The International Cessna^® 170 Association

I own a 53 Cessne 170B that was Modified to a "171" back in the mid 50's. Those not fimillar, this was the nose gear mod. In the 80's, it was returned to its former glory as a tail dragger,(convential gear). As part of the orignal mod, the engine mount was changed because of nose gear mounting structure. When my Bird was restored, the engine mount was modified to eliminate the mounting structure of the nose gear and left in place. My question is this. Was the Thrust Line of the atrcraft changed buy tilting the engine slighty down to accomidate the drag induced buy the nose gear? My bird is rigged netural and faired. I spin a 7655 on the end of an O300C. Cruse airspeed is 117mph/100kts. Engine and prop spinner do not align with the cowl. Engine mounts are in good shape and do not seem to have settled. This is a real head scratcher!

Runk170 wrote:I own a 53 Cessne 170B that was Modified to a "171" back in the mid 50's. Those not fimillar, this was the nose gear mod. In the 80's, it was returned to its former glory as a tail dragger,(convential gear). As part of the orignal mod, the engine mount was changed because of nose gear mounting structure. When my Bird was restored, the engine mount was modified to eliminate the mounting structure of the nose gear and left in place. My question is this. Was the Thrust Line of the atrcraft changed buy tilting the engine slighty down to accomidate the drag induced buy the nose gear? My bird is rigged netural and faired. I spin a 7655 on the end of an O300C. Cruse airspeed is 117mph/100kts. Engine and prop spinner do not align with the cowl. Engine mounts are in good shape and do not seem to have settled. This is a real head scratcher!

So if I read your post correctly, why would you want the thrustline pointing down to accomodate the nose gear? I would check your firewall attach points. look closely inside where the longerons meet the firewall for cracks if the mounts are in good shape as you so state. I would also check to see if you have the correct bushings at the firewall. D

Doug, thanks for your quick reply. I have inspected everything you could imagine. Mounting structure is good at the fire wall, bushings look to be correct and in good shape, proper bolts and washers,the cradle bushings are good as well with no signs of deterioration or off center. Thats why I asked the question to begin with. These would be design speck's set up by Cessna as they were the ones that supplied the orignal kits for the mod back in the day. By the way I'm a retired A+P/IA so I know what I'm looking at as opposed to a pilot messing around where he shouldn't.

Hi Runk,I reread your post. I do not believe that the thrustline was changed. Your speeds are comparable with mine. Can you provide a picture of the problem? Doug

Runk,
If you'd like I could take some measurements of my engine mount so that you could compare these measurements to your engine mount. I would think measuring from the lower firewall mount to some point on the forward end of the mount might answer your question.

Hey Doug, I don't have a pic at this time but will be going out to my Hgr tomorrow. I'll take some pics then. It's a great Bird and flys straight as an arrow. This bird was in a heavy wind srorm about 10 years ago. The previous owner (also an A+P /IA) put it back together. this turned into a 2 year restoration project only to find out just before "First Flight" He was terminal with the big C. at 43 no less. When I got the bird from him it had a total of about 35 hours sense restoration. But the bird was never flown out and rigged. I spent the first yeat of ownership making it "Just Right". But as I said this miss match drives me nuts. Also its the first thing folks see when they look it over. You'll see what I'm talking about when you see it. I don't think this is a flight issue. Could have had an impact if the Thrust Line was changed. As you, I thought that was pretty remote. Later, got to go now..

Runk

So this mount has been modified twice? How original is the cowling?

Hey Runk170,
I have a good C-170 engine mount I'll sell you for three grand. It has an engine and all the accessories and baffling attached.
You can pull the mount and sell the rest!

PM me.

Runk, While I have no hard-data on this, I think you will find there is a 2-degree "down" thrustline on all the 100 series Cessnas. Having the thrustline slightly down provides stability to flight characteristics. This can actually be visible with the cowling removed. It is not necessary to level the airplane to determine. Place a protractor/level on the firewall (or the upper doorsill) and take a reading...then compare that to the "spine" of the crankcase, or the face of the flange or prop. The "flat" forward of the starter mount on pull-starters is also a likely place.

gahorn wrote:Runk, While I have no hard-data on this, I think you will find there is a 2-degree "down" thrustline on all the 100 series Cessnas.

I compared the angle of the spine on my engine to the firewall face and found it to be 88 degrees which confirms your 2 degree down thrust line.

For unknown reasons (old age?) I've forgotten the aerodynamics of this matter taught during my short career as a production test pilot, but a downward thrustline improves longitudinal stability, while a neutral or up thrustline destabilises it (but provides some aircraft with greater manueverability. Many aerobatic aircraft have neutral thrustline.)

I'll try to use my aerospace engineering degree and explain the aerodynamics and help George refresh his memory a bit.

Easiest way to explain it is to picture the plane from the side, at the wing draw a big arrow UP and at the tail draw a smaller arrow DOWN, now at the engine draw another smaller arrow DOWN. The downward thrust helps the stability in much the same way the tail counteracts the major lift component of the wings. With a neutral thrust line the tail has do to all the work which, like George says makes it more neutral stability.

George, did that help?

Jud, if George needs pictures just make sure they are simple and the lines are far a part. Helps me stay in the lines when I color them in.

Seriously, that was a masterfully simple explanation that covered the subject in one paragraph. (You will now have to post several blank posts to take up the allotted space.)

rupertjl-
Is that a reason canard airplanes like the Longeze are more efficient, they don't have that built-in extra load (lift-induced drag) for the wing to overcome? Do all the arrows point up? Just wondering.

Richard,

Man you guys are making me dig out the textbooks, but without going into it too far, yes, both those arrows point up and that does make the design more efficient from an aerodynamic standpoint. When looking at stability though it's more the about c.g and center of lift and making the moments sum to zero in level flight. In a Long-Ez design the cg is obviously between the front canard and the main wing. The main wing produces more lift than the canard but is closer to the cg than the lift being produced by the canard. The canard usually has a higher wing loading than the main wing and will stall before the main wing reaches its critical AOA, resulting in a pitchdown moment, allowing the canard to start "flying" again and lends the design to being a stable platform.

A digram of a seesaw is also a good way to explain it as well. The fat guy only 2 feet from the fulcrum is balanced by the tiny guy 10 feet away from the fulcrum.