What Speed do you get at 75% cruise and best altitude?

[Tony Scarpelli]

Hello,

I have a question and I hope many people will take the time to answer.

The books say that the following are the best speeds for our Comanches at 75% cruise:

180 ~ 139 knots
250 ~ 155 knots
260 ~ 157 knots
260 TC ~ 198 knots

I heard the rule of thumb that best speed for a normally aspirated engine is 1/2 the service ceiling at 75% power.

What speeds do you see on your air craft at best cruise True air speed?

[Harley McGatha]

Remember that Piper rated their speeds at gross unlike Mooney, Beech, etc..

The service ceiling for a 260B at gross is 22,800 ft and I do believe that the comanche is basically the same speed at 12,000 as it is at 8,000 ft. The other airplanes the speed drop is much more drastic.
Hint: If a bonanza, cessna, mooney, etc.. wants to race you pick the altitude.
The bonanza service ceiling is 16,800 ft and the comanche will only loose about 10-12 knots. I like flying in the mid-teens if the winds allow, wearing oxygen.
In my 260B, I file for 165 kts and this is no problem at 2,800# but it is a little slower at 3,100 lbs. up to 12,000 ft.

Harley

[9089P]

Hi Tony,

I agree with Harley's comments on the 260B. We fly at around 2700 lbs most of the time and at 65% because in the west we generally have to go too high to get 75% without going up and down all the time. At 65% and 9500' we can count on 157kts.

As Harley said going up doesn't change the speed atleast up to 13.5. If we are going eastbound with the wind that is what we do. We generally use O2 at 10.5 and above.

As for mooneys and bonanzas, higher is better. Their wing doesn't seem to handle the altitude as well and with bonanzas the hp advantage is reduced as you go up.

Good luck, Don

[N3322G]

Tony,

I fly a Twin but let me add that factors such as rigging, CG and engine tuning/age materially impact cruise speed. My experience comes from racing but applies to your question at 75%.

A great example is the Twin Comanche from France that competed against our Twin Comanche in the 1992 Round the World Race. Same engines, same speed mods but Mom and sister Nancy were averaging 10 kts faster. The French team did an unauthorized midnight engine inspection because they were sure the engines were not stock as required by the race rules. They'd just come from Lycoming 50 hours earlier so they were extremely stock.

So what made the big difference? Two things. Both planes carried extra fuel to make the long legs - Mom's was all in the wing - 10 tanks 182 gallons. The French team had put their extra fuel aft of the baggage as no fuel was allowed in the cockpit area so their CG was much further aft. Second, Mom and Nancy weighed a total of about 210 pounds whereas the French team was at least double that amount.

With respect to engine age, we found that our race experience saw the engines peak speed at about 200 hours. I overhauled the engines that raced twice around the world a little past TBO and found I picked up 5-8 kts.

Hope this helps you.

Pat Keefer ICS 08899

[N3322G]

Tony - forgot to comment on the best altitude question. Best altitude for me is wherever there is a tailwind because a good wind can trump the best power.

My 160 hp engines poop out at about 5500 165-170 kts at 75% but on my Denver Texas flight on Sunday. 5500 is not an option. At 9000 I was poking along at 150 kts for the first half of the trip but then I got 195Kts after I passed through the front so I stayed at 9000 as long as center let me.

There might be a different answer to the 'best altitude' depending upon your goal - best power or best speed or best economy. if you define your goal to your question I might be able to help with additional comments.

Pat Keefer ICS 08899

[N8962P]

My 260B makes pretty much exactly the book IAS numbers. I have a suspicion that my rigging isn't quite right, which is probably costing me a bit, but also have tip tanks and gear lobe fairings, which help a bit.

Typically fly at 65% and expect about 146IAS. 75% buys me 157 IAS. I choose altitude based more on winds than anything else.

[Tony Scarpelli]

[N3322G]

[T210DRVR]

[Bill@66y]

[Bill@66y]

[Zach Grant L1011jock]

Bill,
It will not verify rigging, but it will show you all the high pressure drag items! If bugs hit it, air is making a hard turn and basically the form is being an inertial separator, and bugs get squished. No bugs, air flows smoothly arround the object, as do the bugs (that or whatever form in front caught all the bugs...)
-Zach

[N3322G]

Bill,

Zach is correct. The bug splatter method will not entirely tell you if you aircraft is in rig but it can help. Here is how I use it on my Twin Comanche.

I specifically look at the tail and see if one side of the tail has more bugs than the other. If so, one side is seeing more airflow and therefore the plane is crooked in the air and so goes slower. Now for a Twin that can mean the engines aren't synced up or a few others things may need tweaking. It also means that at my next annual, I will have the rigging checked. Cables do get loose over time - its why we do annual in the first place.

Hope this is of help to you.

Pat Keefer ICS 08899

[Spy Av8r]

[Zach Grant L1011jock]