170 true airspeed

[garybcollins]

I was interested in obtaining some baseline data on the speed of my plane to check the effect of various modifications. Full power runs are about the only way to avoid errors is setting the power. The best tool for this that I have found is a program that uses GPS or Loran speed data from three consecutive runs on cardinal headings. For example, East, North, West. You can start in any direction. The program gives you true airspeed and wind direction and speed. I think the results are a bit conservative as I usually do better than these values. That is probably because on a cross country the plane has time to achieve its full trimmed speed while on these runs I was making turns to the next heading and the air was not very stable. The program is at:
http://www.reacomp.com Click on the Airspeed Applet. It is fun to play with--give it a try.

My 1954 170B is stock. It had the original wheel pants, and about 1100 hrs SMOH. This was done on August 1, 2001 and it was hot on the surface. I should have set the altimeter at 29.92 to get pressure altitude but it was set at the current altimeter setting and I did not record that. The RPM is from the Horizon electronic tach and are correct within 1%. I made runs at each altitude going up and coming down and averaged the results.

Altitude speed(mph)RPM Temp(F) M.P.(in)

2000 136 2705 90 27.6

4000 133 2690 72 26.0

6000 130 2660 64 24.3

8000 129 2660 55 22.8

10000 124 2595 53 21.0

12000 124 2580 52 19.8

I like to fly between 7500 and 9500 feet on cross country flights if possible and routinely get about 135 mph on 8.2-8.5 gph This is about 5 mph faster than the speeds calculated by the program. The first significant mod I have made was to work on the lower end of the speed spectrum by installing VGs. They are supposed to have little effect on the top end but I made one quick series of full power runs at 4000 feet to see if I could measure an effect. Forgot to record the temp. but the M.P. was 26.0 in. which agrees with the table. The speed was 131 mph. That needs to be checked further as I see no differences in cross country ground speeds after the VGs. I ran in the Sun 60 at Sun-n-Fun this year and made 111 knots even though I missed the first turn by almost a mile. If I had been on course I would have been close to 115 knots which is close to agreement with the table, considering it is measured from a standing start. I would like to get some more 170s to run the Sun 60. If we can get 3 planes they will set up a category for us.

[Rudy Mantel]

Gary, that's gotta be the fastest 170 in town !
My 170B is an honest 100k airplane. If the GPS shows over 100 I'm real happy !
I cruise at 2450 RPM at SL up to full bore at 8,000 ft & up with the original McCauley 76-53 prop.
Rudy

[garybcollins]

Rudy,

If my plane is fast, I am really surprised. It was wrecked when it was 6 months old and that repair, while structurally OK, was not done with great care to maintain original contours. That really came to light when I installed a baggage door. Because of the less than perfect repair, I could not get the door to fit well. The right main door also fits poorly, probably because of distortion from the accident. As I was installing the VGs this spring, I also found the left wing is less than perfect. And the right wing is actually off a 172. So it has no reason to be faster than the average 170.

You might check your tach to see if you are really getting the rpm you think you are. My mechanical tach was off over 100 rpm when I bought the plane and would not stay calibrated which lead me to get the Horizon electronic tach.

I did spend alot of time on rigging after I bought my plane and that brought the indicated up from 112 mph to about 118. (I still think in mph). The aileron gap seals (tape type) brought it up to 121. Bartone exhaust extensions and a prop overhaul seem to have helped a bit to make it about 124 now. That is indicated at say, 6500. I am waiting for FAA approval to install 15x6.00x6 tires and some slick Vans wheel pants and hoping they will get the indicated up near 130. On a RV-6 these pants gave an 8 mph increase but I know they won't do that on my slower plane. But going to the smaller diameter tire and putting it in the good fairing may make for a 5 mph gain. My goal is to get 145 mph true out of it with the original engine and if these wheels and fairings work, I should be close. Are you up for the Sun 60 next at next years Sun-n-Fun?

[N1478D]

OKAY, I'll still enter the spot landing contest

[Rudy Mantel]

Gary, that is a fast airplane ! Maybe it came out of the factory kinda crooked and when it was repaired they (accidentally) got it right! And your rigging and all thos improvements certainly helped.
I will check my tach one of these days. The fuel consumption is around 8.5 gph and I always lean.
I do plan to be at Sun n' Fun next year- (it's only an hour and a half flight from my home ) What's the Sun 60 ?
Rudy

[GAHorn]

I think we may all rest assured that if one airplane in the fleet was significantly faster than the rest, that Cessna would have used it to develop performance data and kept it from the rest of us!
I'm inclined to suggest that the airspeed indicator/static system needs calibration.