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Is our RV-8A a little slow...?

Hi folks!

First post here from a freshly minted RV-8A owner. Absolutely love the RV and am really excited to finally be a part of the RV community. I hate for my first post to be speed related, as that's probably a cliche on here, but would like any advice on what things to look for first.

My amateur hunch is the prop has at least something to do with it. It's a fixed pitch Sensenich, and I have to pull the throttle back to keep from exceeding the 2600rpm limit. That's taking off about 3-4" of available manifold pressure (from 22" to 18").

I've included some pics with data from the G3X and a photo of a second test flight data.

RV-8A, O-360 approx 200 SMOH, Sensenich FP prop, test flights done at 1500lbs.

So I guess my questions are:

1. Are these speeds unusually slow for this engine/prop combo?

2. Would switching to a prop that can run 2700rpm make the plane fly closer to book speeds being able to utilize 70-75% HP.

3. Would switching to a C/S prop improve cruise speeds? I read all the time FP and C/S cruise speeds are close... but every fast RV I've seen has C/S prop.

Any info or feedback appreciated. Thanks!


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Someone a lot more qualified will come along shortly but I know they are going to ask if you have all the fairings and wheel pants on. Any other details you can provide?
 
One more bit of information... The wheel pants and leg fairings are on, however we are missing the root fairings (where the main gear legs attach to the fuselage). Not the whole gear leg fairing, just the transition covers where they go into the fuselage.

Have not calibrated the airspeed indicator. I suspect it is close (within a few kts at most) just based on a few flights with light winds, and TAS being very close to gps groundspeed.
 
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I think you have answered your own question.
Yes, if you have to limit MP to 18” when 22” is available, that’s a 20% loss in power, which will translate to something like a 6 or 7% loss of airspeed.
And yes, the gear leg fairings are important for drag reduction.
 
I think you have answered your own question.
Yes, if you have to limit MP to 18” when 22” is available, that’s a 20% loss in power, which will translate to something like a 6 or 7% loss of airspeed.
And yes, the gear leg fairings are important for drag reduction.

Yeah, that's what I thought... but have heard a lot of people like this prop on the RV-8, and not sure why they would if it's limiting cruise speed significantly.

In the second test at 7500', I'm showing 20"MP, but fuel flow is a little high. I feel it should be about 1gph less, but as I did that the engine produced more power and I had to throttle back. Efficiency gain, but no speed gain.

We have the root fairings (if that's the right term) so we will get new numbers once we get them trimmed and installed. I'm trying to be realistic and not expecting more than 1-3kts from that, but I guess we will see!
 
Hi folks!

First post here from a freshly minted RV-8A owner. Absolutely love the RV and am really excited to finally be a part of the RV community. I hate for my first post to be speed related, as that's probably a cliche on here, but would like any advice on what things to look for first.

My amateur hunch is the prop has at least something to do with it. It's a fixed pitch Sensenich, and I have to pull the throttle back to keep from exceeding the 2600rpm limit. That's taking off about 3-4" of available manifold pressure (from 22" to 18").

I've included some pics with data from the G3X and a photo of a second test flight data.

RV-8A, O-360 approx 200 SMOH, Sensenich FP prop (71x69), test flights done at 1500lbs.
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A Metal Sensenich on an O360 does not have a 2600 rpm restriction. The prop you have is under pitched. Van's calls for a pitch of 85 inches for the RV8 with the O360.
 
A Metal Sensenich on an O360 does not have a 2600 rpm restriction. The prop you have is under pitched. Van's calls for a pitch of 85 inches for the RV8 with the O360.


Very interesting on the RPM limit. Everything I read mentions the 2600rpm limit.

As to the pitch, I have not been able to verify what it's pitched at, just going on a piece of paper I found with that line highlighted...would need to take to prop shop to verify.

I am seeing around 2200 static, which I understand is on the high end (fine pitch) of the suggested envelope.
 
You should probably check airspeed calibration before assuming the indicated speed values are correct.

The NTPS-method spreadsheet can be downloaded here:
https://www.danhorton.net/Misc/NTPS_gps-pec.XLS

Note the entries are groundspeed (not airspeed) and track (not heading or course).

In the simple analysis, the indicated true airspeed on your EFIS should match the calculated value from this spreadsheet. Fly at dawn, use your altitude hold if you have one, lock the throttle for at least three legs at three headings.
 
A Metal Sensenich on an O360 does not have a 2600 rpm restriction. The prop you have is under pitched. Van's calls for a pitch of 85 inches for the RV8 with the O360.

I believe I may have misread the pitch. I believe ours may be pitched to 85 as the the model number is: 72FM8S9-1-85

That 85 at the end may be the pitch, if I'm not mistaken.
 
Very interesting on the RPM limit. Everything I read mentions the 2600rpm limit.

As to the pitch, I have not been able to verify what it's pitched at, just going on a piece of paper I found with that line highlighted...would need to take to prop shop to verify.

I am seeing around 2200 static, which I understand is on the high end (fine pitch) of the suggested envelope.

I believe the O-360 doesn’t have that 2600 rpm restriction, only the O320. What are you reading that says otherwise?

Also your oil temp on the pic of the G3X looks like you could warm it up a little.. perhaps some aluminum foil tape? I prefer 185 to 200 to cook off the moisture.
 
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I believe the O-360 doesn’t have that 2600 rpm restriction, only the O320. What are you reading that says otherwise?

Also your oil temp on the pic of the G3X looks like you could warm it up a little.. perhaps some aluminum foil tape? I prefer 185 to 200 to cook off the moisture.


I'm going off what Sensenich has on their Application Guide...https://www.sensenich.com/wp-content/uploads/2018/01/Application_Guides_Aluminum_Propeller_Applications_1349891787.pdf

The last page shows "Placard required to avoid operation above 2600 RPM. Unless I missed something in the literature, it doesn't specify engine type. Seems to be a universal limitation.
 
I believe the O-360 doesn’t have that 2600 rpm restriction, only the O320. What are you reading that says otherwise?

Also your oil temp on the pic of the G3X looks like you could warm it up a little.. perhaps some aluminum foil tape? I prefer 185 to 200 to cook off the moisture.

Duh... just saw the last column and #8 does not apply to our engine!

You are correct. Thank you for making me look this over more carefully!
 
Why 2600 rpm on the o-320?

Now I'm curious. Anyone know why the 2600 rpm redline on the o-320 engines? [And the o-360 180 hp]. Seems like the smaller displacement o-320 is going to make smaller thrust pulses on the prop than an o-360 200 HP. Resonant frequency issue?
 
Duh... just saw the last column and #8 does not apply to our engine!

You are correct. Thank you for making me look this over more carefully!

Nice.. now go fly tomorrow and let her rip! My -4 with the O360 and the same prop you have does 171 knots true at 12,500 18 inches, 2660 RPM 9.5 G/HR. I do have to throttle back a little so she won’t overspeed. WOT is 18.5 inches, and it’ll go over 2700, so I throttle back to an even 18 inches then lean for max rpm.
 
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From Van's......

70CM (150/160 HP) PROPELLERS ARE LIMITED TO MAXIMUM 2600 RPM..... This is not your prop.

Here's how to decode the part numbers: PROP 70CM7S9-0 (81) means: 70 = design diameter in inches, CM = designator for O-320, 7 = bolt diameter in 16ths of an inch, S = requires crank spacer, 9 = length of spacer in 1/4s of an inch, 0=full length blade (not cut down) (81) = pitch in inches.

The Sensenich metal propellers for the O-360 that have part numbers starting with PROP 72FM8S(xx-x) are NOT CERTIFIED.

https://shop.vansaircraft.com/cgi-bin/shop.cgi
 
fly an equilateral triangle (heading 360; 120; and 240) at the same indicated airspeed. Write down the "true airspeed" and "ground speed" numbers. Divide by 3, they should equal. If not, look for static leaks. I was able to bring my speeds into calibration using a small (.040) dam in front of the static ports.

DAR Gary
 
Flew a quick test flight this morning and was seeing ~160kts TAS at 7500' and 70% HP.

These are just figures off the G3X. Need to do some calibration tests and the TAS triangle. I will also do some at 11,500' and see. Will do that this weekend when the weather improves.



Thanks for the info everyone.

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Flew a quick test flight this morning and was seeing ~160kts TAS at 7500' and 70% HP. SNIP

Yep - slower than your plane could be, but consistent with my experience flying my first RV-8A with that FP prop. I did 300 hours flying behind that prop before I replaced it with the nice Hartzell BA prop. It became a new plane. I tell builders using this FP prop was my big mistake on that build, but I was following the advice of those who went before me.

The new RV-8 (180hp IO-360) has the same Hartzell prop and typical LOP cruise is 170kts (heavy) to 175 knots (solo). Fuel burn in between 8.5gph and 7.5gph (8K - 12K’).

I note that any propeller’s ability to transform engine power to thrust is a far more complicated equation than just pitch. I flew and RV-14 with a fancy custom composite two blade CS prop that was ~5kts slower in cruise than expected. It took all the 390 power just fine, I suspect however some was wasted on just thrashing the air.

Carl
 
Yep - slower than your plane could be, but consistent with my experience flying my first RV-8A with that FP prop. I did 300 hours flying behind that prop before I replaced it with the nice Hartzell BA prop. It became a new plane. I tell builders using this FP prop was my big mistake on that build, but I was following the advice of those who went before me.

The new RV-8 (180hp IO-360) has the same Hartzell prop and typical LOP cruise is 170kts (heavy) to 175 knots (solo). Fuel burn in between 8.5gph and 7.5gph (8K - 12K’).

Carl

Thanks for the info.

So you converted your -8A from fixed pitch to C/S? I know this engine has a hollow crank, so it could be done. Is there much involved besides the obvious? Did it require a new cowl or a cowl modification? And what speed bump did you see after converting?
 
Thanks for the info.

So you converted your -8A from fixed pitch to C/S? I know this engine has a hollow crank, so it could be done. Is there much involved besides the obvious? Did it require a new cowl or a cowl modification? And what speed bump did you see after converting?

In a perfect world you should be able to just replace the prop and make a new spinner (uses a different back and front plate than the FP prop). Spinner available from Van’s.

While there should be no big cowl changes, you might have a spinner to cowl issue - it all depends on what you have now. If so, you may need to do some fit work.

The touchy issues are:
- Do you still have the governor to prop oil line that came with the engine. You will need it. If not you can replace with a braided hose.
- You will need to remove and then replace the rear crank oil seal (the one that gets a hole punched in it if using a FP prop). This is not crazy hard but do not attempt without the help of someone that knows what they are doing and who has the special tools to do it right. The seal is aft of a oil supply tube that if you damage it you will be in the hurt locker.
- You need to buy and install a prop governor, governor control cable and prop control on your quadrant. Assuming you only have a two arm quadrant this will need to be replaced. Parts available from Van’s.

From memory I think I saw 170 knots on a good day at an RPM higher than practical using the FP prop. With the CS prop 180 was easy to hit if you want to burn the gas to do it. Note - don’t overlook the value of careful rigging on RV top speed.

On the first takeoff, when your head goes back from the acceleration, you will be rewarded for your efforts.

Carl
 
Note - don’t overlook the value of careful rigging on RV top speed.


Carl

I'll be looking into rigging and finishing our gear leg root fairings before making a prop change.

It would be nice to get the same cruise speed at lower RPM, but I probably wouldn't do the conversion without feeling confident the rest of the plane was sorted, and the prop is the weak link. I know there are other benefits with takeoff/climb performance, but in those regards the plane performs satisfactorily for my purposes. I just feel it should be faster than 160kts at 70% power. I was expecting 170-175kts out of it.

You are the only report I've read of a speed bump going from FP to CS.
 
Mo Speed

You are the only report I've read of a speed bump going from FP to CS.

I can confirm a speed increase, as well as ROC, take off and landing roll, reduced fuel burn, and a tremendous grin factor. And if you're interested in formation flying, it SO much easier with a CS prop.

As a friend said "stop on the side of the road and pick up alum cans, eat PBJ sandwiches, sell yourself...do whatever you need to do to buy a CS prop".

I switched my FP Sensenich (O-360) to a CS after 200 hours...what a different airplane!

Also, it doesn't need to be a new BA Hartzell. Some of the older models work very well too.

Laird
 
Thanks Laird. If you recall, what were your cruise speeds before and after?

It was 20 some years ago, so that information has been overwritten, and I've misplaced my flight test data years ago, but I do remember that it was something like 10mph, maybe more, maybe less.

I do remember that I had the 83" pitch, which was a bit of a climb prop, so I couldn't use all the throttle at cruise. I think they recommend the 85 or 87 now.

Speed isn't the only thing you get with a CS prop, it's all the other stuff that really changes the airplane. I used to say when I added the CS prop, I got the last 30% of performance out of the airplane.

My RV-6 has a max cruise (8000' DA, full throttle, 150deg rich of peak) of 183kts. Generally, I flight plan for 165kts door to door at 9 ghp which I usually beat.
 
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PA vs DA

At the start of your thread, it looks like you’re targeting 7500’ pressure altitude. For the 75% test, shouldn’t it be 7500’ density altitude?
 
CS vs FP again?!

Hey Eric,

I've got a FP (Whirlwind) prop on my 8, and I can tell you it's plenty fast. I cruise at 215mph at 2550, and hit VNE at 2750 at 7500ft. On the other hand I only see 2050 static.

This is with a fuel injected, dual P-Mag o360 with 9:1 pistons. The fastest it's gone is 243mph, but that was with mods that don't work for normal flying. Home made tailwheel pant that covers the wheel AND vertical socket, No external antennas, and a few other doodads.

When I built the plane I was planning on racing it, but then I saw the two RV8s that were in the 270+mph club and gave up on that.

The plane light at 1006 lbs, so that helps with the climb. A CS prop would help with the climb, but I don't think it would help the top speed.
 
Hey Eric,

I've got a FP (Whirlwind) prop on my 8, and I can tell you it's plenty fast. I cruise at 215mph at 2550, and hit VNE at 2750 at 7500ft. On the other hand I only see 2050 static.

This is with a fuel injected, dual P-Mag o360 with 9:1 pistons. The fastest it's gone is 243mph, but that was with mods that don't work for normal flying. Home made tailwheel pant that covers the wheel AND vertical socket, No external antennas, and a few other doodads.

When I built the plane I was planning on racing it, but then I saw the two RV8s that were in the 270+mph club and gave up on that.

The plane light at 1006 lbs, so that helps with the climb. A CS prop would help with the climb, but I don't think it would help the top speed.


Sounds like you have a very fast plane. I'm not intending or expecting this to be a 180kt plane, but I figured it would do a little better than 150-155kts at moderate power settings. It seems most report 165-175kts at similar power settings and altitude.

I am seeing 2200 static, and 2300+ about rotation speed. It's pitched at 85", which seems on the more cruise prop end of the spectrum to me, but 2200 static seems high for a cruise FP. Catto lists their cruise prop pitched at 78". I know there's probably more that goes into it than pitch, but still.

Would probably learn a lot borrowing a Catto someone has laying around and do some comparisons.
 
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Check the RPM with a digital tach, sounds like you may have a climb prop ?
What’s your ground roll, and vertical climb rate ?
 
Check the RPM with a digital tach, sounds like you may have a climb prop ?
What’s your ground roll, and vertical climb rate ?

According to Sensenich, it's labeled a "Standard" prop at -85. They offer what they label a "climb" prop as a -83 and a Cruise prop as -86.

On climb out with full fuel and just me, OAT surface 23C, I'm seeing about 1000fpm with a constant IAS of 125kts from SL up to 5500'.

I haven't recorded numbers at a Vy climb, but I would guess about 1200-1400 on average warm day.
 
I've got a Catto 3-blade FP on my -7 and my numbers aren't that much different from what you're seeing. At altitude I routinely cruise at 2650 and see 168 KTAS or so, with a lot more throttle to give. For a short test, I ran it full throttle at 8000 ft DA and it will do 183 KTAS but at 2800 rpm (obviously above engine redline, prop redline is 3100 rpm). Static is around 2300 RPM.

Point is that of course an well designed fixed pitch will go as fast as a CS, just at a different RPM depending on the pitch. For my typical mission the Catto is a great prop with nice balance of take-off, climb and cruise performance ...but I am one of those currently picking up soda cans on the side of the road to put towards an eventual CS upgrade :)
 
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