M

mark960

Active Member
I am flying an RV9A that was already built when I purchased it. It flies great but doesn't perform as well as others. One concern I have noted, is that in level cruise flight, the forward sections of the elevator are approx 1" above the horizontal stab. (ie. the elevator is pushing the nose down) It doesn't seem to matter how I load the plane, how much fuel or how fast I fly. The plane is typical O-320, CS Prop, typical avionics, etc... Besides the obvious drag of the protruding counterweights, I'm figure something is not rigged correctly leading to other additional drag.

Any thoughts???

Mark Klepper
RV9A
 
Last edited:
That's how my elevator looks in level flight. Almost level, no AP, 1/4 maybe?


elevator%252520counterweight.jpg
 
it will change with loading and density altitude...

the elevator to stabilizer relation should change with loading, air speed, and density altitude... as more lift is needed for level flight (high, heavy, slow) you need more nose up attitude... low, light, fast... less.
 
Vlad's elevator position

Vlad said:
That's how my elevator looks in level flight. Almost level, no AP, 1/4 maybe?


After the response from Ken K to "check the horizontal stab incidence angle." I searched the forum using "horizontal stab incidence angle" and found several discussions and info relating to my problem. From what I've read in the discussions, it looks like yours is where you want it. Apparently Vans wants there to be some deflection. (My guess is so that the elevator is slightly loaded to prevent flutter) My elevator counterweights are significantly higher than yours. I'd be happy with where yours are.

Today I added a 1/16" shim to the 1/8" shim that was already there on the forward brackets for the horizontal stab. I will test fly tomorrow and report back.

Mark K
RV9A
Click to expand...
 
Last edited:
Yep

Now you're on track, Mark. Our 6A was very close to Vlad's... about 1/8" to 3/32" up at cruise. With two aboard, the counterweights were just about in line with the stab.

Best,
 
Results after adding 1/16" shims

So.... I added the 1/16" shims (one per side) on top of the 1/8" shim that was in place. The result is that the elevator horns are now protruding only about a 1/4" at cruise (23 square). As far as top speed is concerned... no difference. Those who are thinking of doing the same, I would suggest not bothering.

So now it is on to other things to see why it is no performing as well as it should.

Current performance at 8,000' density altitude, wide open throttle, max RPM, leaned, flying a box to get average speeds.

Full fuel, pilot only, no baggage
OAT: 70
MAP: 23.4
RPM: 2660
GPS Ground Speed (avg): 155 kts
IAS: 135
TAS: 152 (As calculated on my Dynon)

My RV9A has a factory new 0-320 with 250 hrs, and a constant speed prop.


Mark K
RV9A
 
Error

Mark,
If your Ground speed is 155 and your TAS is 152 than you have an indication problem. Fix that first and then see how you are doing.
 
160 KTAS

Full throttle, max RPM, leaned for max. power (ROP), you should see approx. 75% and 160 KTAS.

Mine does 152 KTAS at that altitude, @ 2350 RPM, LOP (approx. 60% power) and "sips" about 25 ltr/hr.
Doing 161 KTAS (@ 2600 RPM, ROP) it will "slurp" 36 ltr/hr. Guess what my usual cruising speed is............

I see about 1/4" to 1/2" of the counterweights above the stabiliser, depending on the loading, but have not bothered to change anything because the fairing will no longer fit and I will have to redrill the holes in the front brackets of the elevator and make slotted holes....... ouch! (how did you do that?) Now that you confirm it does not make any difference anyway, I am not even tempted anymore.

Regards, Tonny.
 
Expected performance

Looking at Van's specs, I should see some 169kts. That number corresponds with what other people have posted and from some other RV9A pilots I have spoken with.

I don't plan on flying those numbers, but if I'm not getting those numbers at max performance, it means I am not getting the performance/efficiency I should be getting at my cruise settings. At 23 square, I typical see mid to upper 130kts TAS

Mark K
RV9A
 
Wow, I didn't think there was any way I'd REALLY end up with a 160 knot airplane, as planes can often fall far short of manufacturer claims. But you guys have me excited now. I'll be watching this thread with great interest.

Mark, best of luck with your efforts!
 
Mine too!!

I just noticed the other day that in level flight, my counterbalance arm was about 1" above the horizontal stab. Mark is saying don't bother, and if it does not change anything I can understand that, but should I add another shim to adjust that.
I've only got a 0-235 with fixed pitch prop, and I would love to see figures that Mark is getting. Our 9a only has 31 hrs on the hobbs, and the wheel pants and leg fairings haven't been fitted yet. Until they have been fitted I haven't been concerned about the performance figures.

Bob
 
Mark,

JMHO but, I think your expectations are a little high. Looking at Van's specs. for 75% power @ 8000ft I see 187 mph solo or 162.5 kts TAS. The 169 kts is Vmax at sea level I think. You're not far off from Van's numbers.
I normally see in the 160-162kts TAS range at ~8000 ft with fixed pitch and a little less than 2600 RPM. I have to pull the black knob back a bit so as to not exceed the prop RPM limit.

Bill:)
 
BillC said:
Mark,

JMHO but, I think your expectations are a little high. Looking at Van's specs. for 75% power @ 8000ft I see 187 mph solo or 162.5 kts TAS. The 169 kts is Vmax at sea level I think. You're not far off from Van's numbers.
I normally see in the 160-162kts TAS range at ~8000 ft with fixed pitch and a little less than 2600 RPM. I have to pull the black knob back a bit so as to not exceed the prop RPM limit.

Bill:)
Click to expand...

Your numbers are about right, Bill.

I think Mark could come up close to 160 if he ran at best power, about 100 ROP. For sure that's how Vans does it.
 
mark960 said:
Looking at Van's specs, I should see some 169kts. That number corresponds with what other people have posted and from some other RV9A pilots I have spoken with.

I don't plan on flying those numbers, but if I'm not getting those numbers at max performance, it means I am not getting the performance/efficiency I should be getting at my cruise settings. At 23 square, I typical see mid to upper 130kts TAS

Mark K
RV9A
Click to expand...

At 100% power you may see 169, Mark. You won't see it at 8000' where max power is about 75%.

If you have an EGT system, set 100 ROP and see how it rolls at that setting.
 
BillC

You are right, I was wrong. I was reading Top Speed not the 75% at 8000, which is the most power you can get at 8000. (without a turbo). So.... it looks like I am not that far off of where I should be.

Now I am only looking for the missing 7 knots. ie... It's probably one or more small items that need to be tweaked.

Mark K
RV9A
 
Off topic, Vlad, but I love the picture of NYC in the background!

Jim Bower
St. Louis, MO
RV-6A
N143DJ (flying...finally)
 
Just out of curiosity, are you sure your OAT is reading correctly. I was looking at the temp's in NY the last few days and it looks like you should have a lot cooler OAT than 70 degrees to get a DA of 8000 on an 80 degree day.
I had my temp probe mounted in the NACA inlet on the side of the fuse and my temps were off by 15 to 20 degrees. This will definitely affect you TAS on the Dynon.

Just a thought..

mark960 said:
Current performance at 8,000' density altitude, wide open throttle, max RPM, leaned, flying a box to get average speeds.

Full fuel, pilot only, no baggage
OAT: 70
MAP: 23.4
RPM: 2660
GPS Ground Speed (avg): 155 kts
IAS: 135
TAS: 152 (As calculated on my Dynon)

My RV9A has a factory new 0-320 with 250 hrs, and a constant speed prop.


Mark K
RV9A
Click to expand...
 
David-aviator said:
At 100% power you may see 169, Mark. You won't see it at 8000' where max power is about 75%.
Click to expand...

It seems to be difficult to determine what the "official" 9A speed numbers are.

Vans lists 75% power at 8,000 ft at 161.5 kts with a fixed pitch prop (don't know which one).

CAFE gives an accurately tested speed of about 167 kts TAS at 8,500 ft DA WOT, 2,600 rpm and with a rich mixture. This is with the three blade MT prop and if I remember correctly with two mags.

It is some time since I tested my 9A at 8,500 ft DA WOT with a rich mixture but if I remember correctly I did three separate 4 leg runs and using the Excel GPS spreadsheet averaged 169/170 kts TAS. This is with electronic ignition, a Hartzell CS prop, 9:1 CR and at about 2,690 rpm (higher than CAFE).

So there are a number of variables and I think a lot depends on the prop and engine specification/equipment.

Fin
9A
 
Speeds

In zero wind, TAS is the same as ground speed. IAS will be lower depepending on density altitude and position errors.

IAS - number on gauge
CAS - IAS corrected for instrument errors
EAS - CAS corrected for compression [even RVs arn't fast enough to make this relevant :) ]
TAS - EAS corrected for pressure and temperature
GS - TAS corrected for wind.

The HS and elevator create lift, but in the opposite direction. Opposite of the wings. If it wasn't this way, the airplane would be extremely unstable and virtually uncontrollable. But because of this, the wing has to create more lift and thus causes more induced drag. The more counterweight you see above the elevator means the tail is creating less "negative lift", and therefore, in theory, you should go faster. That's why aft loaded airplanes theoretically go faster. In practicallity, it's probably 1/4 of a mph between the forward limit and the aft limit.
 
Last edited:
You must log in or register to reply here.