nose down trim near the limit. It seems to me my horiz stab is not producing enough lift at high speed cruise.

<snip>

I am using the longer arm on the trim tab. I can trim it hands off at 150 rich of peak. But the trim tab is very close to the limit. The real issue is the drag from a deflected elevator and trim tab.

This thread has gotten off in the weeds about all kinds of problems that don't exist...the OP says he's got the longer arm on the trim tab, and we've established that having mis-rigged wings or stab on the RV-14 is highly improbable since the alignment holes are prepunched. You can't add more than 3 deg of flap reflex because that's about where the flaps hit the rear spar. It sounds like the OP can trim it hands off at high speed cruise. If the trim tab is very close to the nose-down limit at high speed (and especially at high speed combined with aft CG), that isn't a problem, that means Vans has designed it with adequate nose-down trim authority.

The OP's complaint is not trim authority but rather the drag caused by a deflected trim tab and elevator. It's true that these deflected surfaces add some drag although I'd be surprised if it was more than a few knots (this should not result in a 'slow cruise speed', but the OP doesn't define what that is). If you wanted to reduce drag you could do the structural mods (and fiberglass rework to the empennage intersection fairing) to rig the horizontal stab a couple degrees higher angle of incidence (raise the leading edge). At some point this should provide perfectly in trail elevator & trim tab at a specific high speed/aft CG condition, but it comes with tradeoffs...in all other conditions (lower speed and/or more forward CG) you'll need more nose-up trim than you do currently. You might run out of nose-up trim authority at approach speeds, and the pitch authority to rotate or keep the nosewheel off the ground on landing will be reduced and require a higher speed than at the factory incidence setting (especially at fwd CG). It may affect ability to stall the airplane at forward CG, and overall pitch stability of the airplane may also be negatively affected. So if you start experimenting with new horizontal stab incidence in pursuit of those last couple knots, make sure you know what you're doing, proceed with caution, and thoroughly test all corners of the ground and flight operating envelope.
 
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Properly trimmed cruise flight should have zero deflection in the elevator.

Zero elevator deflection will happen at specific combinations of speed and CG...it would be fun to have a graph of the zero deflection trim condition (with speed vs. CG axis) to see what that looks like for an RV-14. One end of the plot would probably be at a somewhat forward CG and higher speed, and the other end at further aft CG and lower speed. Point is, both of those are variables from one flight to the next, and the CG changes during each flight as you burn off fuel. You can't expect the elevator to be perfectly in trail during all 'cruise flight' conditions.
 
Saying vs having might be different

I thought I also had the longer arm since my kit was delivered a year after another kit at the field yet mine was shorter. The devils is in the details. I realize Vans never messes up but all issues need to be reviewed. Currently my trim is roughly in this position in flight. Trim Down.jpg

Trim Normal.jpg

Trim Up.jpg

As you can see I took the easier approach and lengthened the control horn to give me a little more length. Obviously it's what is needed in flight and I would like to hear from others what their "cruise trim" looks like. By the way the performance with this set-up I think has been excellent with regularly seeing 175 knots TAS at 9,000 - 11,000 ft.
 

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Picture of trim position (roughly in cruise)

See attached, not sure why it did not attach to previous response.
 

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Trimming hands off at 150 Knots

This is not what this airframe is capable of doing even ROP. ROP on mine is closer to 165 knots. Something is amiss.
 
This is not what this airframe is capable of doing even ROP. ROP on mine is closer to 165 knots. Something is amiss.

If you are getting 165k max speed, then there is something not right.

I would imagine you have verified your speed that is not an error in the speed.

For reference, my first 14A top speed was 187knots and my current 14A has a top speed of 191 knots with the cowl flap open. These speeds have been calibrated and known to be accurate within a fraction of a knot based on 3 way GPS run.

At these speed on both planes, I am not near my nose down trim limits.
 
Zero elevator deflection will happen at specific combinations of speed and CG...it would be fun to have a graph of the zero deflection trim condition (with speed vs. CG axis) to see what that looks like for an RV-14. One end of the plot would probably be at a somewhat forward CG and higher speed, and the other end at further aft CG and lower speed. Point is, both of those are variables from one flight to the next, and the CG changes during each flight as you burn off fuel. You can't expect the elevator to be perfectly in trail during all 'cruise flight' conditions.

These are XC planes so optimum rigging is for the 90% of the time in cruise flight. AND at the highest speeds. Added tab deflection for slower flight has less impact on overall efficiency. Understood that minor re-trimming is needed for shifts in fuel, etc. But NOT full trim tab deflection in the top, most used speed range.

Maybe the idea of 'rigging for best flight' has been lost in the later Vans kits and the 'assemble and go' mentality.
(geez, starting to sound like one of those 'back in the day . . ." builders)
 
165 knots ROP @ 11,000

My numbers are 165 knots TAS ROP at 11,000 and 21.2 In MP., 7.6 GPH
175 - 178 knots LOP TAS at 11,000 ft and 21.2 MP., 9.1 GPH.

I've seen other data and this looked ok. Can you provide your other data at cruise settings? I know this is now going thread drift and if there's another site to review data let me know.

The OP indicated he was at 150 knots with max trim set. I think this is the issue this thread was created for. He definitely has an issue with trim and exploring solutions. My trim as you can see from the pictures is more than ok even at only 175 knots.......
 
There are a number of RV-14(A)s with the same "issue," including mine - here is the original thread from 2018, with updates in 2021 from me and a few others with similar problems.

https://vansairforce.net/community/showthread.php?t=162417&highlight=trim&page=2

That original thread lead to the longer pushrods that we have now ... but, some of us found that even though we do indeed have the longer trim pushrod, for some aircraft it could be a tad longer. HOWEVER, I say "could" and not "should" - since the position sensor in the trim motor doesn't actually read the last ~10% of travel (verified by a communication I had with Ray Allen), although my G3X _reads_ almost full nose down trim at high speed, upon testing I (and others) found that we still have ample nose-down trim remaining. E.g. I took mine to max speed (~180ktas) at various c.g. loading, and pushed the nose-down trim on the stick; I was surprised at how much trim authority still remained.

One person constructed a fix, which is on page 2 of that thread - he constructed a pushrod that is about 1/8" longer. This is essentially what Van's recommended to me, if I wanted to change mine. But, they also said that if I have full trim authority, I could leave it alone - which is what I've elected to do so far. When my plane came back from the paint shop, the pushrod was painted so nicely!

Hope this helps.

_Alex
 
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My -14 has the trim tab exactly in trail with the elevator and that seems to work perfectly for takeoff and climb and cruise, even at 180 knots. The only time I really make anything but micro adjustments is coming in to land which required a fair amount of up trim. I have always been blown away by how little trim this airplane requires over a fairly large speed range. I have the original trim pushrod from 2018.
 
On another recent solo flight, at 9500 ft, I needed about 50% full nose down trim to maintain level flight, CG about middle of range, +2% pitch. The elevator is only slightly deflected down relative to the HS. Cruise TAS is typically 165-168 kts at full throttle. I may take up the task of measuring elevator and trim deflection versus CG for the RV-14A at various flight phases. My guess is that at a more forward CG, elevator deflection will trend toward zero, so I see no need to 'shim' the HS or increase the control arm length on my bird. It would be interesting to hear what others are seeing under similar conditions.
 

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On another recent solo flight, at 9500 ft, I needed about 50% full nose down trim to maintain level flight, CG about middle of range, +2% pitch. The elevator is only slightly deflected down relative to the HS. Cruise TAS is typically 165-168 kts at full throttle. I may take up the task of measuring elevator and trim deflection versus CG for the RV-14A at various flight phases. My guess is that at a more forward CG, elevator deflection will trend toward zero, so I see no need to 'shim' the HS or increase the control arm length on my bird. It would be interesting to hear what others are seeing under similar conditions.

That is my typical cruise speed and I am guessing a slightly more nose down elevator deflection purely based on the dirtiness of the elevator horn. I will try next time to observe and see if I can take a picture such as you have done to see how much deflection it actually is.

But to your point and others, if you have enough trim authority + some reserve, then it is best to leave it alone. This is a true case of "if it aint broke, don't fix it"
 
Wondering . . . . .

To folks with a "full down trim in cruise" RV-14,
is your plane a SB or QB ???
 
Indicated trim

Seems most 14’s cruise at 80% to 100% indicated nose down which is ok.
 

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Review 14’s EFIS

That was not my 14 but another VAF post. Mine shows ~~~80% in cruise trim down but almost impossible to discern the exact %.
 
I assume the RV-14 trim tab linkage is not linear (just like the other RVs). As such, trim indication may be misleading. I suggest doing a table on trim indication to actual trim tab position.

Carl
 
Not a speed run

As stated, not a "speed run". Just mid altitude cruise.
Note trim indicators upper right. Cruise is absolutely neutral.
Slow build. B222E317-4D31-4328-8678-ED9FF68647F6.jpg
 
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