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Jul 10, 2011, 03:28 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Build Log

Asymmetrical UAV/AP


I get on the Asym kick every couple of years. I get out the BV specs and make a few drawings and never quite make it to balsa. The problem is that there has never been a reason other than the novelty of the design. Well, this time it's different.

I need a model for forward oblique photography. My previous configuration (photo 1) would allow a clear forward view, but the high thrust line and pusher prop caused a number of problems. A conventional layout gives good results for orthogonal photos but the motor and prop get in the way of a forward view.

So, that brings the asym into consideration. I always start with the camera in the pod and the motor in the fuselage. That results in a cumbersome pod. Then I ran across a photo of the initial Blohm und Voss Ha 141-0. It had the motor in the pod - well duh!!

I made a sketch (it always starts with a sketch - photo 2) and ran a spread sheet that allowed for CG calculation using longitudinal and lateral position of components. I treated the center section of the wing, pod and fuselage as a fixed distance of 12.5 inches. The position of the lateral CG is 1/2 half the span. The spread sheet lets me vary the span of each panel.

I'd like to put the Lipos on the CG so I can vary the capacity. The wing panels will mount with plywood blades so they fold back when they hit anything (landing conditions photo 3).

I have a set of ribs I cut with a section Norm Masters kindly designed. The area will be 6 sq ft. Weight 64 ounces. ~11 oz/ sq ft. Microdan 2510 motor with 11x7 folding prop. No gear - hand launch only. Canon S95 camera. Attopilot IMU system.

I think I'll actually build this one.

Tom
Last edited by Tom Harper; Jul 10, 2011 at 06:12 PM.
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Jul 10, 2011, 07:50 PM
Registered User
You should also consider a regular twin or a single engined twin boom pusher setup, though.
Jul 10, 2011, 08:08 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Twin boom pusher presents construction and launch problems. Also, vibration from the prop in the downwash.

Twin is inefficient.

What's wrong with asym? It worked for B&V.

Tom
Jul 10, 2011, 11:46 PM
B for Bruce
BMatthews's Avatar
I believe that there's a clinic in the midwest that goes by the name Assymetrics Anonymous that you could sign up for....

Remember that B&V had wind tunnels available to their teams. We don't. So anything you do along these lines carries a big risk with it.

As far as I recall you use electric power for your camera carrying models. With electrics the engine out risks of twin power is eliminated. So rather than mess about with the potential issues of assymetric designs it seems to me you're far better off with a twin motor setup so you leave the nose as prop free for locating the camera.
Jul 11, 2011, 07:42 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Hi Bruce,

What do you believe the risks are? I don't think BV used a wind tunnel for the 141. The prototype Ha141-0 went from concept to test flight in less than ten months. I think they drew it and flew it. It's not as asymmetric as it appears. The CG is offset from the thrust line only about 4% of the span.

It's a flight dynamics problem.

Tom
Last edited by Tom Harper; Jul 11, 2011 at 08:36 AM.
Jul 11, 2011, 09:31 AM
Cognitive dissonance
kcaldwel's Avatar
For an airplane that just cruises around taking photos, I really can't see a big problem - at worst possibly having to put some rudder and aileron trim in, that changes with airplane speed.

Build it!

Kevin
Jul 11, 2011, 01:34 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by kcaldwel
For an airplane that just cruises around taking photos, I really can't see a big problem - at worst possibly having to put some rudder and aileron trim in, that changes with airplane speed.

Build it!

Kevin
Ditto!
Last edited by nmasters; Jul 11, 2011 at 03:32 PM.
Jul 11, 2011, 01:40 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by Tom Harper
I have a set of ribs I cut with a section Norm Masters kindly designed. The area will be 6 sq ft.
I don't remember which section that was but I think it was for a flying wing. If so you should use flaps because those low moment airfoils are also low CLmax so if you don't have flaps you'll be landing hotter than necessary.

--Norm
Jul 11, 2011, 02:13 PM
B for Bruce
BMatthews's Avatar
If it is a flying wing section you'd be better off using a section with a regular camber. Or even the good ol' Clark Y. It all depends on how slow you want to be able to loiter.
Jul 11, 2011, 02:14 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Norm,

No. It was a section for a wing replacement on the red model above.

I expanded my spreadsheet calculations. The result shows that it's tough to make the lateral CG with the motor in the nacelle. So, I switched to the BV 141 configuration.

It appears that BV had the same problem. The Ha 141-0 put the engine in the pod. All subsequent variants had the engine in the fuselage.

Tom
Jul 11, 2011, 02:17 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Bruce,

60 KMH seems to be the ideal. Much slower and cross winds will produce damping problems during navigation. Much faster uses too much battery current.

Tom
Jul 11, 2011, 02:24 PM
REMOVE TRUMP
vespa's Avatar
This configuration solves your field of view problems and looks pretty easy to launch.
Jul 11, 2011, 02:55 PM
B for Bruce
BMatthews's Avatar
But since most of us would want to point the camera down I'd prefer to see the wings of that design mounted to the top of the center pod.
Jul 11, 2011, 04:17 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by Tom Harper
No. It was a section for a wing replacement on the red model above.
I think I remember that thread. The NM-57 right?
Quote:
I expanded my spreadsheet calculations. The result shows that it's tough to make the lateral CG with the motor in the nacelle. So, I switched to the BV 141 configuration.

It appears that BV had the same problem. The Ha 141-0 put the engine in the pod. All subsequent variants had the engine in the fuselage.
Tom--

Blohm und Voss didn't have a massive battery to move around. Can't you just design it as a twin and use the battery in the other wing to simulate the weight of the dead engine? If the battery is lighter than the engine and pod move it farther outboard. Your left and right roll rates might not be the same but so what.

--Norm
Jul 11, 2011, 05:29 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
The asym looks simpler

Current sketch attached

Tom


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