Flap to Elevator Ratios

What should the ratio of flap movement to elevator movement be? Does the ratio change based on tail moment, airfoil, or ???

In 1960, seems like it was 30 degrees flaps and 45 elevator. Since the new planes pop squares with 8 degrees of flap, does the ratio even matter much? I have a Brodak warbird, and the plans show 1 to 1. Is that the norm today?

The Bob Palmer designs such as the T-Bird were considered to be able to fly through a "harder" corner with a 1:1.5 ratio, flap to elevator. They typically didn't feature flaps that were as big, and weren't full-span. By the mid-1960's, so many clones of the Nobler were in existence that in sheer numbers, the 1:1 ratio planes swamped the older type.

(The T-Bird also featured a split flap system in which the WRONG flap deflected more than the other one, at least wrong by current
theories.)

With the larger tails used today, not only do we get more leverage with less deflection, but we are able to move the CG back further which decreases the needed deflection even more. Thus it would be rare to find a modern design that truly needs more than 20 degrees of deflection when properly trimmed.

When the tail surfaces, flaps, etc., are properly designed in relation to the other, the one to one ratio becomes the norm. The only exception would be if you then build the plane exceptionally light. In this case, it isn't so much that you need more tail deflection, but less flap deflection. Thus the ratio could change in favor of the elevator. But, simply setting the ratio for 1:1 should do the average builder/flyer just fine.

LAST EDITED ON Jan-05-01 AT 11:40 PM (EST)

>Gang --
>
>What should the ratio of flap
>movement to elevator movement be?
>Does the ratio change based
>on tail moment, airfoil, or
>???
>
>In 1960, seems like it was
>30 degrees flaps and 45
>elevator. Since the new planes
>pop squares with 8 degrees
>of flap, does the ratio
>even matter much? I have
>a Brodak warbird, and the
>plans show 1 to 1.
>Is that the norm today?

1:1 is in the ballpark for most models, mostly because the design rules of thumb are built assuming it than any intrinsic value.

The ratio you want depends on a lot of things. Conceptually, the elevator controls the pitch rotational rate, and the flap controls the lift. This is a significant simplification, but it makes it easier to understand. What you are after is to create some sort of balance between these two. Pitching around quickly doesn't do you any good if you don't enough lift to make the airplane change the direction of travel, and having a lot of lift doesn't help if the nose doesn't aim in a different direction. So they need to be compatible in some way.

In the extreme, take a Ringmaster (a model with a 0:1 flap/elevator ratio) or even worse, the Air Trails "Wing" that Bob Hazle took to VSC and that was featured in my column. They will pitch around much more quickly than they can change direction. That's why it's so prone to stalls, and has to be nursed through corners with a very soft hand. On the other end of the scale there was an airplane from the late 40's called something like the "flipper" that had flaps but only a fixed stab (1:0). Reportedly, this one would go climb up or down with the fuselage more or less level, and only a slight tendency to pitch.

Within these extremes there's some middle ground of good where it rotates at the same rate it changes direction. How you do it depends on the weight, the airfoil, the size of the flaps and elevators, the mass properties, and on and on. The 1:1 ratio is common more because everybody sized their flaps, elevators, and wing assuming it would be 1:1, and when it came out a little bit wrong one way or the other, they didn't alter the ratio but instead resized the other components the next time, until it converged on the right answer. The bottom line is there doesn't appear to be any reason to think that 1:1 angle ratio with the other components sized to balance the turn is magic. Other ratios with resized components (like .25:1 with very large flaps) may be as good, better, or worse. There's not a lot of data to go on.

The flap=lift and elevator=rate oversimplification is wrong because the flap has a significant effect on the pitch rate, and the elevator has significant effect on the AoA and therefore the lift. For instance, putting flaps on a Ringmaster may not be a great idea because the adverse pitching moment from the flap may not be easily overcome by the short tail. The two effect are strongly coupled. But the concept of treating them separately is very useful for visualization.

As with almost everything else, trimming the flap:elevator ratio based on flight characteristics is not unheard of. Most top airplanes are built with either a series of holes or a slider in the elevator horn to enable you to use other ratios. Since a lot of them are original designs, you're never quite sure how it will fly,or how heavy it will be, beforehand, and you need to be able to compensate. In reality, this feature is not used all that much, partly because the "right" setting is pretty vague and subjective, and partly because most of us are still pretty close to the original rules of thumb developed to make 1:1 work.

It's most useful if you wind up unexpectedly a lot lighter or heavier han planned due to paint or bad estimation. If you planned for 55 oz and it actually winds up weighing 70, the only way to salvage the situation may be to take out a bunch of elevator motion and then compensate by opening up the handle. That way you get a lot of lift for a given rotation rate. On the other end of the scale, I suggested to David Fitgerald recently that since his Kiev airplane was a lot lighter than all the other TP's, we might want to try slowing down the flap to get a little less lift. While the jury is still out to some extent, it appeared to be a step in the right direction.

Trimming it is just a matter of feel, and there's a significant range of "right" depending on the pilot. If you get too little flap, the airplane will feel great and give the impression of a very tight turn, but will be prone to little hops coming out of corners when you try to level out. The reason is that the nose of the airplane is a little inside the flight path, and will thus be a little up when the airplane is moving level, and you have to pitch it down a bit to get it level. It will look like the model is rotating around the spinner. If you have too much flap, the model will have to "swoop" into and out of corners, the corners will give the mostly false impression of being too large, and it will look like it's rotating around the tail. It will also feel unusually heavy on the controls.

The Nobler and even more so, the kit Ares, and the Patternmaster are toward the latter end of the scale, and the Shark 45, most Fancher designs like the Trivial Pursuit/Star Gazer/Pond Scum, and the Impact etc. are towards the former. All of these models have excellent contest records which indicates the range is large. I would assert with no definitive proof that the "crisp" style pattern seemingly favored in competition today is more easily done with the Fancher-style models than the Nobler style. But that could change in a heartbeat, so it's probably better to just make it feel the way that suits you the best on average and then just ignore the apparent whims of fashion.

It does make for an interesting experiment, and it's pretty safe, so fiddling just to try and see the effects may be a worthwhile use of your time.

That was not a reply. You wrote Chapter 1 of your book "How To Build A World Beater". Easy to understand, succinct, and required reading for anyone who is designing CL stunt planes. Technical writing is an unappreciated art form, and you are in the top .1% on this article. All the kids out there will read and ponder.

You also described the next generation of CL systems. Flaps should kick in on squares and triangular maneuvers, and not come into play in the round maneuvers. Somebody will figure out a way to do that, guarantee! You shout that any linear relationship to flaps and elevator movement is inherently flawed and results in a compromise.

Thank you very much! You flatter me. I have in fact written a more than a few book-esqe technical reports for money, one of them something like 450 pages of migraine-inducing graphics ('Flight Experimental Results and Analysis of Propellant Pressure Resonances in the MILSTAR Auxilary Propulsion System", not available at Barnes and Noble or via FOIA, actual cost to the taxpayer probably in the $2,000,000 range if you figure in the cost of transporting nitrogen tetraoxide to 24000 miles). So I have a fair bit of experience.

What sometimes annoys people is that I tend go further than what anyone really want to know. Ask me the time, and I will not only tell you how to build a clock, but also feel compelled to explain the theory of the pendulum, and the metallurgy of the mainspring. I'm sure it's due to something akin to obsessive-compulsive disorder, which is does tend to occur in stunt pilots at a far higher rate then the general populace.

>You also described the next generation
>of CL systems. Flaps should
>kick in on squares and
>triangular maneuvers, and not come
>into play in the round
>maneuvers. Somebody will figure out
>a way to do that,
>guarantee! You shout that any
>linear relationship to flaps and
>elevator movement is inherently flawed
>and results in a compromise.

Way ahead of you there, although I don't think it's entirely clear that this is true. It's relatively easy to make the flaps only move at large control deflections - use two pushrods, one from bellcrank to elevator, and the other from bellcrank to flap. Then just create a huge amount of slop in the flap horn, maybe 15 degrees. Alternatives are cams to drive the flaps with a cam or eccentric (another word with broad appications in stunt) thats on the flat at neutral and runs up the slope as you move the controls.

There are several problems with this idea. Having no or minimal flap motion requires the airplane pitch angle to change for every small correction you need to make in say, a round loop. These tend to be very visible, and tend to hurt the illusion that you're smoothly going around in a circle. Being able to change the turn radius with minimal pitch angle change in the fuselage is a big advantage.

An additional problem with any sort of non-linearity or discontinuity you introduce is that it's non-linear or discontinuous. One of the reasons I think everybody finds my airplane so easy to fly compared to others is that the I spent far more time in the design and trim trying to get a linear control response, even at the expense of what has traditionally been referred to as "performance". If you just look at the "numbers" on my airplane you will find it neglibly different from most of the others.

The goal was to get exactly the same response in all conditions, and at all handle positions. Introducing any sort of non-linearity like greatly variable control load (very very light feel near 0, and disproportionate very heavy loads at large deflection, as the flap-cam would introduce), would make it very difficult to fly reliably. This last might be compensated for with the still-experimental (in stunt, anyway) servo- or boost-tabs, but I would think there still a lot of work to do to see if it really does anything good enough to warrant the extra complication.

I think it's easy to visualize the effect of a discontinuity. Something that suddenly happens is really the kiss of death. As an example, one of the locals built an airplane with flow and load actuated leading edge slats. This worked vaguely like the similar devices on early jets like the Me-262 and F-86, except it worked both directions. The idea was to increase the lift and therefore make it turn tighter corners. It was kind of slick the way it was done. The problem was that it did nothing until it deployed, and then suddenly it did far too much. It was nearly impossible to fly unless you flew it soft enough to never deploy the slats. This is just an example. but anything that induces some sudden or extreme change in handling characteristics, even if it were to improve the performance, is going to tend to induce piloting errors.

This off the topic, but is offered as an aside: An even more entertaining side effect of this system was that sometimes one slat deployed fractionally sooner or later than the other one. Having a Cl of .4 on the inboard wing and a Cl of 1.5 on the outboard wing leads to a lot of running.

You convinced me! My next plane will be the Brett xxx or whatever you have up your sleeve. Send me plans, and I will give you a couple of thousand dollars, OK?

Now be fair. I didn't say discontinuous. I said non-linear. I was picturing logarithmic ratios, which are non-linear, but certainly not discontinuous. It is a case where a word is worth 10 to the minus 3 pictures.

Yeah, I thought you might have written some tech stuff before. You sounded a little too slick. Point is, you had sound thoughts and expressed them well. That is difficult to do.