Elevator position during flight

cpolsley · Post by **cpolsley** » Wed Feb 05, 2003 11:07 pm

Anyone happen to notice the position of their elevator during flight? With one both front seats occupied my elevator appears to be just slightly nose down, about 2 to 3 degrees nose down when compared to the horizontal stabilizer. This position would seem normal if loaded aft CG but thats not the case. Anyone recall the position of theirs or am I the only one who looks aft, actually I was looking for traffic.

N170BP · Post by **N170BP** » Thu Feb 06, 2003 5:31 am

If I recall correctly, mine is also ever-so-slightly down in
level/cruise flight (we're talking just *barely* down....).

Dunno why that is (sorry).

But if you want to make yourself feel better, look up a picture
of a WW-II (Supermarine) Spitfire (preferably an air-to-air shot
of a restored example, where the camera plane is something relatively slow) and note the amount of down-elevator the Spit has during cruise flight. You'll think you're elevator position in flight looks just fine!

Bela P. Havasreti
'54 C-170B N170BP

russfarris · Post by **russfarris** » Thu Feb 06, 2003 6:21 am

Well, I have noticed exactly the same thing - by myself, or with two of us in front, the elevator is a few degrees down in cruise flight. This means that throughout the CG range, the elevator is always down in level flight. There is a certain amount of trim drag associated with this.

My experience designing and flying radio control models tells me that Cessna got the incidence angle wrong on the horizontal tail...i.e., the leading edge is set a few degrees too low. I can't think of any aerodynamic reason for them to intentionaly do this, but maybe I was was sick that long-ago day in aeronautical engineering class. Russ Farris

P.S. That is why airplanes like the earlier 180s, 182s and J-3 Cubs have adjustable stabs; the trim moves the entire surface, and the elevator fairs behind it. Although at the speeds we are dealing with, I doubt if it makes much difference...

N170BP · Post by **N170BP** » Thu Feb 06, 2003 7:30 am

This leads me to ask the question;

Has anyone adjusted their wing trailing edge eccentric bolts to move
the wing trailing edges down as far as they'll go and noticed
any improvement in cruise speed?

I was thinking about trying this.... Maybe move them both
down as far as they'll go, test fly the thing, and if it flys crooked,
move the offending wing back up bit by bit until she flys hands-off
again.

Is there any reason why I shouldn't do this?

Bela P. Havasreti
'54 C-170B N170BP

jon s blocker · Post by **jon s blocker** » Thu Feb 06, 2003 2:35 pm

I am trying to visualize placing the wing position down with the eccentric bolts at the bottom of their travel. Would that not tend to slow you down? I know that they reflex ailerons and flaperons on sailplanes to build speed, but that is by lifting the traing edge of the control surface. I don't know if raising the AOA of your wings will have the effect you want or not. I have to profess I don't know alot about what changing the AOA of your wings will do, I do know that you tend to gain speed by lessening your dihedral and also by putting in MORE washout in your wings,(tricks I learned on my Cubs and Citabria). The payoff is more speed, less stability. Like I said I don,t know what would happen if you tried this, keep us informed, you might be on to something. Thanks,
Jon

N170BP · Post by **N170BP** » Thu Feb 06, 2003 4:44 pm

The way it was explained to me is that adjusting the eccentric
bolts so the trailing edge moves down takes incidence out of
the entire wing (it doesn't just lower the trailing edge). The
net effect is the tail "flies higher" during cruise, and there is
less drag on the wing.

I was also told that some time ago, the T-6 racer guys at Reno would
do this for the same reason (mill the wing center section attach blocks down to take the incidence out of the wing) until they got caught during tech inspections....

Part of my info comes from some C-180 guys/drivers, so I'll ask
them to elaborate.

Bela P. Havasreti
'54 C-170B N170BP

jon s blocker · Post by **jon s blocker** » Thu Feb 06, 2003 5:30 pm

Bela,
That makes sense to me. Some times I need to be hit over the head in order to look at the big picture. Getting the tail to fligh higher was not in my realm of thought, but it does make sense now. Will the effects of the change in flight attitude form a blanketing effect, from the wing, on the elevator, taking away some of the effectiveness of the elevator? Just thinking out loud, we are rebuilding and have our wings off. When they go on it would be an idea worth pondering!
Jon

CAS · Post by **CAS** » Fri Feb 07, 2003 3:20 am

I may get shot down in flames here as I have not consulted the text books and this is all off the top of my head, but ....

It seems to me that if you LOWER the training edge of the main wings you are INCREASING the incidence angle. This may have the effect of making the aircraft fly slightly nose lower. It would also change the relationship between the main plane and tail plane incidence and cause the airflow over the mainplane to be deflected lower (below the tailplane) perhaps reducing its effectiveness.

The purpose of the tail plane is to balance the moments of the lift/weigh/thrust/drag forces and set the aircraft at the required attitude/angle of attack. In a lot of aircraft the net force on the tail plane is downward meaning that the main plane has to make up some extra lift for this added "weight". It seems to me it would be more efficient to have the tailplane add some positive lift and thus reduce slightly the lift the main plane needs to produce .... resulting in slightly higher air speed for the same power setting. This is what happens with an aft CofG - in most Cessna singles anyway.

The 'reflex' flap feature on some Maules and other aircraft is, I think, to reduce the camber of the wing and thus reduce drag and get a bit better TAS. In any case this feature RAISES the trailing edge not lower it.

I would have thought Mr Cessna would have sorted out the best all around incident angle for the wings during the flight test/certification of the aircraft. I don't plan to mess with mine.

Another 3.3 cents worth (check the exchange rate

)

David

Roesbery · Post by **Roesbery** » Fri Feb 07, 2003 4:07 am

Let's see if I can keep it simple. Your CG normally is a bit forward, ie: two in front; to achieve level flight your tail plane will need to exert a downward force ( elevator trailing edge up,forward tips down ). As you add weight further aft the position of the trimmed elevator will move toward a neutral position.Add more weight aft and the trimmed elevator will at some point be a lifting plane ( elevator tips up and trailing edge down ). Somewhere in the area of neutral or slightly nose down trim ( elevator tips up trailing edge down ) should produce your best speed, ( least drag ) both wing planes lifting not fighting each other.

Dave Clark · Post by **Dave Clark** » Fri Feb 07, 2003 1:05 pm

OH YES Roesbery is exactly right and that is why we fly noticeably faster (3-5 mph) with an aft CG. It will also help the speed to lessen the AOA of the wing by dropping the LE down, especially if you're flying near the fwd CG range.

funseventy · Post by **funseventy** » Fri Feb 07, 2003 5:00 pm

Back when I was a Swift driver, I did some racing. And I also remember talking to Edna Gardner White before she past. She raced most of her life and one of her last racers was a 120. In both cases you lower the trailing edge of the horizontal tailand not the wing. In Alaska we'd always crank the excentrics down for better AOI for shorter takeoffs, but it would hurt the cruise performance. The amount that it helped or hurt was minimal and wasn't worth the effort. If you work on the horizontal you can see some significant change in cruise, but it is very hard to get approved. And you also give up a lot of stability and those lovely handling characteristics. Clyde Cessna wasn't an Idiot, if you want to go fast get a Bonanza.

Kelly

N170BP · Post by **N170BP** » Fri Feb 07, 2003 8:47 pm

I never suggested or implied that Clyde Cessna was an idiot....

When you raced Swifts (and modified them to do so I assume),
why didn't you just "buy a Bonanza" to go faster?

Bela P. Havasreti
'54 C-170B N170BP

funseventy · Post by **funseventy** » Fri Feb 07, 2003 10:06 pm

I'm sorry Bela. I didn't mean that you thought that. I just meant in General I love my 170. The swift I raced was almost all original except the C-125 was replaced with a O-300D and it was really light. It fit into a neat class, 108-149hp. Not many airplanes in there will compete witha swift. Including Take-off and Climb over a 60 mile course we won with 153mph. It was fun.

Once again Bela, I never wanted to offend you.

Kelly

zero.one.victor · Post by **zero.one.victor** » Sat Feb 08, 2003 1:37 am

There are now eight,count 'em eight,Swifts based at Diamond Point ,a small airpark just west of Port Townsend. I think it's a desease endemic to the area.I know most of them guys. Swifts are really cool. There are no 2 of the 8 alike--there are a 200 horse Lycoming,a 180 horse Lycoming,3 145 Cont's,2 125 Cont's,and a 150 Lyc with a c/s prop! Swifters are even more fanatical than us Cessna owners are. For the kind of money some of these guys have in their Swifts,they could buy RV's and go really fast.But they wouldn't be near as cool.....
Bela,I know why you want more speed outa yer 170. You ain't ever gonna keep up with that Skywagon crowd.....you just need to meet more 170er's to fly with!

Eric

funseventy · Post by **funseventy** » Sat Feb 08, 2003 1:43 am

Bela, I'm always up for a 170 group exploring. I told Eric I'd meet up with him at Stehekin or somewhere. I'm also up for a trip to the beach strip out there by you on the coast.

Just a side note on swifter's. Whatever they tell you on speed take off at least ten. And that includes me as well. There is something in the Swift water. Or maybe that polish gets absorbed through the skin and affects their vision as it pertains to airspeed. Good group of guys that know how to have fun.

Kelly