Just how fast would a C model fly, if a C model could fly fast?

[flyer338]

I am back to flying and back to  Mooney ownership after 8.5 years on the ground and it feels great. I was going through my hard drive and found something I wrote about testing my previous Mooney (1965 C N78898 - now flying out of Allentown, New Jersey). I flew this airplane about 2,000 hours over more than 10 years. Two years and 500 hours into my stewardship of 898 I began to modify it for reduced drag. Not only did I gain in speed, I also gained in climb, visibility, better ride in turbulence, and a little quieter cabin. Albeit at a cost of more than $6,000 (did the labor myself), and about 90 lbs. useful load.

The drag reduction mods were: flap gap and aileron gap seals, dorsal fairing, tail rood fairings, rudder and elevator hinge covers, cowl closure, landing light cover, brake rotation, 201 wing tips, 201 windshield, smooth belly, and moving several antennas inside. I also removed the rotating beacon and replaced it with strobes in the new wingtips. I offset some of the mod weight by updating the panel and losing the KX-170B and King ADF.

I did some speed testing at various altitudes using GPS to eliminate the ASI error. To the best of my ability, the speeds I am reporting are real. Before modification 898 would cruise at 140 ktas between 7500 and 8500 on 65% power.

Aircraft:

1965 M20C, TTAF 6179 hrs, TTE 1102 hrs (O-360-A1D, 180 hp).

Mods: flap and aileron gap seals, smooth belly, cowl closure, dorsal fin and tail root fairings, rudder and elevator hinge covers, 201 windshield, 201 wingtips, landing light cover, and brake calipers rotated.  Rotating beacon removed; com 2, transponder, and elt antennas moved inside.

 

Test conditions:

Altimeter 30.06, OAT at 1000' +15c, OAT at 5000' and 7000' +21c. Aircraft take off weight 2150 lbs at 48" aft of datum.

 

Test method:

For each altitude and aircraft configuration the plane was flown on a cardinal heading and the airspeed allowed to stabilize.  Actual true airspeed was calculated from the GPS groundspeeds on the cardinal headings (NSEW). All speeds are knots. Calculated true airspeed and density altitude were calculated using the CAL function of the KLX-135A gps. Flight test was done over the ocean to minimize the effects of up and down drafts on the results.

 

Results:

7000' top speed.  Full throttle (22.5" mp) and 2700 rpm. Mixture 100 rich of peak (best power). DA was 9000'.

IAS 140, calculated TAS 159, actual TAS 154.

 

7000' cruise.  Full throttle (22.5" mp) and 2300 rpm. Mixture was best economy (just rich enough to keep the engine smooth). DA was 9000'.

IAS 138, calculated TAS 157, actual TAS 153.

 

5000' top speed.  Full throttle (24.5" mp) and 2700 rpm.  Mixture was set full rich.

IAS 146, calculated TAS 160, actual TAS 155. DA was 6900'

 

1000' top speed.  Full throttle (28" mp) and 2700 rpm.  Mixture was set full rich.

IAS 163, calculated TAS 165, actual TAS 158. DA was 1200'

 

Conclusion: The airspeed indicator is 4-5 knots fast at cruise and about 7 knots fast at full scale (163 knots is almost VNE).  Before modification the max IAS was 150.

 

I would be interested in this kind of detailed test results from other stock and modified Mooneys, especially older short body planes and 201s.

[Brian E.]

The answer to the question is:  more efficiently than any B, C or P brand aircraft!

Seriously--good work and nice write up.  I've been looking for pics of these mods, would appreciate any that you have,

[gsxrpilot]

I used to own a very fast M20C. It was N6XM, but has since met the reaper and is no more.

She was a fully modded C. She had once been owned by an Indy Car driver who did much like you did but without doing any of the work himself. She came with a crate full of receipts for over $100K of work just to the airframe to have fitted every know speed mod available. When I was in negotiations to buy her, I sent her to Don Maxwell for the pre-buy. He informed me later and has since told anyone who asks, that 6XM was the fastest C he's ever flown. 

I could consistently flight plan for 155 knots TAS and would get it.

[Jerry 5TJ]

Check out the Cafe Foundation Mooney for a stretch goal.  Yes, it was an E model, not a C.  But the airframe and cooling mods were applicable to any short-body M20.  The developers were fond of touting efficiency and not speed: The claim was it would do 160 KTAS on 6.7 GPH  

I opine the upper limit of an “unlimited mod cost” M20C cruise speed is around 170 KTAS / 9 GPH / 6,000’.  

For $250K I’ll prove it....

[Pete M]

I had a 64 E that would do 162.75 ktas on 4 way gps runs. Did one run at about 8500ft 23" 2550rpm and another at 10500ft 21" 2550rpm. Both runs averaged 162.75ktas. It had a recently oh'd engine, new cam, new lyc cylinders, new paint and new prop. Ailerons were set at 0 and flaps were set to be out of the breeze. Only mods were flap gap seals. If I did it again i would have sent the cylinders to lycon for a port and polish. Oh, I did have the k&n flilter (challenger?) That airplane was only good for about 146 when I bought it.

Edited December 2, 2019 by Pete M

[flyer338]

N78898 was one of the planes that fell withing the great Chevron bad fuel debacle of 1994 and wound up with a factory reman engine. I flew it down to Lakeport for Lasar to do the work; Paul Loewen flew me back in his E (I think it was a '65) and it trued at 170 knots at 6,500.

Here are a couple of pictures of 898:

[David Lloyd]

You guys are making me feel bad. My much modded C drags along at 146Kts max.  Everything but the lower gear doors.  A full rigging is in order.  Maybe next year.

[Jerry 5TJ]

Mooneyspace nautical miles may be shorter than standard.  

[cliffy]

There's 6076 and there's 5280 and then there's all the rest in between :-) :-)

DL I'll make you feel even worse  my UN modified  D model is only about 6 to 7 kts slower than your speedster and its never had any speed mods  :-)  It does need a good rigging though. 

[Tcraft938]

I REALLY appreciate this post since I only have a few hours in our new to us 63 M20C.  Also I flew it yesterday on a cold winter day and at 8500' -8C I was seeing numbers I didn't believe, 182 mph or 158 kts True 2400 rpm 22.5 MP.  I'm still doing the engine break-in so I have been keeping copious data notes from each flight.  If your curious, attached is a data sheet of the first flight.  I don't have any speed mods but I would like to get a few.  Number one on my list is the cowl enclosure, for better cooling and aesthetics.  It allegedly improves speed 5mph, I would be curious if those on the group saw increases to speed or rate of climb for items such as the cowl enclosure, gap seals, etc.   PS I also attached an excel file that looks much better and can see flights #1, 3 and 7.  Over the first 10 hours or so it seemed to get a little faster at the same power settings.  The latest issue I have noticed is it appears my MP gauge is reading almost 2" lower than actual.  I believe this because when on the ground if I set the altimeter to elevation the MP gauge is almost 2" lower as well as in cruise if I set for book, I'm significantly faster than book and burning a little more fuel per hour.  Annual is next month so will learn what that will entail.  

Flight No	1		Date:	9/3/2019		Tach End	4,574.55
			T/O Time	11:05am		Tach Start	4,573.20	Flight Duration	1.35	hrs
	To of Climb	Cruise	Cruise	Pattern
Time	11:14	11:40	11:55	12:10					Squaks & Discrepencies
IAS mph	157	153	158	120					1		Prop RPM low for T/O approx 2640
True Airspeed	171	167	166	124	-	-	-	-	2		Fuel Flow Inop reads 0.0
Alt	4500	4500	2500	1800					3		Small oil leak appears front of oil cooler
OAT	61	62	66	74
RPM	2500	2400	2400	2500					Notes & Comments
MP inches	23	23	23	15					1		Oil consumption, approx 1 qt
% Power	77%	74%	78%	52%					2		Fuel used, approx 12.5 gallons
Fuel Pres	4.8	4.8	5	4.8					3		Ave Fuel burn 10.8 gph
Oil Temp	182	178	180	184					4		Estimated power based on 180 hp = 80.4%
Oil Pres	72	71	74	69					5		Indicates engine is capable of > 180 hp
CHT
Cyl 1	300	290	272	292
Cyl 2	264	239	228	246
Cyl 3	315	313	298	307
Cyl 4	313	304	288	298
EGT
Cyl 1	1254	1218	1224
Cyl 2	1163	1082	1098
Cyl 3	1256	1244	1246
Cyl 4	1132	1092	1115
Volts	13.8	13.2	13.1	13.1
	Min	Max	Redline
Oil Temp	100	225	245
Oil Press	55	95		25	Idle
Fuel Press	1.0	8.0		3.0	desired
CHT	100	450	500

Break in Data.xlsx

[Prior owner]

A lot of things can be discovered during rigging.....turned out that my aircraft had the wrong elevator bungees installed,, and the trailing edge of the elevator was several degrees "down" when trim was set for take-off.  Prior owner complained of lack of nose-up trim on approach....I'm not surprised!

 Check out the pic of 898 above- his elevator in cruise is perfectly streamlined with the horizontal stab. Is yours? Mine isn't...

[aviatoreb]

2 hours ago, Jerry 5TJ said:

Mooneyspace nautical miles may be shorter than standard.  

Let,

1 mnm= gamma nm = 1.15 gamma sm

where gamma=(1/((1+sqrt(5))/2))^.05 .

[Jerry 5TJ]

6 hours ago, cliffy said:

my UN modified  D model is only about 6 to 7 kts slower than your speedster

Really?  Not even the retractable gear mod?  Impressive.  
 

[cliffy]

OK you got me!

[DXB]

8 hours ago, PilotCoyote said:

 Check out the pic of 898 above- his elevator in cruise is perfectly streamlined with the horizontal stab. Is yours? Mine isn't...

This is an interesting point, but it's not necessarily rigging that achieves this I think.  I thought it tended to align at relatively aft CGs, contributing to the speed advantage in that part of the allowable W/B envelope.  Maybe he's got some cinder blocks in the baggage compartment. 

[Prior owner]

Agreed. I've got 50 lbs in my baggage area and even when solo I still can't get the elevator to lay flat at any cruise power setting- it's always slightly "up".

A small bungee adjustment would give that elevator position at cruise. 

[Hank]

9 hours ago, PilotCoyote said:

 Check out the pic of 898 above- his elevator in cruise is perfectly streamlined with the horizontal stab. Is yours? Mine isn't...

And he has cleaned up his antennas (I onoy see one, way back towards the tail), possibly removed VOR antennas (don't see whiskers, blades or my annoyingly draggy towel bar) and the roof scoop is closed. It all adds up . . . .

[carusoam]

9 hours ago, Tcraft938 said:

 I don't have any speed mods but I would like to get a few.  Number one on my list is the cowl enclosure, for better cooling and aesthetics.  It allegedly improves speed 5mph, I would be curious if those on the group saw increases to speed or rate of climb for items such as the cowl enclosure, gap seals, etc.  

many of the cowl closures have come to a natural end... no longer in production...

There is one Cowl mod that is ramping up production around here... not too hard to find.

Then there is the LoPresti... a great cowl, but if you can afford it... you probably already bought something faster.... 

Best regards,

-a-

[Tcraft938]

I was thinking about the more simple frontal enclosure sold by LASAR.  They claim 5 mph and better CHT.  While I love the looks of bigger projects like LoPresti, I don't think I can justify that kind of $ and labor to improve looks and perhaps nominal increase in speed/efficiency.  Just got down with OH engine and I have a feeling the not too distant future entails doing something with fuel tanks and paint after that.  Perhaps during paint would be the time to incorporate some of the various mods like cowl enclosure, gap seals, one piece belly pan.  Incidentally, my A&P tells me that at annual next month he can guarantee after assigning me the task to do the three belly panels I will be putting the one piece at the top of my priority list.  He keeps joking that he's already ordered it.  LOL  

[DXB]

1 minute ago, Tcraft938 said:

They claim 5 mph and better CHT.  

It does look better but that's about it.  Take the LASAR claims with a grain of salt for sure.  I have one and am waiting on Sabremech cowl.

[cliffy]

Belly panels?  Eh- put it up a little on jacks for room and take a small electric screw gun (know how to use it by breaking the screws loose first then hit the trigger) (and use a good NEW bit) and they all come off in about 15 mins. No big deal. I replace all the screws for new every 3 or so times off. Makes a big difference. Remember, all the screws down there are not the same. mark them with tape and pencil. 

You'll spend more time cleaning them than it took to get them off. Good time to wax them when they are off. 

[DXB]

12 hours ago, PilotCoyote said:

Agreed. I've got 50 lbs in my baggage area and even when solo I still can't get the elevator to lay flat at any cruise power setting- it's always slightly "up".

A small bungee adjustment would give that elevator position at cruise. 

Maybe my thinking is off on this, but I suspect the bungee adjustment would be irrelevant. It would might change the pitch tension in the yoke, but the elevator position needed to maintain level flight would still be the same at a particular CG.  Bear in mind that I don't know squat about rigging, so I welcome correction.  

[Prior owner]

1 hour ago, DXB said:

Maybe my thinking is off on this, but I suspect the bungee adjustment would be irrelevant. It would might change the pitch tension in the yoke, but the elevator position needed to maintain level flight would still be the same at a particular CG.  Bear in mind that I don't know squat about rigging, so I welcome correction.  

My thinking is that the aircraft can be rigged to have the elevator flying at neutral in cruise (simply adjust bungees for less upwards deflection, and the horizontal stab will have to be moved more nose down in cruise by the pilot- the result will be an elevator at neutral position in cruise).

After using the travel boards on my plane, I believe that it is highly probable that their are variations/slop in their manufacture. Although my plane has been "rigged" with them, it doesn't necessarily mean that the plane's rigging is correct.....

Did Mooney intend for the elevator to have positive deflection in cruise? Why when I look at other Mooneys, do I see inconsistencies in the position of the elevator in both takeoff trim position and cruise trim position? Yes, CG plays a large part, but I believe that there is more to this....

Would love to hear from an old Mooney test pilot!

Edited December 3, 2019 by PilotCoyote

[EricJ]

Remember the wing makes a LOT of the total drag, and changing the amount of downforce created by the tail by fiddling with the trim changes the amount of lift the wing has to make, and affects the drag it produces.   What one might gain in making the elevator look cool in trail might be more than lost by added drag in the wing by changing the required angle of attack.    The lift has to come from somewhere, so if you remove some of it (or decrease the downforce) in the tail, the wing has to compensate with more lift and more drag.

I think Rich mentioned quite a bit back that you can find the minimum drag configuration by adjusting the trim through a range and holding the airplane level with the yoke (against the trim forces) and observing the resulting airspeed.   The minimum drag configuration may not be with the elevator flush in trail.  I think this is also why short, mid, and long-bodies may wind up with the trimmed elevator configuration slightly different for each, since the required tail downforce in cruise is different for each.

The guy that built the full-sized Spitfire replica, with wooden wings and a steel frame fuselage, said he thought it was cool that pics of Spitfires in cruise had slight down elevator and he noticed that his did, too.   He took that as a sign that he got a lot of it right. 

Edited December 3, 2019 by EricJ

[Prior owner]

Yes, come to think of it I do remember Rich saying that, and it does make sense. I think you just assigned me a science project for my short body...! At some point, when I have good weather again, I'd like to spend a little time flying the aircraft and seeing what configuration gives it best speed. Until then, I cannot be sure that vanity and aerodynamics might not go hand-in-hand. You certainly have me doubting my original idea though...

Edited December 3, 2019 by PilotCoyote