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-   -   Is our RV-8A a little slow...? (https://vansairforce.net/community/showthread.php?t=199374)

EricTheRV8er 09-20-2021 06:01 PM

Is our RV-8A a little slow...?
 
2 Attachment(s)
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!


https://vansairforce.net/community/a...1&d=1632178807


https://vansairforce.net/community/a...1&d=1632178820

mattsmith 09-20-2021 06:23 PM

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?

alpinelakespilot2000 09-20-2021 06:23 PM

Have you properly calibrated/confirmed your airspeed indicator first using a proper calibration method. If IAS is off so too may TAS be off.

EricTheRV8er 09-20-2021 06:29 PM

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.

BobTurner 09-20-2021 06:40 PM

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.

EricTheRV8er 09-20-2021 06:46 PM

Quote:

Originally Posted by BobTurner (Post 1556832)
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!

gasman 09-20-2021 07:28 PM

Quote:

Originally Posted by EricTheRV8er (Post 1556817)
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.
url]

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.

EricTheRV8er 09-20-2021 07:35 PM

Quote:

Originally Posted by gasman (Post 1556857)
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.

DanH 09-20-2021 07:36 PM

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.

EricTheRV8er 09-20-2021 07:45 PM

Quote:

Originally Posted by gasman (Post 1556857)
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.


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