RV-14A Excessive elevator trim in cruise

[MS19087]

I know this issue was raised under another thread, however nothing conclusive was presented so I am revisiting the topic.

This discussion compares two very similar RV-14As. The biggest difference being prop and paint weight.

RV14A #1 (mine) one has a 3 blade composite Hartzell (Empty weight 1283 - Nose = 375# Mains = 457#+ 451# )

RV14A #2 has a 2 blade metal Hartzell (empty weight 1333 - Nose = 377# Mains = 480# + 476#) This plane has more aft CG.

In cruise, my RV-14A (#1) flies with the elevator trim very much neutral with one or 2 passengers aboard - i am very happy with its trim characteristics.

In contrast, my neighbor (#2) flies with what i consider extreme elevator trim (down) in cruise w/ 2 pax.

A) Is this condition entirely CG driven?
B) Could an incorrectly installed nose fairing have any negative trim effects if not aligned level for cruise?

By the way - his RV is about 5knts faster than mine

Thoughts?

 

[mturnerb]

I know this issue was raised under another thread, however nothing conclusive was presented so I am revisiting the topic.

This discussion compares two very similar RV-14As. The biggest difference being prop and paint weight.

RV14A #1 (mine) one has a 3 blade composite Hartzell (Empty weight 1283 - Nose = 375# Mains = 457#+ 451# )

RV14A #2 has a 2 blade metal Hartzell (empty weight 1333 - Nose = 377# Mains = 480# + 476#) This plane has more aft CG.

In cruise, my RV-14A (#1) flies with the elevator trim very much neutral with one or 2 passengers aboard - i am very happy with its trim characteristics.

In contrast, my neighbor (#2) flies with what i consider extreme elevator trim (down) in cruise w/ 2 pax.

A) Is this condition entirely CG driven?
B) Could an incorrectly installed nose fairing have any negative trim effects if not aligned level for cruise?

Thoughts?

One parameter to check is the calibration of trim in the EFIS - may be just as simple as being sure that EFIS "neutral" trim indication is actually neutral...

 

[Carl Froehlich]

Agree - first verify the trim indication is reflecting reality. If so then excessive nose down trim could indicate:
- CG too far aft.
- Horizontal Stabilizer angle of attack is off, resulting in excessive tail down force. I don’t recall if the RV-14 uses a shim under the forward HS spar or not to set the HS angle of attack. Check the prints.
- Less of an impact on pitch trim would be some rigging problem with flaps. I’d first check that #2 flies with flaps in the reflex position (full up).

Now overlay speed effect. A too far aft CG will be more noticeable at low speeds. A HS with too much down force will be more noticeable as speed increases. On my first RV (8A) I increased the shim under the HS forward spar slightly to get the elevators more in trail for cruise. Be careful, too much and you run out of nose up trim on landing.

That would be another data point. Compare landing nose up trim on both airplanes.

Take some data and figure out what you have.

Side note - assuming both airplanes are properly rigged I’d guess the 5kts faster speed reflects the better efficiency of the two blade Hartzell BA prop converting engine power to thrust at typical RV cruise speeds.

Carl

 

[bjdecker]

Not just the prop...

Don't trust the IAS/TAS indications until you verify the speeds with the NTPS or PEC spreadsheet tool (GPS 3-way or 4-way).

I've seen variations in the actual TAS as high as 7Kts due to paint on/in/around the static ports.

 

[bjdecker]

I don’t recall if the RV-14 uses a shim under the forward HS spar or not to set the HS angle of attack.

Just FYI - on the -14 it is a fixed location dictated by pre-drilled holes in the fore and aft mounting "forks". It is what it is...

 

[rvbuilder2002]

By the way - his RV is about 5knts faster than mine

Thoughts?

His is faster because he has the two blade metal and you have the 3 blade composite…..

Have you confirmed that there is a major difference in the trim tab position, or are you just going by the panel indicator?

 

[MS19087]

His is faster because he has the two blade metal and you have the 3 blade composite…..
Have you confirmed that there is a major difference in the trim tab position, or are you just going by the panel indicator?

Yes - I am aware the speed difference was due to prop choice . . . however i am willing to sacrifice 5 kts for smoother operation with less vibration (IMHO).

We have not confirmed that this is a panel indication error . . . next item to check - thanks!
.

 

[Bavafa]

I have been flying a new 14A (belong to my friend) that has the same prop and same engine as mine but his is unpainted and a bit more nose heavy than mine.
Mine is a bit faster (6-7k) and has more nose down trim than his. My speed has been verified by the 3-Way GPS and his has been verified by the same method.
One other big difference that I see, the cowl flap has a much bigger impact on oil temp on mine than his. If I close his cowl flap the oil temp first go up by around 4-5F and comes back down to the same temp. But the effect on his CHT is greater than mine. There is also higher pressure on my cowl flap to close than on his indicating higher pressure in my lower cowl.

 

[BruceW]

Are engine thrust angles the same?
Washers behind motor mount? (retro from the -7 setup)
What else can change thrust angle?

If HS angle is set by predrilled holes (no change), can thrust angle be the difference?

 

[czechsix]

Have you done a formation flight?

Yes - I am aware the speed difference was due to prop choice . . . however i am willing to sacrifice 5 kts for smoother operation with less vibration (IMHO).

We have not confirmed that this is a panel indication error . . . next item to check - thanks!
.

Mark, have you flown these aircraft side-by-side to confirm that 5 knot speed difference? Paul Dye tested the 3-blade composite Hartzell vs. the 2-blade metal Hartzell on his RV-8 (and also tested a new Whirlwind 3-blade) and wrote a Kitplanes article about it, as I recall they were all very close to the same performance. 5 knots is a significant speed difference and my bet is there are other factors (fairing alignment, power setting--especially leaning technique, fuel flow, % power, optimal rpm for the respective props which may not be the same, density altitude, pitot/static system accuracy, OAT accuracy if you're comparing TAS, etc). Flying side by side removes a lot of those variables, at that point you mainly want to verify you're using similar leaning techniques with the same fuel flow and % power indications to really quantify speed difference. Are both aircraft using the exact same IO-390 model? Ignition systems?

Also you noted the difference in empty weight CG, but for apples-apples comparison of trim position in flight you'd want to factor in pilot/passenger/baggage/fuel (to get actual CG as close to the same as possible) and then fly together and visually check elevator & trim tab positions. As someone else noted, verify flap positions both in -3 deg reflex as well.