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Cruise Speeds with O-360 Cowl

tmcguff

tmcguff

Member
I have a 9A with an IO-320 and an O-360 cowl. I have a 70CM7S9-0-79 Sensenich prop. My cruise speeds are about 8 to 10 kts slower than Van's published speeds. Has anyone with the O-360 cowl noticed slower than published cruise speeds?
 
Since I'll be using the 360 cowl for similar reasons, I'll be interested in the responses you get. In the meantime, I expect you'll be asked for more information, such as:

- What data are you using to base your conclusion on? IAS or TAS?
- What altitude are you flying and what is the temperature?
- Have you flown a rectangular box pattern using GPS to minimize/eliminate the effect of winds aloft?
- Have you flown along side other aircraft, in particular similar RVs to verify your are slower?
 
I've flown a rectangular course using the GPS ground speeds for my calculations. Flights have been around 8000' density altitude. I also have a TAS display on my EFIS that seems to be fairly accurate. Your suggestion of flying next to another RV-9 is a good one and I had that on my list of things to do. There is no heavy wing and it is coordinated in flight without any mods.
 
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One other (semi scientific) thing I'd do is fly at about 1,000' on a calm day and see how your IAS compares with the GPS ground speed. The closer to sea level the better would be my guess but it probably doesn't matter.

I have never calibrated airspeed instruments but given the number of comments I've seen on VAF about it, it looks to be a chore - doable but it takes some effort and attention to detail. Apparently small errors in the static pressure can lead to significant errors in indicated air speed. It would seem to me, the first thing to figure out is if your airspeed indicator is accurate. Flying with another RV with a known accurate ASI and the test above are intended to see if your ASI is in the right ballpark.

FWIW, (and this may not be much) I have a very difficult time imagining there's an 8-10 mph difference between two of Van's cowls. If there was, we'd have read all about it. That difference is about the max you see attributed to the Sam James cowl. My seat-of-pants opinion is your ASI is lying to you, or you have another drag issue that is fixable, or a combination of both.
 
Don said:
Since I'll be using the 360 cowl for similar reasons, I'll be interested in the responses you get. In the meantime, I expect you'll be asked for more information, such as:

- What data are you using to base your conclusion on? IAS or TAS?
- What altitude are you flying and what is the temperature?
- Have you flown a rectangular box pattern using GPS to minimize/eliminate the effect of winds aloft?
- Have you flown along side other aircraft, in particular similar RVs to verify your are slower?
Click to expand...

- Is your airplane completely finished?
 
Don said:
Since I'll be using the 360 cowl for similar reasons, I'll be interested in the responses you get. In the meantime, I expect you'll be asked for more information, such as:

- What data are you using to base your conclusion on? IAS or TAS?
- What altitude are you flying and what is the temperature?
- Have you flown a rectangular box pattern using GPS to minimize/eliminate the effect of winds aloft?
- Have you flown along side other aircraft, in particular similar RVs to verify your are slower?
Click to expand...

Don, there are a couple of things to watch for that I had to deal with using fuel injection and the larger cowl. You'll need to exchange the mixture control arm on the throttle body for a straight arm. If you call Vans on this they can give you the details of who to contact for the exchange. There just isn't enough room for the standard arm in the full rich position. I also needed a longer trim cable. This is because it has to loop forward of the electric fuel pump housing. I also needed longer throttle and mixture cables. Vans doesn't mention these things so I found out the hard way. Hope this saves you some time and frustration.
 
tmcguff said:
I have a 9A with an IO-320 and an O-360 cowl. I have a 70CM7S9-0-79 Sensenich prop. My cruise speeds are about 8 to 10 kts slower than Van's published speeds. Has anyone with the O-360 cowl noticed slower than published cruise speeds?
Click to expand...

We need more details such as density altitude FF, rpm, TAS and leaning technique to see if you really are slower.

Fin
9A
 
Mine is working.

I have the IO-320 under the same cowl. I have a CS prop so there is that difference. I am getting the VAN's published speeds.

I did need to modify the cowl to get clearance with the throttle body control linkage.

Kent
 
tmcguff said:
Don, there are a couple of things to watch for that I had to deal with using fuel injection and the larger cowl. You'll need to exchange the mixture control arm on the throttle body for a straight arm. If you call Vans on this they can give you the details of who to contact for the exchange. There just isn't enough room for the standard arm in the full rich position. I also needed a longer trim cable. This is because it has to loop forward of the electric fuel pump housing. I also needed longer throttle and mixture cables. Vans doesn't mention these things so I found out the hard way. Hope this saves you some time and frustration.
Click to expand...


Excellent information to be armed with. On my immediate to do list is one that says to reinstall the rudder trim cable to be sure it's long enough. I had my doubts. Do you have the cold air vertical induction? I don't know how much difference it makes but I have the horizontal (not cold air) induction. When I ordered my engine I was clear that I wanted the Silver Hawk FI so I didn't need to plumb an extra line and I wanted to keep the FWF as close to standard as possible. I'm not sure if any of this matters yet in terms of cable length.
 
Don said:
Not yet. I'm finishing the fuselage and hoping there's less than a year to go on the build. Why do you ask?
Click to expand...

Don, sorry for the confusion....... I was just adding to your list. If the fairings and wheel pants are not installed, the airplane will not achieve top speed.
 
tmcguff said:
I have a 9A with an IO-320 and an O-360 cowl. I have a 70CM7S9-0-79 Sensenich prop. My cruise speeds are about 8 to 10 kts slower than Van's published speeds. Has anyone with the O-360 cowl noticed slower than published cruise speeds?
Click to expand...
We seem to have the same experience....

IF the O-360 gives only gives extra 5 knots ....
and we can't get to the top of the green in normal cruise with the O-320 and are less than 5-10 knots less than the top of the green...

Don't understand Van's insistance that you don't put more than an O-320 in the 9 :confused:

With our flight experience and flight date .... kinda wish we had the O-360... Remember .... Lycoming says 65% power or less for max life.....
 
360

I believe I'm going to go with a 360 too. Fortunately I haven't gotten to that stage yet so no regrets. At first, I would have argued against the 360 in a 9. But, living here in CO and talking with other experienced builders, I have learned: the extra power would be an advantage, especially next to the Rockies; I should get the approx same performance; and, the 360 engine just won't have to work as hard. And that spell longer life in my book.

We'll see.
 
MauiLvrs said:
We seem to have the same experience....

IF the O-360 gives only gives extra 5 knots ....
and we can't get to the top of the green in normal cruise with the O-320 and are less than 5-10 knots less than the top of the green...

Don't understand Van's insistance that you don't put more than an O-320 in the 9 :confused:

With our flight experience and flight date .... kinda wish we had the O-360... Remember .... Lycoming says 65% power or less for max life.....
Click to expand...

You are talking about indicated airspeed vs. true airspeed.

Vne is based on true airspeed.

One thing I noticed with the O-360 vs. the O-290 is that when pointed downhill in bumpy air, I am pulling off a lot of power to get the thing slowed down. With the small engine, I would probably had to point it straight down before I had to worry.

My typical flight is around 150 to 155 knots with the power at around 55 to 65% and fuel burn between 6.5 and 8 GPH. I'm still learning this engine so I'm only going to post ranges.

However, the climb rate is eye popping. Not F1 / Rocket eye popping but RV eye popping. The long wing and 80 MPH indicated with a FP two bladed Catto cruise prop will get you off the ground in a hurry and going up at crazy fast rates. Just remember to pull back on the throttle or the speed at which your tanks drain will also be eye popping.
 
150 HP O320
Carb
50 degrees rich of peak
2600 rpm
Catto 3-blade composite fixed
James cowl

8000 DA sees 152-154 knots TAS
9600 DA sees 150 knots
11100 DA sees146 knots

All WOT power settings
This is a 9a rigging seems pretty good although I'd love an expert to review
 
Just as a point of reference, I fly a 7A with an O360 cowl and am as fast or faster than my friends 7 with the IO360 cowl. So I would contend the O360 cowl makes little if any difference compared to the IO360 cowl.
 
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