How do you know when you're underpowered?

[Curare]

Hey guys, I've been experimenting with control line recently and knocked up a quick 29" flapless stunt trainer, with an Enya 09 in the front, running full song, running after a number of repairs at 17oz.

I'm trying to figure out whether I'm underpowered or not. It'll loop but it's stalling on the downside if I turn to tightly, and slows right down if I try and turn a corner.

Is this from being underpowered or overweight for the area or both?

[Clint Ormosen]

I'd say both. Put a 15 on it and see what happens. If it's stalling on the downside of a loop, weight is probably the culprit. Or it's extremely nose heavy.

[Curare]

It's not nose heavy,as a matter of fact it's balancing about as far aft as I dare (around 25-30%MAC).,

[Trostle]

With a 29" span, I doubt if you have more than 170 or 180 sq in of area.  This is small, even for a 1/2A stunt ship if you are looking for something that will do more than big loops and some semblance of a wingover.  A 1/2A stunt ship with 200 sq in and weighing no more than 11 oz will do a very nice AMA pattern.  With your 17 oz and small wing area, you are going to be overweight, no matter how much power you put on it.  But, if you keep the balance point at about 20% of the average chord, put lots of nitro (15% or more), run the engine as fast as it will go on about a 7-3 or 7 3.5 prop, and use 52 to 55 ft lines, you should be able to do some BIG loops, but probably nothing under 60 degrees and some lazy eights, but otherwise, sorry, you will have a dog.  It will be fast and can make a lot of noise.  Larry Scarinzi is a master of this kind of stuff, but his airplanes are at least light and balanced right.

The small Flying Clown has about a 25" span, 200 sq in and with a .15 and at maybe 13 or 14 oz can do a decent OTS pattern.  (As proven by John Wright.)  17oz on less than 200 sq in will be a problem other than having a fast sport ship.

Keith

[Curare]

Keith, it's a fairly low aspect ratio wing with around 200 squares give or take, but I take your point about wing loading.

It was actually a trial for cutting a Q&D foam wing (with a Roncz/Eppler polywog style foil) and I was expecting a wing loading of around 10oz/sq ft.

Now that you're bring it up, I think I'm off the mark scaling a wing loading, (should be thinking cubic wing loading?)

I was tossing up as to whether or not to build another lighter one, but considering how far away I am on wing loading, it's probably not worthwhile trying for another profile with that engine.

What kind of weight reduction would/could I expect from a light full fuselage over a profile?

[Curare]

Well, it appears I have egg on my face, the original wing was a 29x8" constant chord  wing, so about 232 sq in, but the foam wing is in fact  35" with a taper so around 245 sq in.

Thinking about this some more makes me wonder; how do you determine the correct amount of engine to tow the airframe around? Is there a calc or is it purely shooting from the hip?

[Jim Kraft]

Part of your problem is the CG. For a plane like that with no flaps it should be around 10 to 15% MAC. So if you want to try a heavier engine it might help. The only thing is you are adding weight to an already overweight plane. Control line planes are balanced quite a bit forward of where most R/C plane are.

[Steve Thomas]

I don't think your problem is insufficient power for weight. I've also got a flapless model of around 17oz, running an Enya 09. With 15% nitro and a 7x4 I can back it off so it runs a nice 4/2, and it will still do a pretty reasonable impression of the full pattern.

10 oz/sq.ft should be fine for a model like this. I think the problem might be more to do with aspect ratio - certainly with your 29" wing, anyway. At 232 sq.in the AR is only about 3.6, which is way low. As soon as you try to manoeuvre you'll be generating a lot of induced drag, which will slow the model right down and make it stagger. The 35" wing has an AR of 5, which is much better although still less than I like for small stunters. My belief, totally unsupported by any scientific proof, is that little models like this do better with quite a high AR (6+). 

Steve

[Dennis Moritz]

Try a 15, if you have one. If it's sluggish in turns, after that, add tail weight. Power overcomes aerodynamic ills in control line, sometimes. If the plane falls towards you, looses line tension, as you do the top part of a loop, you're probably not flying fast enough, for the weight of the plane. Actually, what choice do you have, except more power. Cannot trim out big time flying issues. Light 33" planes are flyable with 1/2 a power. 17 ounces is too heavy. I don't know if an Enya 09 puts out that much more power than a honking 1/2 A. Sounds underpowered from your description. A strong engine will pull a plane through a stalling turn. Part of the reason the plane stalls is it's going too slow. Definitely a power issue. Especially since you are not describing a super tight pull out.

[Bill Little]

I am tending towards adding more power.  While the model will possibly never do great stunts, I believe the more power, the better it will fly to a point.

BIG Bear
RNMM/AMM

[Trostle]

I am tending towards adding more power.  While the model will possibly never do great stunts, I believe the more power, the better it will fly to a point.

BIG Bear
RNMM/AMM

A very famous American Stunt flier once stated that "There is no such thing as an overpowered stunt ship".  In fact, I think he even wrote that in a magazine column.  I will try to find it.

Keith

[Jim Thomerson]

I agree with  Jim Kraft that your CG is way to far back.  On an airplane of that size the CG needs to be between 10% and 15% of MAC, unless you have an unusually large tail volume.  That is why it is stalling.

[PJ Rowland]

I was about to say when your engine stops during the Triangles... but thats an electric joke.

[Curare]

I agree with  Jim Kraft that your CG is way to far back.  On an airplane of that size the CG needs to be between 10% and 15% of MAC, unless you have an unusually large tail volume.  That is why it is stalling.

It's around 29% on a relatively long tail moment. elevator to stab volumes are roughly 40-60. I was aiming to have a large relatively high A/R tail with a narrow chord elevator.

I feel like I'm missing something here. How does having the cg further forward create better stall characteristics? I understand the change to track and feel but I would have thought that a more forward cg would have meant there would need to be more elevator input for a given reaction, puishing you closer to a stalled condition,no?

[Doug Moon]

It's around 29% on a relatively long tail moment. elevator to stab volumes are roughly 40-60. I was aiming to have a large relatively high A/R tail with a narrow chord elevator.

I feel like I'm missing something here. How does having the cg further forward create better stall characteristics? I understand the change to track and feel but I would have thought that a more forward cg would have meant there would need to be more elevator input for a given reaction, puishing you closer to a stalled condition,no?

Can you post a pic of the model.  That will help a ton.

I am certain if I get this wrong someone will point it out and get it straight but here goes.

There is a distance, static margin, between the CG and the Aerodynamic Center of the airframe.  If the static arm is too short then the tail will want to swap positions with the nose in hard cornering situations.  That's a little extreme in but that is what is going on when the CG is too far aft. Or the opposite is true.  If the static margin is too long then more than a usable amount of control input is needed to change the pitch of the model.  So having the CG in the proper "area" to begin with is very important and it is always a good idea to "miss" it a little forward if you are just guessing and work your way back as you trim the model.

The amount of tail area % to the wing is important if you want to figure your "about" proper CG position on the bench prior to flying. Find out what the tail area is example mine is 26% of the wing so to start with I have the CG at 26% MAC.  There is no scientific basis for that starting position but it just seems to work.  From there I fine tune it until it is smooth and even everywhere I go.  Rarely if ever do I move the CG back from that starting point.
 

[Chris Wilson]

It'll loop but it's stalling on the downside if I turn to tightly, and slows right down if I try and turn a corner.

Perhaps with such a rear ward CG and the model now being VERY responsive you are over turning the model and creating the stall yourself.

[Brett Buck]

I feel like I'm missing something here. How does having the cg further forward create better stall characteristics?

   If the CG is sufficiently far aft, the airplane will be unstable at low AoA and will tend to drive itself to a higher angle of attack than you wanted, possibly causing a stall.

    Brett

[Curare]

Ahh, now that's more than possible!

I can't supply a pic but here's the basic plan i started with with the foam wing over-layed as a chain dot.

[George]

Curare,

What prop, fuel, and lines are you using?

George

[Brett Buck]

Ahh, now that's more than possible!

  That answered your direct question, but Keith almost certainly has it - 12.2 oz/square foot with no flaps on a small airplane is probably just plain too heavy for good performance. A master pilot might be able to work around the limitations to some extent but if it's also marginally stable/marginally unstable, that's just going to make it more difficult.

   This is a pretty classic problem with airplanes using .09s and 15, particularly vintage or vintage-type  engines. They have to be built rather heavily to handle the weight and vibration, but have to be relatively small as well to avoid running out out of power. Keith again nails it, this is smaller than a lot of 1/2A stunt planes using Medallion/Tee Dee.049s and much heavier. Can't use an 049 on this one, the weight savings would be negligible, and can't use a more powerful engine because it's too small to handle more engine. Of course, if "you can never have too much power" is true, you should slap on a Saito 91. Or a Cyclon .061.

     The design looks fine, by the way, and you should be able to build that to around 10 oz with an Enya 09, and that would make a decent flier. I would probably make it bigger than that for an 09, but it would work. Build it to about 7-8 oz with Medallion 049, and you would really have something.

     Brett

[Steve Helmick]

I would suggest 15% nitro and experimenting with some APC propellers (partly because they have such an incredible number of sizes), but also Graupner and Thunder Tiger props are excellent. Lots of magic in the right propeller.  Steve

[Dave Hull]

Other things to consider:

1. Power loading, not just wing loading, if you want to compare planes. I don't have an Enya, so have no feel for your power output. I agree with an earlier suggestion of trying some props. I would try letting the engine spin up and see if there is a better combination. (I am tempted to keep comparing your setup with a Jr. Nobler. Picking the right engine for one of these raises the issues that Keith noted.)

2. I couldn't find where you described your lines. I would fly something like this on .012 stranded or .010 solids, maybe 45-52 ft, assuming that there is nothing particularly awesome power-wise about an Enya .09.

3. Aerodynamic details: is there anything about the plane, wing, or setup that is draggy? Is the leading edge sharp? Is the whole ensemble set up with zero incidence, or are all the pieces roughly offsetting each other? Did I read that you have a pollywog shaped airfoil as part of this experiment? How fat? How much travel does the elevator have, and do you have a wide handle? Can you remove the landing gear and do a test flight and see what that does?

4. If you have significant wing taper (in your diagram, LE sweep) then the real center of lift is normally given in percent MAC. For the way you sketched it, it won't matter much, but it will be further forward than what you would expect by just looking at the root chord. So if you are at 30% balance on the root chord and not having some difficulty flying straight and level, there is something causing damping (extra drag) somewhere.

If the plane is still solid and you think it just isn't living up to its potential, I'd stick in a .15 that you know makes some power and give it a try. Why not?

[phil c]

I am tending towards adding more power.  While the model will possibly never do great stunts, I believe the more power, the better it will fly to a point.

BIG Bear
RNMM/AMM

put in an old For a or Cyclon F2D motor.  Depitch an APC 7/3 to 7/2 or so.  That will both decrease the weight and greatly increase power.  One of those engines will easily haul a 17oz. 400+ sq.in. wing, even slowed down to 55 mph or so.

[Jim Thomerson]

My recommendation on moving the CG forward is based of flying lots of small CL stunt airplanes. 

As a side note, I read a couple of articles on power free flight model stability back in the 1950s.  I did some tail volume calculations for free flights of known power characteristics and necessary trim, and came to the conclusion that there was a range of 10% MAC between a forward want to loop CG position and and very touchy want to dive squirrel.   I went out one day with a very stable good climbing freeflight ( the spiral climb is a combination of loop and roll).  I incrementally added solder to the tail. Each addition of tail weight moved the climb angle down, to the last experiment where the airplane actually dived.  Short run and no crash.

[Curare]

All good food for thought, thanks for your input gents. Alas the aircraft has been scrapped, and a version 2.0 may be in the pipeline.

Having a look at the math though makes for some frightening reading.

The engine alone is 4oz, and  I think I'd struggle to make a profile for less than 3 oz with enough strength and side area, which leaves only 4 ounces for the wing, tail and hardware!

Possible?

[Steve Thomas]

'Possible?'

Well, probably not for me, but then again I don't believe it's really necessary. As long as it's fundamentally sound (in terms of balance, straightness and the basic 'numbers'), it ought to fly fine at 16-17 oz.

The Enya 09-IV is a fine and underrated sport 09.  With a 7x4 it's within about 200 rpm of an OS 10FSR, and arguably nicer handling.

[Peter Nevai]

Using the words "experimenting, knocked up, running full song, underpowered, stalling, to tightly, overweight" in only two paragraphs brings to mind WAY Too many wisecrack sexual inuendos to comprehend!!!!!!