True Airspeed Losses

[airkraft]

Recently, I've noticed that my indicated air speeds (and therefore my GPS derived TAS) are about 10 KTS lower than what I am used to.  Does any one have any possible explanations?

The engine behaves normally.  Runs right up to 2,700RPM for T/O and climb.  Burns the same amount of fuel.  Airplane, a 1966 M20 E, just seems to be 10 Kts slow.  I do not think it is a pitot/static ,rigging or gear door issue since I have had the similar speed results before as well as after my recent MSC annual.

Might it simply be a factor of warmer summer temps. aloft?  I do not recall experiencing such drastic performances decreases before.

Just returned from a long X/C, Aspen to Torrance, CA (LA Basin).  At 10,500' going west I indicated an average 121 KTS which with temps. around 5 degrees C gave me approx. 144 KTAS according to the KLN89B.  Seems 6-8 knots slow.  (My airplane is stock except for a 201 style windshield and 2 blade Hartzell Scimitar prop.  Engine is 100 hour since Western Skyways O/H).  Normally I have a hard time staying out of the yellow arc below 10,000'.

CA to Aspen @ 11,500 I struggled to indicate 120 KTS with similar OAT's and a TAS of 142KTS.  I 'm used to seeing 150+ knots true, so I am really puzzled and a bit worried with my recent flight performance.

What do you other E model pilots see for indicated airspeeds at various altitudes.  Do you see such drastic seasonal airspeed indications?

I would really appreciate any comments, advice and suggestions.  

 

 

 

[The-sky-captain]

Aircraft- This is from the MAPA site under "Mapa log sample articles" tab. It's a pretty good read. 

Level Flight Cruise Performance 1964 M20E N6966U, Full throttle, 2500 rpm, 50 degrees rich of peak, cowl flaps closed.

Altitude

OAT

F

MP

RAM

AIR

RPM

Direction

IAS

KTS/MP

GPS GROUNDSPEED

KTS

10000

64

20.1

off

2500

W

128/148

156

10000

64

20.1

off

2500

S

129/149

132

10000

64

20.1

off

2500

N

128/148

157

10000

64

20.1

off

2500

E

128/148

137

 

 

 

 

 

Average GS/TAS 145.5 kts.

 

7000

65

22.7

off

2500

E

140/162

141

7000

65

23.8

on

2500

E

144/166

144

7000

65

22.7

off

2500

S

140/162

136

7000

65

23.8

on

2500

S

143/165

140

7000

65

22.7

off

2500

W

139/160

160

7000

65

23.8

on

2500

W

143/165

165

7000

65

22.7

off

2500

N

140/162

161

7000

65

23.8

on

2500

N

144/166

166

 

 

 

Average GS/TAS (ram air off) 149.5 kts

Average GS/TAS (ram air on) 153.75 kts

4500

76

24.7

off

2500

W

144/166

152

4500

76

26.0

on

2500

W

146/168

153

4500

76

24.7

off

2500

S

143/164

138

4500

76

26.0

on

2500

S

144/166

140

4500

76

24.7

off

2500

E

143/164

149

4500

76

26.0

on

2500

E

146/168

151

4500

76

24.7

off

2500

N

143/164

158

4500

76

26.0

on

2500

N

144/166

162

 

 

 

Average GS/TAS (ram air off) 149.25 kts

Average GS/TAS (ram air on) 151.5 kts

 

 

[Cruiser]

when was the last time it was thouroughly detailed and polished? It wouldn't get you 10kts., but a couple anyway.

[Piloto]

Weight is a significant factor on true airspeed. From full fuel to 1/3 there could be a 5kts difference. High ambient temperature will decrease engine performance too. I would check for pitot and static line leaks and alternate static valve. Check or replace engine air filter. Check also rigging of the landing gear doors in the retracted position. If using a 3 axis autopilot, disconnect it and trim by hand to see if speed increase. If so you may have a bad trim servo. Verify flaps are getting fully retracted. Dirty wing leading edge will slow down the plane, keep it clean. 

José

 

[N9937c]

You might have your airspeed system checked for leaks. Can be done when static is checked. If airspeed is very old could have slight leakage resulting in reduced indication.

larry

[GeorgePerry]

Quote: Cruiser

when was the last time it was thouroughly detailed and polished? It wouldn't get you 10kts., but a couple anyway.

[airkraft]

Thanks all!  I did mention my indicated a/s shortfall to my AI and he did not find any blockages in the P/S system.  I will ask him to check for leaks.  The plane is clean and waxed, even though the paint is rather tired being original and 43 years young (I'd say its better than 5/10). 

A kind pilot in Buena Vista, CO suggested today, as I waited out some convective activity, I do a true airspeed check by flying E, W, N, S and averaging ground speeds to get TAS.  This would rule out TAS loss due to inaccurate indicated/calibrated entries when calculating true in the KLN 89B.  I'll let you know what I find.

[Cruiser]

Airkraft,

I love the security system you have for your plane. (as seen in your Avatar) I makes me laugh everytime I see him. I bet you have no worries about someone messing with the plane when he is on duty. Does he like to fly also?

 

[airkraft]

That's Kylie.  She's a she. Cute, ain't she?  My favorite passenger, never a complaint!

[alun]

here's mine!  "enjoying" a flight in the back seat of my baby.

[The-sky-captain]

Nice pic Alun! I would love to get my dogs in the plane but with the new leather and their combined 150lbs I don't think that it's a great idea to introduce them the the joys of flight just yet!