Your wisdom and advice please ...

A flying buddy and I are designing an airplane and we want to avoid potential problems that may be caused by the air off the wing affecting the horizontal stabilizer, especially with flaps deployed. We are in the early stages of the design to be similar to the Stick series with a shoulder mounted semi-symmetrical wing at about 1 degree positive incidence and 1 degree of dihedral. We can locate the horizontal stab (to be zero incidence) vertically anywhere from about 2" below to 1" above the wing within the design. The elevator hinge line will be 3 cord lengths behind the trailing edge. The total horizontal stab/elevator area will be a usual 20% of the wing area.

This is not to be a 3D or wildly aerobatic bird. Mostly it will be for weekend sport flying.

What are the best or worst vertical positions for the stab relative to the wing?

Thanks in advance for you thoughts and comments.

Bedford

Is there a particular problem you're trying to avoid? Without knowing what exactly you're trying to accomplish with your model, it
would be safe to say that placing the H-stab on a level with the wing, or perhaps even slightly above would keep the stab slightly above the downwash created by the wing. In all honesty...I'm not even sure how much real "downwash" our models are creating. We're simply not displacing a lot of air molecules you know? Of course, a visit to the hobby shop, or the local airport would show that there are variety of wing/stab relationships in use...all work very well.

"Pitch neutrality" with flaps deployed most likely won't be possible unless some form of flap/elevator mix is used. If it were easy to achieve by a simple change to the stab position...Cessna, Piper, Beech, Mooney, Boeing and a host of others would have done it years ago.

Rolling your own models sure is a great way to enjoy the hobby...have fun with the project!

'Race

Sailplanes have been flying for years now with flaps and all they do is mix in some elevator trim compensation to allow for the nose up pitching action from the sudden extra lift. Models with lower aspect ratios and shorter tail moments often see the opposite effect and find they end up with a nose down pitching reaction due to the increase in pitching moment being more effective than the extra lift. But here again some elevator mixed in will compensate.

I say do nothing. Just put it where you eyes like it and where it's convienient to hook up the controls. Stick to the usual wing and tail incidence angles for the type of model. Mix in some elevator later on if needed. It'll be fine.

Thanks guys. I am not really concerned about pitch trim change when flaps are used. I know that affect is coming. Depending on wing position (height) pitch trim can do a number of different things. I look forward to flight experiments around what the flaps are going to do.

My question was regarding the wing altering or disturbing the flow of air over the stabilizer. In my view, it should be relatively straight, but with a bit of down flow in regular flight configuration. With flaps deployed, the air with likely be more turbulent, making the pitch control erratic if the stab is in the wrong position. Our guess is the best position is high on the fuselage, which will be a bit above the wing.

Bedford

In the Sept 1995 MAN, Andy Lennon wrote an article about this. It depends upon your wing planform, aspect ratio and your coefficient of lift at cruising speed (!).

I'm going to have to read the article again to figure it out. Do you have more info?

Well, snitch, the wing span is right now 96" and cord length is a constant 14-3/8" for an aspect ratio of 6.7. Not so easy to determine the lift coefficient. That will depend on load factor and angle of attack in flight. Don't know how to tie down an estimate for those. My target is for a dry weight of about 15#, but a fuel load of up to 2.5# (50 oz tank - another mission for this bird is cross country flight with up to 1 hour's range).

Bedford

Judging from the graph, I make the centreline of the wake at roughly 4.5" below the wing TE with lift coefficient of 1.

The angle of the downwash at level with TE is about 5 degrees, if I'm reading this correctly.

Thanks, snitch. That supports our theory of the higher the better. There is always T-tail, but that has some severe structural penalties we aren't ready to absorb. We will just have a straight back tail with the stab mounted on top. Simple is best.

Thanks again everyone.

Bedford

Higher is not always better, especially if high alpha flight is on the menu.
The Britsh had severe problems with the BAC 111 due to deep stall conditions associated with a T-tail configuration.
Tests indicated that at high angles of attack near and above those associated with wing stall, the low-energy wakes of the stalled wing and fuselage-mounted engine nacelles impinged on the horizontal tail and significantly reduced its stabilizing effect. Also, the low-energy wakes severely reduced the effectiveness of the horizontal tail as a longitudinal control. These characteristics manifest themselves as an insidious poststall condition in which the angle of attack of the aircraft would increase to very large values (in excess of 40 degrees) in response to the loss of stability, and the pilot would be unable to recover from the condition because of the loss of horizontal tail control effectiveness. During this entire sequence, the attitude of the aircraft fuselage would vary over a relatively small angular range, and the uncontrollable aircraft would descend steeply in an almost horizontal, wings-level attitude with a high rate of descent to an unsurvivable crash.

/Red B.

In a sinle post let me know: wing span , chord, taper ratio, the airfoil used, the weight of the model loaded and unloaded, the expected leingth from quarter chord of the wing to quarter chord of the tail, the airfoil for the tail, incidence of the wing, and I will compute for you an accurate cg location, horizontal tail planform, tail incidence, and give you an accurate static margin for your aircraft, based on "sport plane characteristics" as for the verticle, your on your own.
Nmking09

oh and the reason that some models pitch up with flaps is not due to the inreased lift, it is mostly due to the change in the downwash angle, this changes the effective incidence of the horizontal causing a trim change. The actual pitching moment due to flaps is nose down, just like a highly cambered airfoil. This changing of downwash and upwash angles is why you don't see many canards with flaps.

nmking09,

I will try,

Wing span ... 96"
Chord ... 14-3/8"
Taper ratio ... 0 (no taper)
Air foil ... hard to say, it is borrowed from my Dynaflight Super Decathlon, semi-symmetrical, 2-1/4" thick.
Loaded ... 17.5#/dry 15#
Length from 1/4 chord to 1/4 chord ... about 47"
tail foil ... flat
wing incident ... +1 deg.

Thanks,

Bedford

High Alpha flight is going to get you - no matter where the stab is placed .
On a model anyway -I don't fly the BAC111 so - I am only guessing on that one
last Sunday - a friend was flying my scale model of a 260 Extra - doing high alpha flight right over the runway -at about 30-40 ft and the engine abruptly quit ( fuel tank disloged and fuel flow stopped .)
He had no chance to drop the nose and regain forward speed so -he just held full up (about 50 degrees on elevators)
The model weighing 13 lbs - on 1150 sq inches - simply fell absolutely flat - with no more than 30 ft forward flight -right to the runway. This drove tailwheel into rudder - mains thru fuselage engine upward etc..
,It showed - again -that stabilized high sink rates -which look like a controlled descent - are really killers .
On our foamies the inertia is so low -they just thump down with no damage. but same thing on angles almost vertical drop -
I don't se how stab placement could help/hinder this .
The business of downwash effect on the models is really far over imagined -in my book.

The high alfa tendency of T-tails that Red B. described is a very common phenomena among T-Tail business Jets. The wing !QUOT!washes out!QUOT! the tail and renders it ineffective. On an extra the tail is, for the most part, in-line with the wing. For a T tail what would actually happen at high alfa is the elevator basically would not work and cause the aircraft to violently pitch nose down. Or at the very least cause the plane to begin oscillating, possibly causing a possible lack of control. My spacewalker actually has this problem on landing, and it's not even a T-tail. I can tell because when I slow down to land I have to be very careful not to over correct the oscillation.

Bottom line, for structural reasons and aerodynamic reasons, leave the tail close to in-line with the wing.

Nmking09

beepee I'll run the numbers and get back to you on the tail sizing.

About 3 year ago I built this airplane using the R/C M plan called cloud 9er as a guide. It weighs 8.5 lbs. with a 71" span power is a Super Tigre 75. I estemate the top leval flight speed at 80mph and slow flight speed at under 20mph. It has split flaps that create a lot of drage. I operate the split flaps with the gear channel. Then it has flaperons controled with the flap and aleron channels mixed. The wing has a semisemetrical airfoil that is built flat then the tips turn to flat bottom with a hole lot of washout. I built thr fin and rudder larger than the plans show. It will do most aerobatic manuvers but it is unstable during inverted flight it is a real chalinge to fly an inverted figure 8 but it can be done. to see it go to http://greenbush.org/~mike.fisher/dad/yellowbird.html I realy enjoy flying it I think it could be saled up.

Ok after crunching some numbers, here is what i have come up with.

span of the horizontal stab = 1.5ft
chord of the h stab = 9in

elevator ratio (precent of the stab that is an elevator) = 40

incidenc of the tail with respect to the fuesolage = +1.52deg

CGlocation = 30 percent chord, aft of the leading edge

nuteral point = 47 percent of the chord, aft of the leading edge (this represents the cg location at which the plane becomes unstable)

As for the verticle placement of the tail, keep it in line with the wing as much as possible, especially if you are doing some aerobatics.

If you can't make the tail to those specs then let me know what you are thinking of and i'll re-run the numbers if you like.

I make no guarentee that these numbers will yield an aircraft that will be perfect, they just represent my most accurate calculations using basic aerodynamic stability equations. These equations come from my coursework studying aerospace engineering, and have been proven through 4 competition aircraft to date.

If you have any other questions, let me know.
Nmking09

Thanks, Nm.

No one should expect a perfect airplane. They are all compromises. Good design results in the right high points and minimized low points. Mostly now, we just have to start chopping logs and creating parts.

Will let you know how it proceeds.

Bedford

beepee,

I had an old 60 inch trainer wing that I built a seaplane type fuselage for. Mounted the engine above with a 50 ounce fuel tank. It had an OS FP 40 for power. About 4.5 pounds empty, 7.5 lb with full fuel. It would fly for about 2 hours at about 50 mph. It didn't have ailerons, used rudder and wing dihedral for roll control. The large mass positioned high up gave it some real bad handling qualities (inertial coupling problems).

Subsequently I put wheels on it, put the motor on the front, and a large water dropping bucket underneath. It handled fine like this, and would lift another 4 pounds or so.

The most enjoyable horizontal tail position I have found is just slightly above the wing. Extending the fuselage a bit really helps eliminate blanketing. I tend to design a smaller tail and place it farther back.

What size power will you install?

Multiflyer

Mutiflyer,

Flying with a Quadra 42. About as reliable as a rock. Very important when you are out on a cross country.

Bedford

Bedford,

I scratch designed a weed whacker sport plane that I’ve been flying for years. It’s my all purpose fun plane and flying test stand. 80” span, 14” chord, about 14-15 lb empty depending on what’s installed at the time. Our field is at sea level with a 500 foot paved runway. I had a Quadra 35 on it at one time and wanted to see what it would lift. I hung a 1.5 gallon water dropping bucket underneath and had to add two 5.5 lb barbell weights before I ran out of runway length to get airborne. It still had some climb performance to use up. I’m guessing it might have hauled another 5 pounds if I had more runway?

It got me thinking about range. The Q35 drinks about an ounce a minute at full power, and does at least 60 mph wide open. 60 is a mile a minute, at an ounce a minute, is a mile per ounce. 21 pounds of gas at 6 lb/gal is 3.5 gallons, which is 448 fluid ounces. So 448 miles covered in about 7 and a half hours. And this is with nothing done to maximize efficiency. Yours should out do that by a good bit?

Are you building it for, fun or for an event??

Bryan

I designed a similar aircraft for dropping milk jugs full of water. I installed the stab 3" below the wing and never had any problem with pitch control, it did pitch up slightly with rudder. I flew as slow as possible when loaded to avoid pulling Gs. The aircraft trimmed to speed well when the cg was 15% mac. Flaps may be overkill, I used full span flap-flaperons and and they were fun to play with but not much value. It landed slow without them and really slow when deployed, aileron authority was almost non existent. I think the biggest asset to high alpha control is in keeping the cg forward .

multiflyer,

We have a cross country event run annually by a local club and this new bird is intended to make the most of our past experience competing in it. Being once a year, the design will have to serve multi-function. One of the design characteristics we are going for is very wide speed range. We have flown a scale Cub previously and it was hampered by limited speed during the up-wind legs. Our new design is to be very clean without struts, tail braces, or any more fuselage than necessary. As clean as it will be, we want some help bleeding energy on landing, thus the flaps. Side benefit of the flaps is the washout affect when deployed.

Cross country flying is about the most fun you can have with an RC transmitter in your hand, especially in a competitive environment. We have talked about long distance flying, but have not put a working plan together. In the end, the airplane is no more than 25% of an ability to compete. The drive team is critical to success as they have to meet an unknown drive time and maintain precision following a map. Anything can get in the way of success. With all the best planning, equipment, and skill, there is still nothing like having luck on your side. Landing wheels first with a whole airplane after three 40 minute grulling flights is one of the greatest feelings you can experience. You should try it sometime!

Bedford

Kweasel,

I often prefer setting up a plane with a more forward CG. I observe that many RC modelers fly with, and many kits/ARFs recommend, a more tail heavy CG than really needed. Tail heavy has advantages for maneuvering, but reduces “feel” for airspeed. A most important thing to be predictable.

Flaps on models are very misunderstood. Unless they are some really special full span slotted and so on, flaps on a model don’t lower stall speed enough to make any real difference. For RC flying, flaps offer more benefit by improving stall behavior, like adding washout, and especially adding drag.

Problem with flaperons, as you have observed, is roll control suffers. This is because adverse yaw is magnified. Aileron input with flaperons deployed creates large differential aileron throw the wrong way! Flaperons can be made manageable by including a good amount of aileron to rudder mixing that activates as the flap action is deployed. But still these don’t produce lots of drag. If you drop them enough to start making high drag, the roll control problem gets really bad.

Drag is great fun. It allows steep approaches to spot landings, or great powered approaches. I recommend “split” flaps to anyone. They are the easiest to build. Just a surface hinged to a stick, then glue the stick into a slot cut into the bottom of the wing. The flap does not have to be exactly at the trailing edge. Just about any size, and installed anywhere from about 50% chord on back, works just great.

Multiflyer

Beepee,

There is a group that puts on a marathon event in the desert out here. Basically how far can you fly on a certain amount of gas. Fun stuff. Like you say, having a wide speed capability is very useful for following a car. They run their course along a closed circuit, and there is always strong wind in the desert in the afternoon. It’s very nice if the plane can handle car speed +/- wind speed. Their course also involves a few thousand feet of altitude change that plays heck with the mixture settings. Very challenging. How is your event structured, what is the goal?

As I mentioned in my post above, try split flaps sometime. They are really great.

I did fly my seaplane from a friends boat on the ocean one time. We flew it alongside the boat for about an hour and a half covering about 40 miles following the coast line. That was a kick. I would like to do a good cross country flight someday. Don’t have any idea what or how though?

Multiflyer

multiflyer,

I believe I have read of your desert event in RCM a number of years ago. Sounds fun. Our event is run by Space City RC out of a rural field NW of Houston. It is a three legged event and you are expected to be able to handle at least 45 minutes in the air between stops. There are two RC club fields further out of town to the west that are used for intermediate stops. The organizers drive the course (different each year) and time the legs driving the speed limit. The total time is logged secretly and the competitors have to try and match it. Your time is measured from wheels up, to wheels down.

The course maps are given to you about 45 minutes before the first plane leaves and are not difficult. Though getting off track, worse yet lost, is a very real possibility. Generally, a team is made up of 4 members: driver, navigator, pilot and co-pilot/spotter. It can be done with less (stressful!), but we prefer at least one extra hand. This year's event had a couple of interesting twists - one team's drive crew did not show so they flew with another, two planes from one truck; another flew a helicopter. Another first this year: everyone finished.

Flying is straight forward. We put the plane in front of the vehicle at about 150 ft or so AGL, a relatively comfortable observation height while sitting in the back of a pickup truck. At times (slow speed and tail winds) the bird will get much higher. At other times as when we are pushing the aircraft to its maximum speed, it will drop low, down to as little as 40 ft before we get the drive crew to adjust speed and allow us to grab some altitude. There are stop signs and multiple turns along the course and these are communicated from cab to flight crew so we know what is coming.

As I said before, great fun!

Bedford

bedford,

That’s an interesting take on cross country flying. Sounds like fun for all since it doesn’t require exotic design or engine to participate. One problem we have out here is it’s now against the law to ride in the back of a pickup without an approved seat and seatbelt installed. Basically part of the seatbelt laws passed some years ago. Does each leg of your event take less than 45 minutes? And are you supposed to land at each stop??

Bryan