Hi,
At the moment I am studying the possibility of doing an early British jet (Gloster E-28). I notice that the plane had a positive stab incidence of 1.75 degrees. The plane seems to have a long tail moment which might have necessitated a lifting tail. Wing incidence was 1 degree positive. Would it be beneficial to emulate the real thing or should I stick to 0-0 tail incidence?
Moreover, why would it be beneficial for the real plane to have a positive stab incidence but not for the model?

Would appreciate your input as this has always been a burning issue at the back of my mind.

With thanks.

Reuben

It also has a long nose, I would set everything at 0 on that design, as a side not this aircraft lends itself to an adjustable stab AOI through the use of a stab tube and leading edge adjusters.

Bob

I would go with the scale incidences so that it will fly tail high which would be appropriate for the aircraft.

It's going to fly at whatever angle of attack the wing assumes. The fuselage and tail will wind up angled however the wing angles them. It'll then either climb or descend until you trim the elevator (if needed) to get it to fly level. That of course will either increase or decrease the drag which will increase or decrease the airspeed. The changed airspeed will then alter the lift the wing produces and the wing will decide on a different angle of attack. etc

Yeah, it goes on and on...... What doesn't change is that the wing decides and everything else is along for the ride. No matter what the incidence of the tail, the pilot will be required to trim the plane to fly level based on what the wing is deciding.

Airplane designers know all that and choose the wing and tail's incidence angles to produce the efficient flight at the speed and load they want that efficient flight to occur. They usually place the tail so it's producing the least drag at that speed and load.

When we model real planes, our environment is much different. And we can simply trim the elevator to give level flight no matter how efficient or inefficient the AOIs are.

Once you've got the trim you want with a model there isn't anything more to worry about usually. As long as we steer the sucker around with the elevator we really don't even have to bother trimming it. We're doing that by holding the stick where it needs to be to do whatever pitch we want. The tail will then generate whatever lift is needed to hold the wing where it's necessary to do what we want. We often dial in trim so it'll fly "level" and so we don't have to hold the stick off center.

Confusing? It's not a simple thing to work out the AOI for a tail. I probably used a wrong word here or there, in fact.

But bottom line is that with models, the AOIs aren't super critical. Get the wing AOI wrong and the plane flies tail high or tail down. Get the stab AOI wrong and the plane will need more pitch trim than it would have otherwise.

No real dangers.

To adjust the tail flying position.... both the incidence of the wing and stab have to be adjusted. To adjust only the tail changes the relationship between wing and stab and that can be quite critical and will likely force significant elevator trim, which has a nasty habit of messing up the harmony of power on/off trims.

That is very true there da Rock. All that given, appearance is next in line. Nothin worse than, lets say a Corsair, flyin by with the tail near 2 inches low. So I say build them more accurate to the full scale. Don't want this Gloster flyin by lookin like it's dog paddling in the air.

It really doesn't need to be confusing, in as little as a weekend you can make several versions of your model as profile gliders and learn allot simply by changing the models wing and tail incident along with the best C/G before you invest the time and build your R/C version.

Bob

Hi guys,
Thanks for all your feedback. I note all your points. I had in mind to make a variable stab, as has been suggested but it I will not then be keeping to the scale fidelity of the plane as the stab would have to be "cut off" from the main fuselage. So, whilst it is a very good idea to have an adjustable stab I am hesitant to go this route. I have looked at videos on Youtube of two scale models (true these were different scale sized models) whilst the smaller version was darting during take-off, the larger version (1/3 scale) took off vertically (if you know what I mean). True that in the latter case the wind seemed to have been strong thus aiding lift of the plane. However I am under the impression that the plane may have had positive incidence at the stab in the way it took off. Here take a look at the larger version:
http://www.youtube.com/watch?v=H6q-m...e=results_main

Can I presume that 0-0 stab incidence would be the safest bet?

Thanks again

Reuben

The AOI of the tail isn't going to have much effect once you've got your pitch trimmed.

If the AOI is close to optimum, the elevator trim will be so close to zero deflection you won't be able to see it.

If it's off one way or the other, you will be able to see elevator deflection at rest when you're close to the model. You'll basically have a cambered airfoil back there. It'll produce the lift needed, in the direction required, to keep the wing at it's AOA at your usual flying speed.

Most of our models don't get pitch twitchy from having the tail AOI inefficiently set. And most are quite tame once we've got the CG in the right place and our pitch trim adjusted. The elevator might not be perfectly in line with the stab, but the model usually couldn't care less what it looks like back there.

How those planes in the videos took off were more apt a function of too quick a throttle application, their power loading, their CG location, and how accurately the pilot worked the sticks.

BTW, where did you find the AOI info on that airplane? That kind of info is really hard to come by in the usual sources available to modelers.

da Rock... from my observations, if elevator trim reaches 1/4" deflection there is often a disparity between power on/off trim. If the trim is down to counter climbing under power, the landing glide slope is too steep with the opposite for up trim.

Mixes can resolve the disparity but it seems better to obtain a fairly neutral elevator as most often power on/off trims are happy.

Thanks for your reply. I am taking the information from the book entitled: JET PIONEERS - GLOSTER AND THE BIRTH OF THE JET AGE (Photos enclosed)
I totally agree with what you stated so if I should stick to 0-0 it should not be a problem as any fine flight adjustments will be taken care of by the elevator trim. Hopefully this will be the ideal setting.

Meanwhile I am enclosing extracts from the stated book and a basic outline of the cad drawing.

Reuben

0-0 is how I'd do it. And... if there is minor elevator trim, it isn't a big deal... and some of that can be countered by trimming the ailerons up or down slightly.

I've upgraded radios recently and the new has two additional trim pots, one does just that, it trims the ailerons up/down slightly whereas the other trims dual elevators for loop tracking.

Yep that is what I thought as well as the best route.
It is also my intention to upgrade as although I converted to 2.4 I do wish to have one of the latest radios on the market. We will see.
For the moment, I have my hands full with other projects but I do wish to start thinking about this project especially with retract system as this plane had knee type struts while the front wheel was angled forward by around 104 degrees which may be somewhat difficult to get at 1/3rd scale.

Thanks all for your feedback. It is appreciated.

Reuben

From that great video I'm seeing a couple of things.

First is that the builder/designer did a masterful job of keeping the weight down to the lower limits of what is acceptable for a model of this size. It shows in how easily and slowly the model achieves lift off and how slow it flies and the rather dainty way it touched down.

As for the stab angles it brings up the idea that is often tossed around that the best way to measure the wing to stab angular difference is to use the wing sections zero lift angle as the reference rather than the simple geometric center line. So if the full size and model use a cambered section then the stabilizer could well be sitting at the proper angle for flight even though the stab is at some negative angle to the wing airfoil's geometric center line.

In any event daRock's explanation given above is what will happen in flight. You will set the CG to some location that you feel is valid then you'll end up trimming the elevator to let the model achieve level flight at some power setting. The final angle of the elevator trim will be set solely by the CG position and chosen/required flight speed to fly level.

The manner in how the model responds to changes in power or flying speed due to diving, etc will be set by the CG location and the elevator trim location that is associated with that CG location. If the model seems to respond too strongly to changes in speed due to diving or adding power then the best response is to shift the CG rearward by some amount and re-set the elevator trim to regain level flight at the cruise power setting. The resulting change in wing to stabilizer angle will make the model less pitch stable. But as with so many things in life too much is often just as bad as too little. The key is to find the happy middle ground.

So all in all if you intend to use the scale airfoil and it is a cambered one then you might want to stick with the scale wing to stabilizer angles. But on the other hand if you go to a 0-0 geometric setup that would hardly be a bad thing either. In the end you might end up with a little down trim for level flight or you may not.

And finally a little background on stabilizer lift. Whether or not a stab on any particular model is a lifting stab is set solely by the CG location. It doesn't matter if you give the stab a positive angle of incidence or give it a lifting airfoil or stick it on the end of a long tail moment. It's still the CG location that determines if the stabilizer is lifting or not. Almost all full size aircraft have the CG range ahead of the 25%MAC point. As such the stabs on them are always lifting negatively to counteract the torque of the CG locatoin trying to pitch the aircraft down. On the other hand most of our models and some specific full size aircraft have their CG ranges behind the wing's 25% MAC position by some amount. On these the stab is always lifting upwards to prevent the wing from wanting to pitch up.

The reason that this is still a stable setup is that the aircraft has an overall planform center of lift (that's likely the wrong description but it's the best I can come up with) that we call the neutral point or NP. A design which is balanced at the NP will show signs of being neutrally stable. That is, you can put it into any attitude and assumging it has enough thrust to not stall it will stay in that attitude until it hits something. But most designs are set up to have the CG be located at some small distance ahead of the NP in order to be positively pitch stable.

So you can actually have some extremes such as found on old 50's era free flight models where the large stabilizer on the long tail resulted in the NP being at or even slightly behind the wing's trailing edge. And the desireable CG location was thus sitting at or just a hair ahead of the wing's trailing edge.

Or look at a canard or tandem wing setup. Such planforms are still analyzed to determine the NP by considering the foreplane as the "wing" and the rearplane as the "stabilzer". The resulting NP location on a canard ends up back near the leading edge of the rearplane. In effect the proper way to look at such a planform is that it's a "conventional" layout that has a really small wing and a really big stabilizer that happens to be strongly lifting because of the very rearward CG location.

As I stated in post #2, 0 degrees incident on everything, and an adjustable AOI on the stabs will most likely work just fine, especially if you keep it light. If you really want to nail things though, then you need to walk the walk, and build some small testing specimens as I already stated earlier. We did this often some 25 years ago while I was still employed at Dryden Flight Research Center in Edwards AFB CA. We would test fly these specimens from on top of the loads lab that was the tallest building NASA had out there at that time, we would make adjustments on our glider versions until we had a good starting point, then we would move on to R/C versions and even full scale application from there. There comes a point that the designer can only speculate so much, the rest is getting in there and doing it... I wish you the very best on your build!

Bob

Thanks again. Your explanations have been helpful and shall take them into consideration especially the fact that making a chuck glider may prove beneficial to the end result. For the moment I am still bent on having a 0-0 stab incidence with a 1- 1.3 degrees AOA on the wing root with a 1 degree washout. In so far as airfoil to be used I am considering the NACA 23012 for root and tip. Stab would be a standard NACA 0010 or similar. Dihedral will be 4 degrees. This is the basic set-up I had in mind but as I said everything still on pc so changes can be made until the cutting starts which I do not envisage to be that soon. However, its good to think about what may be down the road.
In so far as the designer of that video model plane is concerned he and his father are reknowned modellers in the UK as they have made some very nice models over the years. They are both very talented persons. Unfortunately, I have not been succesful in contacting the original designer. BTW that model was eventually sold to a modelshop.

Again, thanks for your explanations. Up to now this has been all very interesting.
Thanks.

You'll be fine. But I'm not sure if you got one of Bruce's points. The AOA depends on how you measure it. Your documentation shows that the plane had a "high lift" airfoil. So when it says the wing incidence was 1 degree, what exactly does that mean? In relation to some line drawn through the airfoil? Or the zero lift line? The stab certainly does not have a high lift section. How is it's AOA measured? It may be that in actual effects in flight the wing and stab are really 0-0, or maybe the stab is slightly negative.

Nevertheless, as others have said, you'll be fine because you will be quite close no matter what you do, and then you will trim it out.

I think it's always best to use the angle of incidence for the wing that the full scale plane uses because then it will look right sitting on the ground. But these are good questions that a lot of scale builders don't often give a lot of thought to.

Great project! I saw the original in the London Science Museum.

Jim

What is the engine thrust line?

I'm in the process of building a Zlin 526. The wing is semi symmetrical at the root and is under-cambered at the tip. The stab is zero degrees and the wing is four degrees positive. The wing also has six degrees of washout. The engine is set a zero degrees of thrust. This is supposed to be exactly how the full scale is set up.

Ken

Mike if I am getting this correct the engine thrust line is 0-0 as this is a turbined plane. Jim, the degrees quoted above are approximately the same as per original. The wing incidence angle is being taken as a parallel line to the centre line of the plane and is angled upwards from the trailing edge of the airfoil. I do not know if you can see it in the drawing.
Again thanks to all.

Is this drawing a good representation of the aircraft? I notice the wing has a positive AOI to the FCL, however the stab is sitting at a 0 AOI to the FCL, at least on this drawing, I only ask because the appendices you posted states there is a positive AOI of 1.75 degrees on the stab, if so that is in direct conflict with what I am seeing on this drawing. Also looks like the Stab has a semi symmetrical airfoil, am I correct in what I am seeing?

Bob

Bob you are very correct the original drawing on the book had a positive stab incidence as you indicated but I changed it to 0-0 for my drawing. Moreover, the stab airfoil is just a trace which will be replaced by a cadded version. This also applies to the wing airfoil.

Just an afterthought. I was thinking of settling for a 1 degree washout. Do you think that this will be a reasonable washouot airfoil tip angle or would one prefer a higher degree of washout gradient?

Not having ever seen the planform view, and relying on the drawing of the plane, it looks like the wing is somewhat like an elliptical planform. Elliptical wings stall equally from root to tip. They don't have a tendency to stall from the tip. Your 1 degree of washout should insure that you definitely don't have washin, but very probably isn't needed.

One reason they went to all the trouble to build elliptical was what they get from them without extra effort. Quite a British attitude, by the way.