Jim.Thompson

Size tug plane recommendations for 99 cc Stihl engine.
[HR][/HR] I'm planning on designing and building a tug plane for aerotowing gliders. I let this be known to a pal of mine and he immediately gave me a big MS660 Stihl chainsaw, which has a 99 cc engine in it. (big bore kit fitted).
Some questions for the readers of this forum:

1. What size plane is nominally matched to an engine this size? I am aware that weight will be a factor, and I will not know the weight of this engine until I strip all the surplus chainsaw stuff off it.
I am totally ignorant of powered radio control aircraft as I'm a glider designer/builder/flyer.

2. What is the reason for using (typically), very thick airfoil sections on powered planes, especially aerobatic planes (like around 12% +). Is it for the added drag that moderates speed through a variety of aerobatics?

Apologies in advance for the simplicity and basic level of the questions.

Jim.

Pelle Gris

I´d recommend a sturdy boxlike structure around 100" span. Straight wing, fully symmetrical airfoil around 15-18% and large flaps if you have a short runway.
Also do build too light! In my experience a tug only profits from a somewhat high wing loading as it gets more stable in tow and you are able to fly in high winds.

I currently fly my own design tug. It´s 97in span, chord of 18,5in and weighs in at about 33lbs ready for takeoff. It´s powered by a King 140RV boxer. Total length including engine is 85".
I´m currently finalizing v2 which will have a slightly stretched wing (103in) as well as larger flaps.

The airfoil is one I have stolen from another design and it performs perfectly. Unable to stall even with full elevator at idle, and thick enough (18%) to provide some drag when landing.


The firewall is located right at the leading edge of the wing and a suitable engine box is attached. The length is adjusted for the weight of the engine used.
Eventually my new v2 will be powered by a homemade 186cc inline tripple, if I can get a reliable engine out of that project, and the other parts set I´m milling for a friend will be powered by a 3W 210cc boxer.

Pelle Gris

I meant to attach a drawing of the airfoil too, but apparently it failed. So here it is.
CG is placed at 29-30% chord and both engine, wing and tailplane is set at 0° incidence.

Jim.Thompson

Many thanks for your well detailed response Pelle.
Your plane is almost exactly what I have narrowed down in my mind the design format for mine. I especially like the Engine box that can be adjusted in length to suit the weight of the engine chosen. What did you construct the box out of? Plywood? I also appreciated your notes regarding the airfoil thickness; I am only experienced with designing and building slopers with thin airfoils. I can see the advantage of the thick section for this application as you describe.
I'm not sure what you mean here though:
"Also do build too light! In my experience a tug only profits from a somewhat high wing loading as it gets more stable in tow and you are able to fly in high winds."
Are you recommending light or heavy? Did you mean to say, don't build too light?
My model will be composite, but only because that is what I am geared up for and experienced in. It will be quick and easy.I'll probably put a bit of taper in the wing, just for appearance sake (although there is nothing wrong with constant chord wings), and maybe a bit of shape in the tail surfaces. Otherwise, it could be passed off for your model. I will start drawings tonight, quite possibly.I intend to blog it on rcgroups.

Pelle Gris

Hi Jim.

Sorry for the confusion. Apparently my "n´t" keys are non functional...:-)
What I meant to write was DO NOT build too light. A slightly heavy towplane does a bit better in adverse weather conditions. And living in Denmark which I do, you get used to flying in windy conditions, as that is the norm here.

I based my build on the SDD Greenley, designed by a guy who´s an active LMA member in the UK. The plans are available online if you do a search. (LMA SDD Greenley).
I did however build my wing without dihedral and with 0 downthrust contrary to what´s shown on the SDD Greenley plans, and that has proved to be perfect for my towplane.

I have towed everything from 10lb modern gliders to 50lb vintage gliders and everything in between, and none have proved to be a problem for this towplane. The record is towing a 35ft Nimbus to 1600ft in less than a minute. MUCH faster and easier than the previous tow using a Giant Stinger powered by a piped G-62. :-)

Gtarling

Greenley and Super Greenley plans here - http://scalesoaring.co.uk/TUGS/Plans/TugPlans.html

Jim.Thompson

Thanks for that clarification Tinus. I will not build it too light, very good. Easier to build heavy I have found.
And thanks for the link to the plans Gtarling. I will download them. I like the note about the advantages of the super thick airfoil and the slow flying characteristics that it provides; very suitable for a big powerful tug. I'm now convinced about that aspect. What is the % thickness of that airfoil, do you know?
I'm undecided about the employment of dihedral or not though. It really doesn't need it, but might provide a small measure of stability.
It will be a simple matter to build this model, or one very similar to it, in composites. The fuse can be glass/foam/glass sandwhich panels, pre-fibreglassed inside first. Then when boxed up to form the fuse, it could be rounded and then glassed on the outside etc. Foam cored, conventional vac bagged wings - 2 part. Tails the same.
Suitable quality ply is expensive and hard to obtain where I live.

Pelle Gris

The airfoil I´m using is the one from the SDD Greenley plan. The chord is made a little shorter though as an aid to transport. 18% thick.

The original plans showed the wing with dihedral but as I mentioned previously I built it straight.
But even then the design is extremely stable, and even in high winds and gusts it still flies as though there were a gyro on the ailerons.

I also mounted my engine at 0/0. No right- and no downthrust, which has shown itself to be perfect.

Jim.Thompson

Thanks again Tinus.
I've been looking at the airfoil polars for the SD8020 airfoil @ 15% and it's looking very favourable, especially at low RE numbers, which is what we are interested in. I'm just a bit shy of going to the 18% like you have, but for no logical reason, just a bit too far out of my "comfort zone", for want of a better expression. I might still use the thicker section after further thought and discussion. I could not find any details of the King 140RV boxer engine in your plane. Do you have a link?
I've started a build thread here:

http://www.rcgroups.com/forums/showthread.php?t=2523079

Pelle Gris

Hi Jim.

The King 140 is a German design, used frequently for towplanes due to it´s ability to swing large propellers at low rpm without issues. I am sure that most other engines in the same category delivers more hp, but at higher rpm. For a low noise setup you want to limit the rpm as much as possible keeping the blade tips below mach 0,6 or so.
The sweet spot for the 140 is, according to my information, around 5000 rpm static, so the 30x16x2, spinning 5300 rpm, is actually a little too small, which also shows as it´s very noisy when in the air. I will most likely try a 28x16x3 prop at a later date to lower the tip speed and thereby the noise.

http://www.kingflugmotoren.de/
The site is only available in German I´m afraid, but since there is limited information on the engines anyway it does not matter much.

I´m sure that the SD8020 is a good airfoil and there is no need to use a thicker airfoil. I only copied the airfoil on the SDD Greenley plan.

I´ll check out you build thread.

Jim.Thompson

Yes, understood. This has prompted me to consider de-tuning this engine of mine by lowering the compression ratio.
I've tried to find out what it is stock, but curiously, it is not listed in the specifications. I wonder if any one reading this might know?
I don't particularly want to calculate it, but only because I'm lazy!

Pelle Gris

If I were you I would focus more on the silencer and prop and keep the engine stock. You need the horsepower for a towplane.
My guess is that your engine delivers max power at rather high rpm, so I would look at using a low diameter/high pitch 3- or 4-blade prop instead, keeping the perimeter speed down while retaining the hp output.

The King 140, being a rotary valve design, is timed for low rpm / high torque and operates very well down to 4-4500 rpm if needed, while a piston ported engine like yours will be much happier at 6K+.

Jim.Thompson

The 3 blade prop option is something I have not considered. Do you know where I might be able to obtain one? What size would you suggest to start with?
The Stihl produces its max power at around 9000 rpm.

Pelle Gris

I normally use Fiala props
Made in the Chech Republic and very good quality. They load the engine very little compared to most other wood props and you are therefore able to use a larger prop than usual.
They can supply 2- and 3-blade props from inventory, and 4-blades on request.
http://www.vrtule-fiala.cz/index.php?lang=en

I have also heard good things about SEP props from Germany.
http://sep-ls.de

If you want carbon Mejzlik makes very nice props too.

Personally I would shoot for around 6.000 to 6.500 on the ground. Then it should still provide ok power, and the tip speed can be kept low.
Not knowing anything about your engine it´s hard to recommend a suitable prop, but my guess would be something like a 24x12x3 to 26x12x3.

I can email you the prop calculator I use myself. If you have measured rpm with a known prop it can guide you in the right direction when selecting another brand or size.
Just let me know and shoot me your email add.