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Radio Control Forum => RC Building Tips and Problem Solving => Topic started by: Steamplane on December 05, 2021, 03:01:14 AM

Title: Biplane glow engine thrust angle?
Post by: Steamplane on December 05, 2021, 03:01:14 AM
Hoping for some input please, will my 1400mm Tiger Moth need down and starboard thrust angle added to a Saito 62 four stroke?

Title: Re: Biplane glow engine thrust angle?
Post by: TimWescott on December 05, 2021, 03:19:37 PM
For a first cut, treat it like you would a scale monoplane.  So, yes, down and right thrust.

Personally, I'd make the motor mount so that I could adjust things, because no matter what you think you're going to do, you always want to tweak it later.

Title: Re: Biplane glow engine thrust angle?
Post by: billdennis747 on December 05, 2021, 03:32:48 PM
There are many RC Tiger plans on Outerzone and in general they have about 2 degrees down and 1 - 2 right, which is what a FF would need. Having said that, some have none so I presume they just trim it out on the  sticks

Title: Re: Biplane glow engine thrust angle?
Post by: Jack Plane on December 05, 2021, 04:44:17 PM
I'd start with just a smidge of right and down, no more than 0.5 to 1.0 degrees each, which can normally be achieved with four washers:  (looking at the firewall) two at top right, one at top left and one at bottom right.

If you can hold off on fitting the cowl until after the initial flights to check this is about correct (if not use thinner washers etc), you'll then be able to line up the cowl for a permanent fit.

The way to test thrust settings is to fly straight and level at half-throttle cruise speed (or just below) then open up quickly.  If the nose tucks down/right you've got too much, etc.

What's the kit/plan?  Does it not indicate anything as a starting point?

Title: Re: Biplane glow engine thrust angle?
Post by: Steamplane on December 05, 2021, 06:39:17 PM
Thanks for replies. The problem I face is mounting the engine mounts on the firewall, the holes need to be up and left to compensate so the cowl sits straight. As these holes will only be 2 or 3 mm from the no offset position I wont be able to reposition easily after initial mount so I need to get it right first time. I'm trying to avoid the trimming it out thing. Nothing on the plans and I've seen maiden test flights screw port quite badly on takeoff. Getting close to covering and I've been really carefull with keeping surfaces true, thrust angle my last obstacle.
I've never flown a tail dragger before so been studying up on rudder control for takeoff and landing.

Title: Re: Biplane glow engine thrust angle?
Post by: Jack Plane on December 06, 2021, 03:32:15 AM
Then either go 0º-0º and learn to use the rudder, or err on the side of caution and go 0.5º-0.5º (which is fairly minimal but at least starts you off in the right direction without overdoing things) and learn to use the rudder anyway.

I imagine the Tiger Moth is going to need rudder for co-ordinated turns with the ailerons, so learn to use it.

By the way this isn't the most active forum for RC advice - you'd be better off asking the question on RCGroups or RCM&E's Modelflying forums.

You didn't say which plan/kit you were building...

Title: Re: Biplane glow engine thrust angle?
Post by: Konrad on December 06, 2021, 11:32:26 AM
Ok, a lot going on here!
First some basics. There is no one thrust angle that works for a speeds and climb angles.

Right thrust with a clockwise spinning prop (viewed from the cockpit), is used to address "P" factor (Prop asymmetrical thrust) NOT engine torque! This is comes from the angle of attack the wing is seeing for any given speed and lift.  This angle of attack effects the direction that the air enters the prop disk. The higher the nose to the direction of flight the more left thrust the prop produces, hence the need for right thrust to counter act this. On the ground with a tail dragger this is often at an extreme angle with the tail on the ground. Also at slow speeds and high angles of attack.  Same goes for when the plane climbs. All these conditions need more right thrust or more right rudder. All this means is that there is no one correct  right offset value that is correct!  With that low RPM four stroke and likely high pitch prop. I'd set the engine right thrust at about 1.5° but more likely 2°. and fit the cowl with that.

As to down thrust, that is highly dependent on the airfoil, speed and weight of the model. I assume you will be using a high lift airfoil. And as most biplanes are slow the wings will be at a high angle of attack. So I'd start with 1.5° to 3° of down thrust.

As I said all this is subject to a lot of variables that actually are changing throughout the flight. In my more performance focused ships I set up for a bit of right and down thrust if for no other reason than I don't want any up or left thrust!

What is your flying experience? Do you know how to trim the thrust angles?

As to this forum there are a lot of good guys here that know their stuff. No need to go elsewhere and have to deal with all those ads.

Title: Re: Biplane glow engine thrust angle?
Post by: Kevin M on December 06, 2021, 01:59:55 PM
I have a few R/C biplanes in this approximate size and power range. Some have been zero-zero, others I have incorporated 1.5 -2 degrees down thrust and 1-1.5 degrees right thrust. Sometimes clearances on engine installation have been the deciding factor, if the engine is closely cowled there is sometimes not too much leeway to alter thrust angles. To be honest I'm not sure it makes all that much difference if you are an experienced R/C flier. There is no one setting that will cover all phases of flight, and learning to use rudder to co-ordinate instinctively is the most helpful factor. Right rudder on take-off is normal for most R/C aircraft, and rudder to co-ordinate turns is necessary on pretty much all the biplanes I have flown. Aileron-rudder mixing and aileron differential can help but in my opinion practice in using the rudder is the best solution. Most of Konrad's comments agree with my understanding, especially if using a fairly close-to-scale aerofoil section.

Title: Re: Biplane glow engine thrust angle?
Post by: Steamplane on December 06, 2021, 04:06:27 PM
Thanks, I've found this advice very helpful.
I didnt mention the kit origin because it doesnt deserve it, I've had to make many modifications to compensate for poor design but I knew that and took on the challenge for the problem solving and learning. To be honest I was being lazy and bought a cheap kit already cut out and some materials supplied. I also wanted to learn silk covering. In future I'll use one of the plans I found here. I've learned a lot and really enjoyed fixing all the problems, undercarriage and P factor compensation being some of my last build learning/challenges/obstacles. I was hoping for some forum help and sounds like I got it, thank you. I'll go with 2 deg down and right.
My powered RC experience is mostly with a Cessna but I have an obsession with the Tiger Moth.
Cheers, Dave

Title: Re: Biplane glow engine thrust angle?
Post by: TimWescott on December 06, 2021, 07:20:30 PM
Personally I'd go with touch of down and a touch of right thrust.  That fits with the fact that the original was a trainer (in fact, I wouldn't be surprised if the original wasn't rigged the same way).  It also fits with the fact that if I'm going to build an airplane that's designed to fly like a Piper Cub, then I'm going to want to fly it like a Piper Cub -- I'd build it light, fly it slow, and expect it to be an easy relaxed flier once I got it off the ground successfully.

As for the taildragger part -- yup, that can get fun.  If your flying field allows it, do the first takeoffs standing right behind the airplane to make it easier to keep it straight.  When it's on the ground, always be gentle about giving it more throttle or more elevator -- those are the two things that'll induce a yaw.  Ideally (and to be scale-ish) you want to reach flying speed after a pretty good roll, then just watch the thing gently fly off of the ground.  They really only get problematic if you try to horse them around.