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 Post subject: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 21:36 
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Joined: 12/20/10
Posts: 35
Aircraft: Panther Navajo
Just finished up a several year restoration on a 1956 C172 with an AVCON Conversion. Everything is new engine, prop, power flow exhaust, Sportsman STOL kit etc. Flew it for the first time the other day and the plane leaped off the ground handled great but the top end speed is only indicating 110 MPH. Had the Avionics shop recheck the pitot system and confirmed all is good. The rigging on the airframe is straight. One thought that was brought to my attention was the engine incidence. Does anyone know what the engine incidence is on an AVCON conversion or have any other suggestions on what to look for. Also what kind of indicated airspeeds are you getting from the conversion.

Flew again tonight. At 2000 feet the winds were almost nothing. Flew four different directions with the GPS showing ground speeds almost the same speed all four directions. Tanks at half full. Two 200 lbs Guys in the front seats. Noticed the elevator just slightly down, elevator trimmed pretty close to middle. I would of thought with two people in the front seats the plane would of required some up elevator to stay level not down elevator. Plane seems to be making full power on takeoff. The engine analyzer shows 18.5 gph full throttle and 2650 RPM on the prop. The engine is sitting straight forward 90 degrees of the firewall and now has .3 degrees of down tilt. We added some washers under the engine mount to tip it down and back to the left. The neighbor in the hanger next to me has a 58 with the stock Continental so I measured his engine incidence. It didn't change anything maybe 1 MPH


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 21:41 
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Joined: 05/08/09
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Location: Stuart, FL (KSUA)
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What rpm will it make at 2000 wide open throttle?

Maybe you are under pitched on the prop.


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 21:48 
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I'm curious about the prop too, and the timing.


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 21:49 
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Aircraft: Panther Navajo
It is the AVCON Conversion. 180 Hp with a Hartzel Constant Speed. I have varied the RPM's from 2500 to 2200 at 24" MP and the speed only changed a couple MPH. I can make 2650 RPM on take off at 18.5 GPH. It climbs like a home sick angel. We maintained 1200 fpm to 2000 agl at 75 MPH


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 21:59 
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Aircraft: bonanza 35j
I have a 68 avcon conversion that does 125kts/145mph at 23/2300 at 6500 ft. A friend had a fast back 175 with avcon that was much slower. I do remember the engine mounts on mine were unusual in that the diagonal were the same, but top mounts were different and bottom were different, ie x y on top and y x on bottom. I assume you have the constant speed prop. I added the power flow as well which increased gas consumption and climb, little or no effect on speed. 68 was the first year of lycoming from the factory and required new engine mounts for the conversion.


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 22:00 
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Location: Stuart, FL (KSUA)
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I was thinking it was fixed pitch.

Hmmmm :scratch: what else could it be....?


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 22:02 
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Username Protected wrote:
It is the AVCON Conversion. 180 Hp with a Hartzel Constant Speed. I have varied the RPM's from 2500 to 2200 at 24" MP and the speed only changed a couple MPH. I can make 2650 RPM on take off at 18.5 GPH. It climbs like a home sick angel. We maintained 1200 fpm to 2000 agl at 75 MPH

Whats static rpm? Sounds like you may have a climb prop on it. I have a bit of time in a 1960 172 that we did an avcon install on, and I don't remember it really being much faster than stock, just climbed a whole lot better than the anemic continental.


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 17 Jul 2019, 23:09 
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Username Protected wrote:
Sounds like you may have a climb prop on it.


He said it's a constant speed prop. It must be rigging. :scratch:

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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 00:51 
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My wife has a 1957 172 with the 180hp conversion, STOL kit, fixed pitch prop and it indicates 100-110mph in cruise as well.

I think hers is/was marketed as a "Bush" conversion. I'm not really sure how that compares to an Avcon, but I think they are related.

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Last edited on 18 Jul 2019, 19:08, edited 1 time in total.

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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 07:21 
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Joined: 12/20/10
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Aircraft: Panther Navajo
I have a 68 avcon conversion that does 125kts/145mph at 23/2300 at 6500 ft. A friend had a fast back 175 with avcon that was much slower. I do remember the engine mounts on mine were unusual in that the diagonal were the same, but top mounts were different and bottom were different, ie x y on top and y x on bottom. I assume you have the constant speed prop. I added the power flow as well which increased gas consumption and climb, little or no effect on speed. 68 was the first year of lycoming from the factory and required new engine mounts for the conversion.


Is there any documentation on the engine mounts. The reason I ask is the paperwork I have which is only the parts list shows four lord engine mounts that are the same. I don't have the exploded view diagrams from the original installation that may show washers in different locations or the mounts as referred to. I took off engine mounts that were completely different than the STC paperwork had listed. I asked my IA about this and was told to get the mounts listed on the STC and of course they were the most expensive mounts lord sells. There is not much room in the cowl to shift the engine around. We did tip the engine down last night one washers width and didn't see any difference maybe one MPH.


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 11:37 
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I believe engine mounts are typically rigged about two degrees in yaw to account for P factor. You indicated that your is straight at 90 degrees to the firewall.

Do you need rudder trim in level flight?


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 11:49 
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Aircraft: Panther Navajo
Rudder trim currently is controlled by the tab on the rudder which is bent to adjust. We have not bent the tab yet. The plane fly's pretty close to straight without any adjustment. If anything I may have to give it a tad amount of right trim


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 12:12 
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Joined: 12/26/16
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Location: Hernando, Ms
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Username Protected wrote:
I don't have the exploded view diagrams from the original installation that may show washers in different locations or the mounts as referred to.


From Drawing 17200

AN970-716 washers used between engine & Lord mount on lower mount only.
AN7-36A bolt (2) lower.

Doubt it will affect speed though.

Sully

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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 12:32 
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Joined: 12/20/10
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Aircraft: Panther Navajo
Any chance you could take a picture of that drawing and send to me


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 Post subject: Re: Slowest 56 C172 with AVCON Conversion HELP
PostPosted: 18 Jul 2019, 15:38 
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Joined: 04/18/10
Posts: 304
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Aircraft: bonanza 35j
As I recall, the Avcon conversions on older 172 required a blister over the #1 valve cover in most installations, my 68 got by without it but had to use a lower profile valve cover to keep the cowl from cracking in that location. Air plains in Wellington ks. has a lot of knowledge in this area and said they dropped the CSP in that it was not worth the trouble modifying the cowling. I stopped by and they gave me several hints (i.e. Another Valve cover) to correct the problems of the lyc 360 with CSP.


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