My Mooney is slow..... 1970 M20E

[N9405V]

Help, my new to me Mooney seems slow.  It seems to be about 20mph below what book numbers say it should be doing in cruise.  It also has a bunch of Lasar speed mods (cowl closure, flap and aileron seals, etc.).  I'm perplexed.  When we flew today CG should have been right in the middle of the envelope (380 lbs of pilots, full fuel, 50lbs of cargo) and at 2500 ft, 2500rpm and 21" MP, we are getting around 140 mph IAS.  Outside temp was about 75 degrees.  According to my math and the cruise table we should have been upwards of 160 mph.  Gear I think are retracting all of the way.  Cowl flap were closed, but the cabin air vent was open and the step is fixed.  The plane consistently burns 9-10 GPH and makes 2700 rpm on take off. 

 

I'm thinking something is out of rig or I've got a lot of just drag.... Any other ideas??

 

Oh, and it has the worlds slowest electric trim......

[Matt Michael]

Tach could be incorrect. Could be out of rig. ASI could be wrong. Static leak/blockage...

[carusoam]

Ideas that come to mind...

To go faster it helps to push the throttle all the way in.

MP is usually a bit higher at the altitude you have selected.

You didn't mention mixture. 50°F ROP is good for speed tests, not so much for engine health.

Am I close?

Best regards,

-a-

[carusoam]

There is a recent thread of speeds for an F.

It shows several people defining what you are looking for. Find the thread and you shall find nirvana...

Best regards,

-a-

[Hank]

That low, I normally run 23"/2300 in my C, and see in the high 130s with my 3-blade, 201 windshield and guppy mouth closure.

Your E should be faster. My critical number (MP + RPM) is 47, so with 46 I'm under 75%. Your E is probably around 70% there. Since you say 9-10 gph, I suppose you are running ROP, as I am.

Things I would check:

-baffling and doghouse for cracks, silicone folded over wrong, etc.

-is the plane coordinated? Does it fly straight, with wings level, if you let go of the yoke? Auto-crab will rob speed.

-are you leaning properly? Full rich is no way to fly unless everything is fully forward. Did that once recently for ~10 nm to get out from clouds blowing in, as I didn't want to take time to file. 1000 agl, WOT, 2700, Full Rich gave 165 MPH indicated.

-full gear retraction (I used to drag one wheel down several inches in winter, it cost 10 mph. New pucks cured it.) what does the floor indicator show?

-check rigging with travel boards and experienced mechanic. Include flaps (some people adjust flap up stop screw instead of rigging properly, and fly in crab ever after.

Let us know what you find. Pretty sure you won't be the only one with the same problem(s).

[clh]

OK,  I am not by any means an expert, Mooney's really don't like low and slow.

 

I would suspect your tach first. 2500', 2400 rpm and 21"  only show 144 mph ias in my POH at  11.1 gph ROP.  That is a pretty big hit for 100rpm.

 

If you are running LOP, you could be getting that performance. 

 

It also could be  you rigging, but I think you will need to open the throttle and go higher to get into the "Mooney zone"

 

craig

[N201MKTurbo]

If it is slow it is usually the rigging. The first thing I would check is the gear. Put it up on jacks and make sure they are retracting up tight. A gear door hanging down a 1/2 inch will cause a huge amount of drag.

[carusoam]

Now I get it.

You were flying along at 2500', recorded the IAS and are wondering why it doesn't match the ancient collection of pseudo data in the owners manual.

By using this method you are adding the ancient instrumentation to the equation as Matt has pointed out.

Keep in mind when the data was compile in the 60's the plane was empty and other standards were followed.

What book values were you using?

It is a real technical challenge to get book speeds. It has less chance of doing so accidentally...

1) get the right book.

2) get the right engine settings.

3) get the right rigging set up.

4) fly at the best altitude for power vs resistance.

5) use WOT.

6) set the mixture.

7) check your APS FF vs EGT balance. There is a name for this...?

8) set your W&B for light and back of the envelope

Then compare to recent reports of similar airframes.

Comparing to old 'book values' is a hopeless cause. See Marketing vs Engineering discussions.

Best regards,

-a-

[Hank]

Actually, Anthony, my old C with vintage instruments comes pretty close to matching the speeds on the curled, brown pages at the back of the stapled-together Owner's Manual at the altitudes I've bothered to check. It's given every 2500' from sea level to 12,500, and I usually traverse 3000-10,000 but sometimes higher or lower. I flew half the length of the Outer Banks at 1200-1500 msl while 3 of my wife's cousins went flightseeing with me to Kitty Hawk [powered way back . . . ].

 

I suspect he's LOP, dragging a gear, or and/out of rig and flying in a crab.

 

But what do I know about the care, feeding and output of a fuel-injected engine? Could be injector issues for all I know . . .  

[DanM20C]

The cruise airspeed charts in my poh are true airspeeds, not indicated. So you are missing it closer. Your true would have been pushing 150. I would first take a look at how accurate your tach is. I recently checked mine with the prop balancing guy and it was less than 50rpm off. He said most of the time the old mechanical tachs are at least 100 off.

[MB65E]

Your Fuel flow should determine power. At 2500 ft I would think you should see 15.0 gal with everything at full tilt. Pull the cheek cowls and check to see the throttle and mixture are hitting the stops. My guess is they are not and were rigged to idle correctly. It's hard to get the throttle correct. Re manufacturing of the dump attachment was the only way I could get mine right. Mine will be cooks close to redline at sea level flat out...

Let us know.

-Matt

[Bob - S50]

I'd have your airspeed indicator checked.  Ours was off by as much as 8 knots when we got it (depending on the airspeed).

 

Bob

[cliffy]

I suspect that your premise is wrong in checking your speed. I believe it's the power setting you chose

 

You really need to check it with WOT, 2700 RPM and at 2500' full rich mixture before you do anything mechanical.

Get a couple of data points at 5000' and  7500' also with WOT and 2700 RPM 50-100 rich of peak mixture. 

 

Also (IMO), IAS is worthless for this. You need to figure TAS  Either do a 3 way GPS average or get out the old wiz wheel 

and figure it with that. Even using IAS instead of CAS will give you good data within a couple of MPH. 

 

Also your ASI may in fact be wrong making the GPS test even more applicable. Do it both ways and if large errors show up

check your ASI out for accuracy. 

[psb]

I would just offer this:

 

My ugly, unmodified '68 F runs extremely close to book speeds and fuel consumption up high when TAS is properly measured. I've never checked down low. So I'm not so sure that those who say you can't hit book speeds are correct. The book numbers in my experience are reliable.

[N9405V]

Thanks for the replies!  I'm using the Owners Manual that came with the plane for my numbers.  We'll look into all of the ideas over the next few months.  I've got the annual scheduled for the first of December.   Really, it may just be pilot error.  :-)

[bonal]

Just completed a 3 and 4 heading check @ 8100FT found ASI to be off by about 5 knots. I would do a GPS check before anything else.

just a thought.

[FloridaMan]

Ditto on the GPS check. I'd also recommend cutting open your oil filter next oil change. I'm a guy on the internet and not an A&P and I'm far from an expert, but I've heard that when the IO360 has a cam lob start to round itself off that the symptom is that the engine is a little "tired." See: http://www.cujet.com/html/engine.html

 

My modded M20F at 6500ft with full fuel indicates 160-165 mph ROP. 

 

I don't think that making full RPM is indicative of making full power. I would assume that the prop governor would just adjust the prop to take less of a bite out of the air and that it would still reach 2700 RPM. 

 

Wait... did you say you're only getting 21" MP at 2500ft? You should be well over 25" at 2500ft. 

 

Also, if your step is fixed and it was a retractable step, is the hole where it retracts covered? When I lost my vacuum pump on my F, I lost a considerable amount of airspeed due to what I perceive being the step being out and that big hole adding drag. 

[Guest]

My second E model would exceed red line(189 mph) straight and level in cool air a max RPM and full throttle.

Clarence

[philiplane]

A ground speed check isn't very useful no matter how many legs you try, compared to just getting the airspeed indicator checked for calibration. The ASI always indicates impact pressure the wing sees, but GPS courses are only as good as the pilot gathering the data.

 

In my avionics career experience, most ASI are inaccurate due to leakage in the system. The indicator itself is usually quite accurate even after 30 years.

[WardHolbrook]

As others have mentioned, I would check the rigging, the tach and the pitot/static system. Without an airdata computer, you can come up with a pretty good approximation of TAS by averaging your GPS groundspeed on the 4 cardinal headings. It's not exact, but it's close enough and gets most of the pitot/static errors out of the equation. Optical tachometers are pretty cheap and make it easy to determine the accuracy of the one in your panel. While you're at it, if it's been more than 10 years since your airplane was last weighed it wouldn't hurt a bit to get that done too. Oh, and most of those early POHs were wildly optimistic. It seems that they were written by the marketing department, not by the guys in Flight Test. (Nowadays, the POHs are written by the guys in Flight Test, but edited by the guys in the Legal Department. Sigh.)

Edited November 1, 2014 by WardHolbrook

[Parker_Woodruff]

Hold on a sec. You are using only 21" manifold pressure if I'm reading your original post correctly. Temp is a bit warmer than standard.

I would want to see your charts, but I don't think the speed is too far off for that power setting. IAS is also going to show a few mph slower due to your altitude.

What speed would you get wide open throttle or at a nice 75% power?

Try Wide open throttle & 2500 RPM and lean to 9gph OR until you're comfortably LOP. What speed does that give you?

[cliffy]

For those so inclined to check their airspeed indicator here's a way

 

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

[N7186V]

Help, my new to me Mooney seems slow.  It seems to be about 20mph below what book numbers say it should be doing in cruise.  

I'm in the same boat with my F.  I'm not happy at all.

[Hank]

You do realize the difference between "indicated" airspeed [what you see in front of you] and "true" airspeed [what is written in the book]?

 

When I show ~135 mph at 10,000, I'm not running way slower than book speed. With 2% per thousand feet loss of indicated airspeed, that mean 135 + 20% = 135 + 27 = 162 mph, I'm right there where I should be by the book.

[N7186V]

You do realize the difference between "indicated" airspeed [what you see in front of you] and "true" airspeed [what is written in the book]?

Yep. Didn't pay attention to the True Airspeed label on the column. Wishing I could take back my post.