Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Biplane Thrustline

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Old 03-17-2012, 05:26 PM
  #1
AussieBen
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Default Biplane Thrustline

Hi all

I’d like to pick your brains on a little project of mine. I am assembling an old 50cc Pitts Special ARF (73in span), this is my first gas setup. The make is unknown and the airframe is quite old, (it lived in a box in someone’s shed for sometime I think). It also lacks any sort of manual. Regardless, it looks to be a very nice airframe and I have stripped it back from its original covering to address seen and unseen hangar-rash.

My main issue, among many, is concerning the mounting of the engine (DLE-55). Due to the lack of a manual and crosshairs on the firewall, I am unsure as to where the thrust line should be (vertically). At the moment I am using the cowling as a guide to where the engine should go, but I am having my doubts. The engine fits in the right position in relation to the cowl, when the stand-offs are butted right at the top of the mounting box. I am aware of the influence an incorrectly placed engine can have on the thrust/drag couple amongst other things, but unsure how significant it would really be, especially in a bipe. Should it be right in the center of the two wings? Should/can it be above or below to a certain degree? How do I determine the best position? etc. Just thought I’d ask people who may have experienced this issue, before diving into the unknown :S.

Below is a picture showing the current relationship between the H-stabiliser line, current thrust line and midpoint of the two wings.

Any help appreciated.

Cheers

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Old 03-18-2012, 05:30 AM
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Rodney
 
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Default RE: Biplane Thrustline

I have no experience with that particular model. However, in some dozen or more 1/4 scale bipes I've flown, all required down thrust, anywhere from 3 to 5 degrees, the more powerful engine, the more down thrust required.
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Old 03-18-2012, 06:36 AM
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Default RE: Biplane Thrustline

On an aerobatic bipe typically- the thrust is zero except for a little right the kit you have appears to be not all that old-

On low powered bipes we saw some needed a bit of UP thrust to hold vertical lines - the real problem was speed bleed ing of because of excess weight and or not enough power

For good all around flying the main thing is weight - avoid weight buildup !!
I don't know which bipes may have needed downthrust . perhaps some I have never seen.
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Old 03-18-2012, 07:09 AM
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Default RE: Biplane Thrustline

Hi AussieBen
I have nothing technical for you. I have had three biplanes. I believe you are correct. Good luck.
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Old 03-18-2012, 12:29 PM
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Default RE: Biplane Thrustline

Here is a partial list of those that I have had that all needed downthrust; 7 Aeromasters, 2 1/4 scale Moths, 2 Phaeton 90's, 3 Phaeton 40's, 2 large Lazy Aces, 1 60 sized Lazy Ace, 1 Giant Aeromaster. The list could go on for some time and ALL required downthrust and some right thrust to fly well. Incidentally, all flew best when the upper wing was at about 1.5 degrees less angle of attack than the lower wing.
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Old 03-18-2012, 11:58 PM
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Default RE: Biplane Thrustline

Thanks guys for your help. I'm probably just being a little paranoid (I do that sometimes ). I just like to check these things before they give trouble down the track. I Always find experienced builders can often spot an issue that is completely overlooked so its always good to ask. I might just mount it 'as is' with 0 up or down and a little right thrust, fly it, and make the adjustments.

Rodney, interesting tip about the lesser AofA on the top wing. I always thought is was supposed to be slightly greater than the lower wing, clearly not in all cases as you have proven. Regardless, it will be interesting to see what modifications I need to make there .

thanks fellas



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Old 03-19-2012, 05:44 AM
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Default RE: Biplane Thrustline


Quote:
ORIGINAL: Rodney

Here is a partial list of those that I have had that all needed downthrust; 7 Aeromasters, 2 1/4 scale Moths, 2 Phaeton 90's, 3 Phaeton 40's, 2 large Lazy Aces, 1 60 sized Lazy Ace, 1 Giant Aeromaster. The list could go on for some time and ALL required downthrust and some right thrust to fly well. Incidentally, all flew best when the upper wing was at about 1.5 degrees less angle of attack than the lower wing.
All of those are quite docile types- fun to fly .
The aerobatic bipes - are setup quite differently
as for mismatched incidences
not desireable on aerobatic types
I had an Aaeromaster probably in 1975- nice model but completely different as compared to a truly aerobatic setup on a Bucker Jungmann -which was the basis for the Aeromaster design -as I recall
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Old 03-19-2012, 04:49 PM
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Default RE: Biplane Thrustline

G'day Ben,

If I'm understanding you correctly your question was regarding the vertical position of the engine, how high it's mounted on the firewall. All the replies seem to be about up or down thrust, ie the angle it's mounted to the firewall. Two different though interrelated things.

I suspect you would notice very little difference in flying if you lowered the engine the small amount to line up with the elevator or to be mid way between the wings, even if one of those positions is the 'correct' one. You won't be able to establish that without flying the model anyway.

Given that shifting the thrust line down will require significant adjustment and or modification of the cowl I would position the engine to match the cowl fly and be happy. You could always change it at a later date if you need to.

Good luck

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Old 03-19-2012, 05:36 PM
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Default RE: Biplane Thrustline

Thanks Dave, Yes you have hit the nail on the head as to the question I was asking. There is a lot of information available out there about up/down/left/right thrust. However there is very little about vertical placement of the engine. Maybe small adjustments on the vertical axis don't have as greater effects on the flying characteristics.....I don't know. I do think you are right though, perhaps just mount it, see what happens. I'm quite comfortable test flying a plane that needs further adjustments, I just wanted to make sure I wasn't setting myself up for disaster at take-off.
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Old 03-19-2012, 07:51 PM
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Default RE: Biplane Thrustline

Yep thrust line height affects the thrust drag couple, think of a vintage model with a high pylon mounted wing with polyhedral and a low mounted engine compared to a low wing aerobatic model with no retracts or wheel pants. Open the throttle and one will tend to pitch up while the other will pitch down all things being equal (which they aren't)

There are aerodynamic as well as the force couple issues as well, but it's only really important to the IMAC and precision aerobatic guys. RMH will be able to tell you all about that.

I had a .60 size Pitts (Midwest I think). It flew beaughtifully with the engine in the scale position, I'm sure you will be fine for the first flights.

How's the weather in Adelaide, did you guys get flooded or was that mainly the East coast?

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Old 03-19-2012, 10:09 PM
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Default RE: Biplane Thrustline


Quote:
ORIGINAL: gerryndennis

all things being equal (which they aren't)
That's the bit I always worry about, lol. Aerody is such an interesting subject, but gives you the biggest headaches at times, as things are never what they seem, hmmm

Weather here in Adelaide has been amazing (although gonna be stormy the next few days apparently). Yea all that flooding has been over on the Eastern side, poor buggers cant catch a break with the weather!
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