Another impossible dream. Maybe not.

[Deskpilot]

Hi guys. I'm back in the design mode but this one just might fly. Not so much a dream but a possible cat 95 contender.

 

Based on Andre Starck's AS-37, this should plan out fine if built light enough, just imagine the fuselage as being open frame. A 60hp motor driving 2 props vie tooth belts.

 

 

 

 

 

 

 

 

[Pilot Pete]

Looks like the props need to spin outside the cockpit area . Hate to see what would happen if a blade went awol

 

 

[Deskpilot]

Yep, already thought of that. but then, how often does a wooden 2 bladed prop break in half?

 

 

[Pilot Pete]

Not often, but I'd hate to be the one to wear it if it did happen. I would like to have the props at least in front of the torso area.

 

 

[Deskpilot]

The pilot position is a guess at the moment so it may move forward. Haven't finished working out the lift distribution, w & m, c.g. and all that important stuff. Main thing is, to build it light enough. Not sure what wing profile to go for, single sided or double, symetrical (original was NACA63015) what power plant etc, etc.

 

My basic question is, do you see any major problems, if so what and why?

 

BTW, thanks for answering Peter.

 

 

[Pilot Pete]

Had a bit of a look at the AS-37. It seems to be a proven design. Unusual to say the least.

 

 

[Guest pookemon]

Yep, already thought of that. but then, how often does a wooden 2 bladed prop break in half?

Once...

 

 

[Guest pookemon]

One thing to consider when thinking about breaking props would be bird strikes.

 

 

[pylon500]

OK, the concept shows promise.

 

Tend to agree with others that the props are not going to be in a comfortable position.

 

Could they be pushers behind the top trailing edge?

 

Point 2, a V tail will have a lot of trouble trying to handle asymmetric thrust.

 

Also at that short a wingspan, the V tail will give you a lot of adverse ROLL with rudder application, an inverted V tail can solve this.

 

Personally, I would go with a flat tail with twin fins, or a simple T tail.

 

Does the end plate joining the wingtips really serve any purpose?

 

Short wings like that could be cantilever, and simple tip plates will give you apparent extended span.

 

Would be a lot less complex with a single pusher, but I see we're doing this just to be different.....

 

If we want LITTLE, have a look at this;

 

 

This was hard to find, but have a look here;http://chuckisawesome.org/?p=996

 

Arthur.

 

 

[Guest pookemon]

I would think that with the props where they are, balance (CofG) would be hard to achieve. Pushers would be better for this (moving the weight of the engines back).

 

Personally I think it should have twin turbines... :)

 

 

[Deskpilot]

Thanks for your reply Arthur. Much appreciate your comments.

 

First off, I'm not trying to go 'small'. I have seen the X-14 before and if that floats your boat, you're welcome. But not for me.

 

To answer your questions, and perhaps Pookeman's here's my take on it. As it stands, I have know idea as to the finished empty weight so I'm expecting to be right up there as near as damn it, with a TOW of 300kg. With the current wing area, that gives me a wing loading of about 33 kg/sqr-meter (30 max). That's outside the 95-10 category limits so I'm going to increase the wingspan. BTW, I haven't included the sloping 'winglets' in my calc but they will offer some extra lift. Andre Stark included them to not only box in the airstream, but to assist lift in turns. I don't know a lot about the Knoller-Betz effect but from what I have read, the space and stagger of the wings are set at 2/3rds cord and the winglets at 40 degree from vertical. Stark used 45 degrees but I think that leaves it open for the lower wing to fold upward to the top wing and therefore is useless strength wise. Even 40 degrees may be too much.

 

I'm undecided as to what airfoil to use but as symmetricals were originally used, it might be more appropriate to go that way again. The underside shape is probably quite important to extra lift generation of the lower wing. As to whether I should go tractor or pusher, in an ideal world (weight wise) I would go contra-rotating props in a tractor config to give as smooth an airflow as possible over an airfoil that maintains a laminar airflow for at least 40% cord. Failing that, weight distribution might dictated a pusher set-up. A useful aspect of the K-B config is that the combined wings provide a very wide C.G. travel with respect to A.O.A. changes.

 

I choose the V tail-plane purely on looks in that it mimicked the winglets. I've toyed with a higher position but really, it all come down to weigh, not that higher means heavier. On that tack, I'll try a high, inverted V for looks. Twin stabs wouldn't look right on a 'semi bi-plane'.

 

It's said that a little knowledge is a dangerous thing and I'm well aware I'm well into that situation. Still, Nothing ventured, nothing gained.

 

All comments accepted and respected (well most of them will be)

 

 

[Pilot Pete]

I notice there are some of these designs that have pusher prop setups. Maybe it was done to help with pulley location in the larger leading edge part of the wing.

 

 

[Louis Moore]

Deskpilot, what program is it you use to build the designs?

 

Well done by the way, I think it looks like a great little machine, would love to see it flying.

 

 

[Deskpilot]

Deskpilot, what program is it you use to build the designs?Well done by the way, I think it looks like a great little machine, would love to see it flying.

"Google SketchUp" Louis. Free to download but needs a bit of practice and experimentation to work well. (I'm not an expert) The program was initially intended for boxy buildings etc.

 

Here's my latest construction stage. Getting there, slowly. The pilot, seat and wheels are downloaded models, manipulated to suit my scale. Temporary ribs shown, at 10inch spacing. Intended to be all aluminium.

 

 

 

[Deskpilot]

Time to up-date my design. I took Arthur's advice and revised the tail plane. Tried V and inverted V but didn't like the look so I went with the higher T plane. At first I took the 'all flying' route but that proved too difficult to build lightly so now have the more conventional style (temporary skins on whole t'plane). I've also changed my airfoil to a symmetrical design. This gives more internal space for the pulleys. Also gone for pusher due to weigh distribution and safety. So nice to know you guys care for me. Thanks for the advice gentlemen.

 

 

 

 

 

[Louis Moore]

Sign me up for the first kit, I am in cause it looks AWESOME

 

The standard tail also means you can do away with the mixer boxes too! Where did you plan on placing the engine, also is there a real purpose to the large sections connecting the two wings, would it not be lighter and easier to just use struts?

 

 

[facthunter]

Sideslip too much without them? Nev

 

 

[Louis Moore]

Looks like there the aileron attach points on closer inspection

 

 

[Deskpilot]

Yep Louis, thar's ailerons on them sloping wing sections. Andre' Stark profiled them as proper airfoils but I haven't bothered. He states that they add lift in a turn. Theoretically yes and I'm not going to argue the point. The other main point is that the Knoller-Betz effect depends on the wing spacing and boxing in the air flow. Please don't ask me to explain it as it goes way over my head but thought it was worth a try anyway. I think a venturi effect creates a faster airflow over the lower wing and therefore more lift for a given surface area. I'm sure Arthur will tell us if I'm wrong !!??

 

As to the engine, I'm currently thinking an inverted 2 stroke twin so that the crank shaft is high without having the whole engine behind my head. I've never messed with 2 strokes but they must be easier to invert than a 4 stoke. There's still a lot to do with regard to the chassis and suspension. NOTE, for simplicity and weight saving, I have used a 'billycart' style steering. This means that the rudder cables will have to be crossed to get correct rudder control. There will also be a simple brake block that can be pressed down onto the front wheel tyre, not meant for emergencies but might prevent one from ramming a hangar wall.

 

 

[Louis Moore]

Doug, I like the billy cart steering a lot but not sure if ultimately it would work. The end result will be a steering mechanism that functions one way in the air and the opposite on the ground, not really sure how well that will go when you get into strong crosswind situations, your rudder application against the wind will cause a different direction on the nose wheel! Sounds a little cross tangled and might end up tearing a nose wheel out or running into a fence!

 

 

[Deskpilot]

OK Louis, lets take it a step at a time. To turn a Billycart left, you push with your right foot. If my rudder cables went straight to the rudder, this would cause the plane to try to go to the right (as per normal flight controls), opposite to where your steering the wheel. Therefore the cables are crossed to correct the situation. One just has to remember to fly it billycart style as well. ie, right foot forward to turn left. With the wheel in full view at all times, this shouldn't be too difficult to finesse. If others think this might be too dangerous, then I'll have to find a light weight cross-over for the wheel control.

 

What I haven't thought out yet, is how to make the wings detachable for transport. I had some thoughts on a removable aft fuselage and t/plane, then rotating the wing to be inline with the chassis. Do-able but might be weighty, plus you'd need a really big trailer to take all of it in one hit.

 

 

[Louis Moore]

I love the design Doug, But have to be honest and say the backwards rudder while flying seems a little too dangerous to me, be interested in others thoughts though. Not sure about detachable wings, sure other builders will have ideas on that

 

 

[pylon500]

Knoller-Betz effect

Can't say I've actually looked up what this is (I'll get around to it), but just from a basic aerodynamic layout,it looks OK.

Some points though, yes I agree with the others re the billy cart steering.

 

I built a trike many years ago, but as a normal aircraft pilot, the idea of billy cart steering didn't appeal, so I made a pedal arrangement that pulled from the bottom of the pedal hence turning the nose-wheel fork in the right direction.

 

As a bonus I mounted an inverted T shaped item just in front and above the tire, held off the tire by a large spring.

 

Thus if I pressed one pedal, it pulled the fork to steer, if I pressed both pedals (against the spring) the T bar pulled down onto the tire, giving me brakes!

 

Now, other points;

 

Had you thought of a tail dragger?

 

Symmetrical section, pro's and con's, no pitching moment (good);

 

Flies at higher angle of attack (how much rotation clearance do you have?);

 

Thicker (how thick do you need?)

 

With the area your looking at, I would stick with a semi lifting section, look at a Foxbat section, has good stall habits, low pitching moment and a bit of thickness.

 

Power plant setup, a long toothed belt like that will have a lot of 'whip' which needs to be controlled, and if running from a two stroke, will need reduction, so how about an upright motor behind you on the bottom using a toothed (or poly V) up to the wing with twin sprockets on the same shaft running chains in tubes to the prop shafts?

 

Sounds ancient, but remember chains have less losses than rubber belts, and you've absorbed the impulses back at the reduction point.

 

Looking at your 'cutaway' of the tail, why bend or break the main tail spar, and then only support upwards with cables?

 

Remember, there is a download on a tail inflight and the joining brackets to hold angled tubes would be wasteful in weight terms.

 

Maybe a tail half up the fin with cables top and bottom

 

Still don't see the point in the angled tip fins?

 

Would be better to hang the ailerons on the lower wing, less complicated to connect, and just use end plates for tips.

 

Is the prop spacing to get them in clean air, or just look good?

 

I know you don't intend to get asymmetric thrust, but if put as close to centre as possible, is lighter and more controllable if you do lose one.

 

You also have not shown any drag (thrust) bracing within the wing to hold the props

 

Any intention to wire brace the wings, or just use big tubes?

 

The end fins wouldn't do anything to support the wings so you need a strut or wires (or big tubes, I'm talking around 4"Ø here)

 

Do you really need that square tube for the nose?

 

Just continue the twin tubes all the way, have a look at the Mitchell P-38.

 

Please don't think I'm trying to shoot you down.

 

I think your onto something, (although I designed something similar many years ago, look in top right here; https://plus.google.com/photos/113292981019876413104/albums/5212775815502683841/5213148201835122130?banner=pwa).

 

I'm in a position that once I've seen something, then I can see where improvements could be made.

 

Arthur.

 

 

[facthunter]

You can also have each engine pointing outwards, a bit. At the correct angle there will be no assymetric effect with a big fin, You need contra rotating engines. Nev

 

 

[Deskpilot]

Thanks for your reply Arthur. Hopefully I can answer all your comments.

 

Can't say I've actually looked up what this is (I'll get around to it), but just from a basic aerodynamic layout,it looks OK.Some points though, yes I agree with the others re the billy cart steering. Read on.

I built a trike many years ago, but as a normal aircraft pilot, the idea of billy cart steering didn't appeal, so I made a pedal arrangement that pulled from the bottom of the pedal hence turning the nose-wheel fork in the right direction.

 

As a bonus I mounted an inverted T shaped item just in front and above the tire, held off the tire by a large spring.

 

Thus if I pressed one pedal, it pulled the fork to steer, if I pressed both pedals (against the spring) the T bar pulled down onto the tire, giving me brakes! Nice one.

 

Now, other points;

 

Had you thought of a tail dragger?

 

OK. Up to now, no I hadn't considered a tail dragger, but..... as with all my designs, I get carried away somewhat and loose sight of the original intent. Same thing has happened with this one. For instance, this is my chassis and suspension at the moment. I think you'll agree it is getting way too complicated and more important, too heavy.

 

 

Therefore, I've decided to take a look at a T/D and, seeing as this is a cat 95 design, my tricycle cert won't hold me back. I've done some T/D conversion training but didn't complete due to low funds.

 

Symmetrical section, pro's and con's, no pitching moment (good); Really not sure on this aspect. I'm following the original design as I think there maybe some inherent value in the symmetry, particularly between the top wing lower side and the bottom wing upper side.

 

Flies at higher angle of attack (how much rotation clearance do you have?); How big an angle? Also, how big should the angle of incidence be to the fuselage?

 

Thicker (how thick do you need?) Could be thinner for less drag I suppose.

 

With the area your looking at, I would stick with a semi lifting section, look at a Foxbat section, has good stall habits, low pitching moment and a bit of thickness. Again, not sure. Like you, I must try and understand this K-B effect more fully.

 

Power plant setup, a long toothed belt like that will have a lot of 'whip' which needs to be controlled, and if running from a two stroke, will need reduction, so how about an upright motor behind you on the bottom using a toothed (or poly V) up to the wing with twin sprockets on the same shaft running chains in tubes to the prop shafts? If I can get away with a low mounted 4 stroke I'll go that way. My only reason for mentioning the 2 stroke was with view to inverting it. I like your proposal but have been turned away from chains due to weight and, apparently, noise. Chains would certainly be easier to install and or change.

 

Sounds ancient, but remember chains have less losses than rubber belts, and you've absorbed the impulses back at the reduction point.

 

Looking at your 'cutaway' of the tail, why bend or break the main tail spar, and then only support upwards with cables?

 

Remember, there is a download on a tail inflight and the joining brackets to hold angled tubes would be wasteful in weight terms.

 

Maybe a tail half up the fin with cables top and bottom. Point taken, didn't think about any downward pressure. The larger tube wasn't intended to be the major spar but more a way of putting some airfoil shape in without resorting to ribs. You'll notice that it is tapered from centre (2") to tips (1") I can easily raise the fin height and then add wires to take the down pressures. Will probably have to increase the fin L.E. angle for looks.

 

 

Still don't see the point in the angled tip fins? Back to K-B effect. In fact, I may have to put and inboard 'wall' as well.

 

Would be better to hang the ailerons on the lower wing, less complicated to connect, and just use end plates for tips.

 

Is the prop spacing to get them in clean air, or just look good? Purely aesthetics at this time although, I was also thinking that as much of the lower wing top surface should be in clean air as possible (when I was thinking tractor config)

 

I know you don't intend to get asymmetric thrust, but if put as close to centre as possible, is lighter and more controllable if you do lose one.

 

You also have not shown any drag (thrust) bracing within the wing to hold the props Would that (assume wires) run say from L.E. center back to T.E. ends, either side of the prop drive tube?

 

Any intention to wire brace the wings, or just use big tubes? AH, there-in lies a problem. I was hoping that the wing BOX would be more or less self supporting. I have looked at external wires but, due to the large offset(stagger) I can hold the top wing down but don't have enough height in the fuselage to hold the bottom wing up. At the same time, I don't want a myriad of wires.

 

The end fins wouldn't do anything to support the wings so you need a strut or wires (or big tubes, I'm talking around 4"Ø here) Don't want to go there if possible.

 

Do you really need that square tube for the nose? Ref above.

 

Just continue the twin tubes all the way, have a look at the Mitchell P-38.

 

Please don't think I'm trying to shoot you down. No problem Arthur. I appreciate your comments and am willing to take them on board. I'm not a prim adona and can accept useful criticism.

 

I think your onto something, (although I designed something similar many years ago, look in top right here; https://plus.google.com/photos/113292981019876413104/albums/5212775815502683841/5213148201835122130?banner=pwa).

 

I'm in a position that once I've seen something, then I can see where improvements could be made.

 

Arthur.

Cheers, Doug