M20J Speed Improvement?

[Jayhawk_aviator]

Have a relatively new to me M20J.  Engine is strong with high 70s compression, and prop was just overhauled.  Extensive annual where gear doors / cowl flaps were rigged, and all flight control rigging was checked.

The inner gear doors were not installed when we bought the plane (and the fairing that go in front under the wing are missing as well).  As part o the annual we peformed the SB (forget the number) which removes the ram air (since it doesn't really really do anything on the Js).  The brake calipers are installed in front of the axle...not sure if that is an STC or something else.

Question is this...not seeing near the speeds expected.  I haven't done a true 4 way TAS test yet, but at 9500' (10 deg C) flat out (21ish inches MAP, 2600+ RPM, 10.2 GPH, one on board, 3/4 fuel) I was seeing around 152ish knots.  Book says we should be around 15 knots higher in that configuration.   

Other than the inner gear doors which I understand adds about 5ish knots (?), anyone have any suggestions on what may be happenning and why the numbers are so far off?

[carusoam]

Post a photo of your instruments while doing this.

Include ASI, GPS, JPI, AI, Altimeter...

You may be surprised what other people can see that you may be missing...

Best regards,

-a-

[markejackson02]

I figure on 155 kts high up and 150 down below 6000' @ 11GPH.  Those speeds sound about right.

[N201MKTurbo]

I agree, your 152 is about right.

[bradp]

Sounds about right.  I do LOP on the majority of trips and plan 148 true on 8.5-9.2 GPH LOP.  That ends up being somewhere in the neighborhood of 60-65% power depending on atmospheric conditions near 8k.  Ballpark I get about 155 true on 11.2 GPH 100-ROP around the same altitude.  

(Btw take a look at the POHs from some of the later model Js.  Best economy calls for 25-C ROP.  Ouch.  Yikes.  )

Low hanging speed fruit includes ensuring the gear doors are flush to the lower wing skin and ensure that you are rigged.  

Everyone has their own prerogative but I would advise throwing $$ at avgas or training instead of the elusive 1-2k (or 38kts) speed mod.  

[ArtVandelay]

Last time I was at 9500', OAT 14, FF 7.8, RPM 2300, MAP 21.7, HP 55%, IAS 128K, TAS 150K, I obviously run LOP, and flightplan for 150K

[Steve Dawson]

I've had the best of 158 KTAS at 8000' at 2500 RPM. I'd try rigging your plane first by checking that the gear doors are fitting properly, the flaps and ailerons are set for low drag and that the engine isn't drooping below the top of the cowl. Try putting weight in the cargo area to set the CG farther back. 

[N9201A]

Does anyone meet or exceed book? I flew with one pilot who claimed he "always" did...then he discovered his ASI was reading fast.

Your 152 number is in the ballpark to me (78 J X 19 years, several mods).

[Steve Dawson]

I should mention that I've checked my speeds using the 3 and 4 way GPS test also. 

[Steve Dawson]

Just now, bluehighwayflyer said:

It sounds like you would top out at around 160 KTAS at 8,000' DA, too, if you pushed it up to 2700 RPM.  The additional 200 RPM  is good for 6 percent more power, or 12 additional horsepower.  We are all actually remarkably close with our numbers. 

If I remember properly it was 163 but I was also fooling around trying to get the best fuel flow/power too and didn't spend enough time on it. At that time I didn't have on my step either but I don't think that changed the speed by more than a knot. Secondly I'd just had rebuilt cylinders installed too.

[ArtVandelay]

Last time I was at 9500', OAT 14, FF 7.8, RPM 2300, MAP 21.7, HP 55%, IAS 128K, TAS 150K, I obviously run LOP, and flightplan for 150K

If you look at the 55% HP @ 2200 RPM, at 2300lbs which is what I weighed...Im dead on book.

[Jayhawk_aviator]

Thanks guys.  I'm really surprised to hear that.  It seems like I'm at least 10 knots below book.

Regarding the inner gear doors...anyway have a good estimate on what those add?  The speed is better on the later (flat?) inner doors as opposed to the clamshell?  Anyone know the part number or reference on where to get?  I think I see that Mooney Mart has those?

 

[jetdriven]

~80 ROP is best power mixture and seems to be the only consistent way to repeat speed runs with any consistency.  

I can get 172-173 knots at 1000' MSL, I haven't done one at 8000' in a while.  But these airplanes are fastest at sea level and get slower the higher you go.  

[INA201]

This is a good thread too regarding inner gear doors.

I'm definitely getting 5 knots from adding them.  I haven't done a 4 way test yet but will post when I get back to it.  However, it seems that I'm at 156 knots now vs 152 knots prior to putting the doors on.  Im flying a 1978J with no step, ram air, multi piece belly, and standard everywhere else.  I'll see if I can do a run this afternoon and post it.

[Jayhawk_aviator]

Good to know.  I have a step and one piece belly for what its worth.  Interested in the old vs new door assessment.

[markejackson02]

On 9/13/2016 at 1:08 AM, Jayhawk_aviator said:

Thanks guys.  I'm really surprised to hear that.  It seems like I'm at least 10 knots below book.

Regarding the inner gear doors...anyway have a good estimate on what those add?  The speed is better on the later (flat?) inner doors as opposed to the clamshell?  Anyone know the part number or reference on where to get?  I think I see that Mooney Mart has those?

 

I put them back on when the plane was painted, they added a couple knots.  Since I fly only from paved runways, I leave them on.

[bradp]

I've had my rear fairings aft of the gear off for refurbishing.  Lost probably 2-3kts ...

[Guest]

On September 13, 2016 at 3:31 AM, jetdriven said:

~80 ROP is best power mixture and seems to be the only consistent way to repeat speed runs with any consistency.  

I can get 172-173 knots at 1000' MSL, I haven't done one at 8000' in a while.  But these airplanes are fastest at sea level and get slower the higher you go.  

Byron,

For those speeds I'm assuming full power?  My E model would exceed red line at full power at 2000 ASL.

Clarence

[Bob - S50]

On 9/13/2016 at 0:31 AM, jetdriven said:

~80 ROP is best power mixture and seems to be the only consistent way to repeat speed runs with any consistency.  

I can get 172-173 knots at 1000' MSL, I haven't done one at 8000' in a while.  But these airplanes are fastest at sea level and get slower the higher you go.  

That, of course, assumes you are using maximum power.  If you used a fixed lower percentage of power such as 65% or 75%, you actually go faster as you go higher until the thinner air makes it impossible to generate the desired power level.

[INA201]

I did a three way run yesterday @6500( 7800 density), 23.4 mp, 2500 rpm, 100 ROP, light turbulence, and afternoon flight.  My highest cht was 380 on number 4 cylinder.  Came up with 153 knots.  I've done a four way run before and have seen 156 knots under similar conditions.  My guess is doing this in the morning with still air would probably yield a little better airspeed avoiding compensation for vertical air movement to stay level.  Also, this is more of a cruise speed test run vs running at 2700 rpm, 50-80 ROP which might get this plane up to 160 knots or so.  Hmm, might give it a try.  Used this calculator below for the first time.  http://www.csgnetwork.com/tasinfocalc.html

[Jsavage3]

Your bird sounds a little slow to me. What mixture setting were you using that gave you 152 knots? The J that we owned for 5 years always gave us 157-159 KTAS (per the true airspeed function on the GTN-750), about 137-139 KIAS, at 8000-10,000 feet, 65% power at 22"MP (WOT) & 2500 RPM or 21"MP (WOT) & 2600 RPM, about 9.5 gph at 50 ROP, ram air open and cowl flaps closed. The highest KTAS I ever saw in that J was 161 KTAS.  Above 10K, the KTAS started taking a pretty big hit and the climb performance above 12K or so quickly became a frustrating wrestling match. We did have the power-flow exhaust and better (Challenger think K&N) air filter, so maybe that's what helped us... I still think a good running & properly rigged J should get close to 160 KTAS.

Edited September 15, 2016 by Jsavage3

[jetdriven]

4 hours ago, INA201 said:

I did a three way run yesterday @6500( 7800 density), 23.4 mp, 2500 rpm, 100 ROP, light turbulence, and afternoon flight.  My highest cht was 380 on number 4 cylinder.  Came up with 153 knots.  I've done a four way run before and have seen 156 knots under similar conditions.  My guess is doing this in the morning with still air would probably yield a little better airspeed avoiding compensation for vertical air movement to stay level.  Also, this is more of a cruise speed test run vs running at 2700 rpm, 50-80 ROP which might get this plane up to 160 knots or so.  Hmm, might give it a try.  Used this calculator below for the first time.  http://www.csgnetwork.com/tasinfocalc.html

That TAS calc is all based on IAS which isn't super accurate. For example at cruise speed, the CAS is about 2 mph slower that IAS. That's before instrument error. Do the 3 track GPS groundspeed method. It's very accurate.

Clarence the 172 knots is maximum power 80 ROP and 2700 RPM.  Pulling back to MP settings and RPM further complicates things.  Depends on density but it's around 195-200 MPH indicated airspeed. 

[Guest]

Last year I took the 400 out for a max power run at 2500', the best I could get was 240 mph indicated.  I will do it again soon after a few mods.

Clarence

[Jsavage3]

3 hours ago, jetdriven said:

That TAS calc is all based on IAS which isn't super accurate. For example at cruise speed, the CAS is about 2 mph slower that IAS. That's before instrument error. Do the 3 track GPS groundspeed method. It's very accurate.

Jetdriven, your feedback is appreciated, but, for clarification purposes, after 5 years of owneship and hundreds of opportunities to evaluate my ship's speed in cruise flight, to include the 4 track GPS method on several occasions, my reported 157-159 KTAS is accurate information. Again, thanks for your feedback. J

[dlthig]

3 hours ago, M20Doc said:

Last year I took the 400 out for a max power run at 2500', the best I could get was 240 mph indicated.  I will do it again soon after a few mods.

Clarence

I find this post offensive and insensitive, considering the IO-720 lashing the horses. I've got a few hours in a "B", but my buddy reported $10K annuals that I can't stomach. You know I respect it, but lack the testicular fortitude to go there.