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Old Apr 07, 2012, 10:06 AM
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don

thanks for the info on props. all my life i felt the same way but recently some arguments like wake ingestion, such as the guys of synergy aircraft talk about, have created doubts.
also the argument that the propwash is pushing back on anything it impinges on subtracting from its 'pull'. some say the prop is very forgiving of the 'dirty' air it ingests, and some talk about the 'fin effect' that improves stability if prop is at the rear.

even burt rutan may have fallen out of favor of pushers.

what i like about pushers is the pilot sits farther forward ahead of the wing for a great view and passenger sits close to cg.

http://www.youtube.com/watch?feature...&v=lpMzb3DdFEA
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Old Apr 07, 2012, 11:05 AM
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Originally Posted by captarmour View Post
...i felt the same way but recently some arguments like wake ingestion, such as the guys of synergy aircraft talk about, have created doubts.
Their website is full of claims, but short on substantiation. Looking at the claims objectively, they don't appear to hold much water, if any. Just because something looks different does not automatically make it better.

They don't give much info in their discussion of prop effects, but their discussion of the effects of downwash on the tail that they do show has some logical errors.

Their list of "proven technologies" are by no means proven, nor automatic, and even where the technologies do work (such as winglets), the benefts are very dependent on careful design and optimization, with a small benefit in even the best cases, and a significant detriment if the design is anything less than optimum, or is operated outside of a limited efficient operating range.

Just looking at the pictures, I can see some significant problem areas.

Quote:
also the argument that the propwash is pushing back on anything it impinges on subtracting from its 'pull'.
Little bit of truth, large dose of exaggeration. Yes, the propwash of a tractor prop does result in a small amount of increase in airframe drag, but in most cases the loss of prop efficiency in an equivalent pusher installation substantially exceeds it.

The promotors of the "pusher efficiency" myth tend to be airframe designers whose understanding of propellers is lacking. They see a prop as something that you can just bolt anywhere on an airfame and it makes thrust. They do not understand that the propeller has "needs" of its own, and the detrimental effects the airframe can have on a prop.

Quote:
some say the prop is very forgiving of the 'dirty' air it ingests,
Speaking as a former full-scale propeller engineer and currently a UAV propeller engineer (among other things), with experience with both pushers and tractors, I can assure you that if anything, the opposite is true.

I know of one very high profile pusher aircraft where the airframe distortion caused the inflow to 25% of the prop disk to be 15 degrees different from the rest of the inflow. Imagine you were in a sailplane that was constantly and very rapidly pitching up and down through a 15 degree range of pitch angles. What would that do to the stresses on the airframe? What would that do to the L/D? That's what was happening to the prop. The blade root stresses were extremely high, and the efficiency was so poor that they had to make a very large increase in the installed power to meet their speed guarantees. This in turn caused an increase in fuel burn and fuel weight, and a major hit to the plane's operating costs and useful load.

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and some talk about the 'fin effect' that improves stability if prop is at the rear.
This part is true. A propeller in an inclined flow field (i.e.: not parallel to the prop shaft axis) makes lateral forces that act like a small fin and stab combination. Note, this is separate from the phenomenon called "P factor", and from the effects of the swirl in the slipstream impinging on airframe parts such as the fin or fuselage. If the prop is ahead of the C/G it is destabilizing, while behind the C/G it increases pitch and yaw stability. However, the effect tends to be fairly small in most cases. OTOH, in planes with weak pitch and/or yaw stability (such as many flying wings), it can be significant.

On the prop-driven Northrop XB-35, the propellers assisted the plane's yaw and pitch stability. When they converted it to jets in the YB-49, they had to add small vertical fins. Part of that was due to the loss of the fin effects from the propshaft housings, but some of it was due to the loss of the stabilizing effects of the props themselves.

Note also, a contra-rotating (two props on the same shaft axis) propeller system cancels out the yaw, but not the lateral forces that cause this stabilizing/destabilizing effect.

Quote:
what i like about pushers is the pilot sits farther forward ahead of the wing for a great view and passenger sits close to cg...
Yes, that's a nice feature, but it is not automatic with a pusher installation, and there are other ways to achieve it that may not have the other tradeoffs.
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Old Apr 07, 2012, 11:37 AM
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Charles, my last test flight was very short and only in a straight line. However, it was better than any previous flights simply because I was able to put power on and off without causing major changes of pitch, and elevator control into the landing was good.
All the model dimensions are in the original plans by Steven Wong given in the link in post 6018.
My only aerodynamic change is to increase the total canard chord to 85mm, of which 20mm is elevator.
Steven used a motor and gearbox, but did not show these, or specify the thrust line, on his plans.
The CofG determined by tests has eventually come very close to that specified on the plans.
I used the online canard CofG calculator to confirm this position and it came out 10mm ahead of the plan position, which is pretty close.
Regards,
Gerry.
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Old Apr 07, 2012, 01:21 PM
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Hello Gerry!

Sounds like progress. I followed your link to Steven Wong's thread. I'm really surprised that he had such success with such a small canard, not to mention the little prop. Anyway, I'm sure you're right to increase the chord on yours. 85mm is pretty well minimum IMHO for this sort of a plane.

I think you said that your slab wing is 18mm thick. What about the leading edge? I think it's really worth while getting a good shape for the LE, and the TE as well, even with a flat plate in between. You could get some washout by drooping the LE at the tips and making things more symmetrical further inboard.

Trevor -- good to see your contribution. I know you understand dive tests. One of my models, a canard, has a high mounted pusher because I fly it from water. The thrust line goes a little above the CoG. An interesting result occurs when I stall it. It just stops without pitching backwards or forwards. The only way to get control again is to give it some throttle - down goes the nose immediately, before any speed picks up. An example of what you were saying about the difference between pitch changing from thrust and pitch changing from speed. I have the elevator trimmed for fairly low speeds, which means I have to make a big effort to keep the nose down if I want to fly fast. Maybe that, and the stall behaviour, suggests I should move the CoG forward, but that would spoil the fun!

Cheers

Nick
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Old Apr 07, 2012, 02:37 PM
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here's my twin setup- after trying a pair 50 mm EDF, I found props are just fine! speed 300's!
http://www.rcgroups.com/forums/showthread.php?t=709644
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Old Apr 07, 2012, 03:08 PM
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Gerry, I checked out the plans and found the CG to be near mid point between elevators and ailerons. Wong said he had both wings set at zero which would seem to eliminate any canard stall protection and cause the model to fly like a delta as I see it. I believe that the larger canard with added incidence is in your favor. Making it fly to your specs is where the fun is.

Charles
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Old Apr 07, 2012, 04:42 PM
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thanks don,

another reason to like tractors ahead is in the case of an accident you dont have an angry prop coming to get you(me)

i agree that the prop already under stress must beaten up even more by 'dirty' air.
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Old Apr 07, 2012, 05:43 PM
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Nick, the main wing is 12mm thick and I have shaped the LE and TE, but it is still symmetrical throughout.
Charles, I built the original model as per the plans ie. zero incidence on wing and canard. Without claiming to understand the finer points of canards, from what I have read a larger canard area and a few degrees of incidence seemed to be generally agreed,so I build them in on the mark 2.
It has become a bit of a challenge to get the model to fly the way I know it can, from Steven's videos.
I have now put a new motor and a 10x6 prop on it, giving me 750g of thrust with 140W of input power, compared to 550g and 250W with the 7x4 prop.
However, I am a bit apprehensive of the larger prop because during early flights I sheared two motor shafts when landing.
Thanks all for your help and encouragement.
Gerry.
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Old Apr 07, 2012, 05:49 PM
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Quote:
another reason to like tractors ahead is in the case of an accident you dont have an angry prop coming to get you(me)
My grandfather was a pilot in the "Great War" (WW I). He flew a two-seat pusher and was shot down. His best friend was the observer in back, and was crushed and killed by the engine in the crash.

My grandfather talked just a little about his activities in a training squadron in Canada later in the war, but never said a word about his combat experience. I only found out about that from my grandmother after he died. He grew a thick mustache to cover some of the scars from the crash, but apparently some of those other "scars" never healed.
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Old Apr 08, 2012, 06:30 AM
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well said my friend...

2 things on my mind,

1. i know a thin sharp LE swept above about 55 degrees form stable vortices and sharp or not give dihedral effect which comes into effect with increasing alpha.
what about with the same LE above and TE sweep(arrow delta) of say 30 degrees? does TE sweep increase or decrease dihedral effect?

2. my design would have engine and cockpit/cabin area mounted relatively high above the center 'wing' between pontoons somewhat like a hydro. the same wings described above mounted outside the pontoons but with at least 10 degree dihedral to increase ground clearance.
would the high CG negate some of the combined dihedral plus sweep induced dihedral? is there a relationship/rule of thumb for CG/dihedral/stability?

as usual appreciate your insights.

cheers to all
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Old Apr 08, 2012, 10:27 PM
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summer preview

here you go gang- it's only 140" (Falsparspan)
twins to be mounted, along with wing/ canard locations.
Johnny
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Old Apr 09, 2012, 01:56 AM
What could possibly go wrong?
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Nice one Johnny!

I like your dog

Nick

I flew my big Starship yesterday, and crashed it -- fool. Reviewing my actions, I think I must have been too sparing with the throttle. I took off gently, then used the elevator to try and gain height.

Rule number one: use the elevator to control speed, the throttle to control altitude. And that applies to taking off as well as landing. doh!
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Old Apr 09, 2012, 06:11 AM
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o nooooo how much damage nick?
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Old Apr 09, 2012, 10:16 AM
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Disappointing. But not terminal. The re-enforced wings proved their strength and the nacelles and motors are fine. Mainly, I will need to rebuild the nose and the front section.

I think it's worth another try.

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Old Apr 09, 2012, 11:46 AM
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man o man it is beautiful, all the best.
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