Elevator size
 

Am working on parts and plan for a slow flying old-timer. My book says when converting to aim for the elevator to be about 20 percent of total Hoz-stab area. That seems both large and small to me. Very same A/C was flown fine with much-much smaller elevator. but then too my booklet says to aim for 30 percent.

The total area of the Hoz-stab in now about 130 SI.

What size you think it ought to be?


Wm.

RE: Elevator size
 

Designs that are planned for simple flight can get away with the smaller size elevator. Unless you plan square eights or snap rolls, lots of quick force won't be needed. Perfect example is the Tiger. Try and get one to snap roll. I think the stock elevator is about 15% on the sucker. It's my understanding it was designed as a "enhanced" trainer. I had 3 of 'em and just getting the wing to stall wasn't something that tail could do easily.

Designs that are pointed at extremely efficient flight often have slightly larger than small elevators. Gliders are good examples. They don't need snap roll ability but want efficiency. They aren't going to get max efficiency from a short chord elevator that has to deflect a lot to get the job done. The less deflection, the better the efficiency.

You don't expect to need a lot of effect for "heavy work" like snaps and such, right. But want your slow flyer to stay in the air with minimal power? If those are your parameters, you won't need 40-50% elevators, but also don't want less than say 20%. I'm guessing.............

Say 30 sq.in for elevator and 100 sq.in for stab and you got a 23% elevator.

RE: Elevator size
 

I was hoping to get into the19 to 16 percent range, with extra deflection. That 30 percent seemed big in light of what was used before.


Wm.

RE: Elevator size
 

What 30 percent?

30 square inches is 23 percent of the 130 square inches you mention. You mention however the "total area" of 130 sq.in is the horiz-stab. I was guessing that "total area" was actually for the entire tail. But it'd be better to clear up the numbers.

What was used before in the entire horizontal tail?

But bottom line is that anything is going to work, especially around 20% for the elevator. Going smaller requires greater deflection. Going larger requires less. Less deflection should result in better efficiency. But the difference in efficiency isn't going to be much. It'll only matter on something like a glider and then not a lot.

RE: Elevator size
 

If you go extra wide for anything close to a 50-50 or 60-40 split you'll just end up needing to reduce the throw enough to get it to avoid being overly sensitive. For a flight trimming function such as found on old timers even 15% of the chord is pleanty. If you want to go with more to make the hinge like end up at the orginal spar location just for appearances then that's fine too. But be prepared to drastically reduce the elevator throw if you end up with a sizable movable to fixed ratio.

RE: Elevator size
 

I will then leave it as estimated.

But, how much throw (up and down) then? I had a similar but very small SI elevator on this oval shaped Hoz-stab back when, and due to deflection it aimed it either up hard or down hard. I was originally hoping it would sort of twist or aim the A/C in the proper direction, not direct it as hard as it did. That deflection was about 1/4 inch on a new elevator of about 6 SI then.

With increased area now, I do not what to get into "over control".


Wm.

RE: Elevator size
 

After a lot of different models from all flying to really narrow elevators I've found that if the trailing edge moves the same amount that all the different setups work about the same. So if a 15% wide elevator needs plus and minus 1/4 inch of travel at the trailing edge that if you make the elevator 50% of the chord it still needs the same plus or minus 1/4 inch. You see, it seems to come down to how much does the overall airfoil angle of attack change.

RE: Elevator size
 

Interesting observation about how much travel that is needed on elevator. Could this be because that a certain deflection on the trailing edge gives the same amount of camber in the airfoil? And to generate a given amount of lift a certain camber is needed in the airfoil?</p>

RE: Elevator size
 

It adds both camber AND angle of attack at the same time. And yes, because both an airfoil's camber and angle of attack are set by the line between the leading and trailing edges it's the throw at the trailing edge that counts for far more than where the hinge line is located.

RE: Elevator size
 

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[img][/img]My scratchbuilt Cassutt has a 51" x 14" = 714 sq in. symmetrical wing and stab of 12" x 7" = 84 sq in. This gives a 12% ratio. It flies like it is on rails!