Blue_Moon_

I need advices on the thrust angle on my Yak. Its a very old kit, so the thrust angle is not indicated. The engine is a lot bigger then the recomended 60size two stroker.

Because im setting up this plane to be an aerobatic plane, i think its very important to get the right thrust angle.

Yak 18 PM ([link=http://www.aero-concept.com/fiches/18PM.html]picture / information[/link])

Wingspan: 71"

Engine: saito 100



Please advice! (down/up/right left angle)

TexasAirBoss

My Feedback: (22)

OK, here is my OPINION.



My advise : start off with 1 to 1 1/2 degrees down and 1 1/2 to 3 degrees right.


Fly it and see. Adjust as needed.

Blue_Moon_

thanks! what effects the offset the most?

Rcpilot

My Feedback: (78)

The problem that you'll run into is two-fold:

First:
Right thrust.
If you don't have enough right thrust, you'll notice the plane pulling to the left when trying to do vertical lines, hovers, and torque rolling.

How to Check:
Bring it in about 1/4 throttle. Flat and level. When it gets past you--get it pointed straight up and hammer the throttle. Release all the controls. Watch and see how bad it pulls to the left when it climbs vertical.

You can do this same test again and then correct it with right rudder input. This can give you an idea of how much you need to correct the plane BEFORE you make any adjustments.

How to Fix:
Add some thin steel washers behind the engine mount on the left side. This will push the engine to the right and help negate the left pull.

Second:
Downthrust:
You'll need downthrust to help keep the plane tracking straight in level flight as well as vertical flight.

How to check:
Again, bring it in about 1/4 throttle and when it gets past you--point it up and hammer the throttle. Release the elevator. Watch to see if it is pulling back towards the canopy or if it is pulling away from you towards the wheels.

Anothe way to check:
You need downthrust to keep the nose up in level flight. A propeller want to fly and track straight. With insufficient downthrust--the nose will fall and you'll need lots of up elevator trim to compensate and keep the nose up.

Fly it straight and level with 0 elevator trim and see if your nose is dropping or rising.

A plane with not enough downthrust will drop the nose very fast when you throttle back to idle. Even a Yak, Edge, Extra, Cap should glide fairly flat and level until it looses most of it's speed. If you can fly along at 1/2 throttle or better--then suddenly chop the throttle--your plane should be carrying enough forward airspeed to glide reasonably well (no elevator input) for some distance before it starts to mush and drop the nose.

How to fix:
If it's dropping the nose, then you need to add some downthrust with a washer behind the top of the engine mount.

Hope this helps. Much easier to explain in person than type it out. Sometimes my fingers don't hit the keys that match what I'm thinking.[:-]

Blue_Moon_

im very satisfied with that answer! But my problem is that the firewall is fairly weak so i only got one shot to drill the mounting holes at the right place. If i drill the holes in the wrong spot, then i cant "re drill" because that will probably make the firewall too frigile.. []

If the holes are drilled in the wrong spot, the prop. wont center in the cowl because the holes are drilled in too far or too close to the firewall centerline...

1.5degres down and 1.5degre right thrust:

drill holes, cm from firewall centerline = "root out of" ((12.5^2)-(12.5sin 1,5)^2) = 0,88cm


Is 1.5deegres a good start? Perhaps i can do the firewall holes slightly larger then the bolts so that i can adjust the a bit if nessesery...

Blue_Moon_

How many deegres are "normal" for a bird of this size / engine of this size? (saito 100)

blw

My Feedback: (3)

The problem isn't that hard to cure with a very good fix. First, determine just how thick the firewall really is. It is probably 1/8" ply. It is fairly easy to get ply at the local hobby shop to epoxy a second layer on to the original. It sounds like that needs to be done anyway. I've done it a few times. If you have access to the area behind the firewall you could also epoxy some triangular balsa stock to the firewall and fuselage sides for additional strength. Anyway, try to cut the ply the exact size of the existing firewall and make sure that the second layer glues firmly.

In any case, one way to set the engine mount holes is to stand the plane up on the tail and put the engine mount/engine over the firewall to test locations. You can easily slip the cowl over the engine and see how it lines up. Once you have everything lined up well, take off the cowl and mark the firewall thru the engine mount holes. You can safely drill them for the size of your blind nuts (t nuts). Don't over size them for angles. When you have it all mounted you can remove the mount and put something behind the left side holes to give you a little bit of right thrust as a starting point. You may have to adjust later to get it right. Some people use flat washers or thin layers of firm rubber to get the angles they need.

The reason for right thrust is to offset a pull to the left due to prop wash hitting the rudder and pushing the plane to the left when you start advancing the throttle. The plane may already have some built in. As for down thrust, I would suggest that you don't do this. The plane looks to be a design that should track well and down thrust will present many problems in the air that you probably won't want to deal with. The plane should fly well witn no down or up thrust at all.

Some people mix out the right thrust issue by mixing their radios with a rudder to throttle mix. This is where your radio adds a little bit or right rudder automatically as you advance the throttle. Someone at your club should be able to help you begin to program this into your radio if it has mixes.

Good luck...

jaka

Hej!
0 grader!! Motor, vinge och stabbe.
Oftast fungerar det bra på alla lågvingade kärror.
Vad menar du med att: "främre spantet är svagt"??...det går väl att limma dit ett starkare spant utanpå?
På en så pass stor kärra som din Yak (avsedd för 10cc-15cc motor?) är det lagom med ett 8mm plywoodspant. Gör så att du antingen byter ut det klena spantet (vad det nu är gjort i för material?) eller limma på ett ytterligare plywoodspant utanpå det gamla med Ca lim.
Att mäta 1,25 grader hit eller dit går nästan inte och är heller inte nödvändigt att göra.
Jag rekommenderar att du skaffar en Robart vinkelmätare så att du kan mäta in åtminstone vinge och stabbe...motorriktingen fixar jag nästan alltid till med ögonmått, men kan naturligtvis med fördel även göras med vinkelmätaren.
Det är viktigt att komma ihåg vad som är viktigt när man konstruerar och bygger ett flygplan.
Att allt är rakt och planet byggt lätt är viktigt! Att motorn sitter rakt eller är neråtriktad eller högerriktad en grad spelar ingen roll när det gäller flygegenskaperna.


Hi !
I would go for 0 degrees on both engine, wing and stabilizer, most low winged airplanes are set up that way. A little right trust is always good to have though. But it is nearly impossible to messure 1,25degrees even if you use a Robart trust meter.
I do recommend using a Robart trust meter when ever you can though.

Jan K
Väsby MF

Blue_Moon_

Yes, the firewall is made of 1/8" plywood. Is this too weak for a saito 100 ?

I have added a couple of pictures of the firewall.

[link=http://www.elldes.se/Niclasprivatamapp/YAK/firewall/firewallsm1.JPG]firewall1[/link]

[link=http://www.elldes.se/Niclasprivatamapp/YAK/firewall/firewallsm3.JPG]Firewall2[/link]

blw

My Feedback: (3)

I was mistakenly under the impression that someone gave you the plane and that it had been built a long time ago. It looks like you have done a very nice job building it. Old firewalls sometimes need extra ply to strengthen them, but yours looks very nice. Maybe some people would suggest 1/16th ply (I don't have the metric size) with a layer of glue for the 100, but I don't think I would add anything, but you say it is very weak. Are you sure it is weak for the engine?

I think you will have problems if you add down thrust. Be sure and check the plans to see if they built in right thrust with the fuselage sides. Maybe they didn't. Adding thrust later is very easy.

Blue_Moon_

blwblw, thank you very much im very glad that you think the fuselage looks ok. =)
im 100% sure that there's no built in thrust offset in the fuselage.

Im not sure if its strong enough for a saito 100. The reason why im so confused about this is that the recomended engine is so much smaller then the one im going to put in. As you can see in the pictures, i have added a couple of triangle balsa blocks inside the fuselage. But i guess i will be fine if i do as you say, add a 1/16 layer to the firewall. I dont want to add this layer from the outside because if would not look very nice. But i guess its just as good to add it from the inside... then bolt the engine thru the both layers.

jaka

Hej!
3,1mm (1/8" ) är för klent om du ska bulta fast ett plastfäste eller aluminiumfäste!
Ett 6mm-8mm plywoodspant är mer lagom!
Som bilderna visar är det mycket enklare att limma på ytterligare ett 3-4mm plywoodspant direkt mot det gamla spantet från utsidan. Det skulle jag gjort. Jag rekommenderar att du använder medium Ca lim. Det är starkast.


Hi!
3,1mm is to weak if you intend to bolt a plastic or aluminum motor mount for your engine.
6-8mm is more in line what a 15-17cc fourstroke or twostroke requires.
I think it is easier to just glue another piece plywood onto the existing firewall from the outside than from the inside, just use medium Ca glue for this.

Regards!
Jan K
Väsby MF

blw

My Feedback: (3)

Maybe Jaka is right. (I can't believe I'm agreeing with him on something) I think you could either CA or epoxy another layer of ply on the outside and make it look good. Just smooth off the edges & corners like you did the original firewall. It will look just as good. I would suggest epoxy so that you have a sandwich of ply-epoxy-ply. That would be very strong.

Blue_Moon_

I will add a 1/16 layer to the existing 1/8 ply and hope this is enough...



I think the firewall will just as strong if I add the 1/16 layer from behind instead of adding it from the front. I hope im right.
On my previous model, a topflite 1/7 mustang, i could see the firewall laden outwards at full throttle.. not a nice sight.

cafusin

I own and builded a Carl goldberg Extra 300 and because of lack of time I gave it to a guy who did the finishing touches and covered it. The problem I found is that he did not add any right thrust. The plans ask for two degrees of right thrust and no down thrust. Instead of moving th engine mount to the left (viewing it from the cockpit) and adding some washers (as the plans indicate) he just centered the engine mount so there is no right thrust. I asked him about it and he says there would be no problem, that he has build several planes included an Extra 33% without any right thrust ignoring the indications of manuals and plans. He owns a hobby shop, is a proffesional builder and fixes planes for a lot of us at the field. He has experience but I don´t feel comfortable to have the right thrust different as what the plans indicate. I´ve builded my planes for several years but now I am very short of time so he finished my plane. The problem is that if I want to correct this, I have to redrill the firewall and modify the position of the cowling which is already bolted and lookes really nice.
Should I expect any bad tendencies from the plane or can I fly it with confidence and maybe after the first flights make my decision to redrill and change the thrust?
Thanks for any input.

da Rock

Actually, even if you're going to redrill the firewall and move the motor mount, you won't have to reposition the cowling at all. Matter of fact, you don't want to move the position of that cowling as it ought to be an excellent center reference for you. You would want the new location of the prop thrust washer and the spinner to be right where they are now, centered.

If you have room in the cowl opening to test the new thrust angle without any rebuild work, do that first obviously. You might find that changing the thrust just a couple of degrees isn't worth it. He might be right.

blw

My Feedback: (3)

One of the top aerobatic rc pilots, and I forgot who, wrote an article in 3D-Flyer about doing the same thing with no right thrust built into the plane. Since he wins so man championships I guess he knows what he is talking about. What he suggests is to add throttle to rudder mixes to your radio. I haven't followed his method yet. I still wonder about it....

rustypep

My Feedback: (6)

I have a trainer that has a thrust problem that someone may be able to help me with. After reading this thread, I think it may have too much down thrust. The plane has a flat bottom airfoil which is very efficient but it requires a lot of down elevator trimmed into it to fly it level. If the plane wants to rise with zero trim, does this indicate too much or too little down thrust? What do I need to do? I did check the incidents of the stab and main wing and they are both correct. The engine mount is wood and was preinstalled and I also wonder if it is off. This is the first generation Hanger 9 ARF trainer, the Easy Fly 40. Thanks for the help in advance.

blw

My Feedback: (3)

Well, you opened a can of worm sort of topic with your trainer question. My trainer had so much down thrust that it woulc bend the nose gear over and the prop would hit the ground on takeoff. To me, that was too much.

The reason people put in down thrust on flat bottomed/ample dihedral wing trainers is to compensate for the airfoils tendency to climb by itself with power applications, and to descend when that power is reduced. In other words, it makes an easier to fly trainer. But, it introduces a lot of problems too.

I would rather deal with pitch control via the stick than have all of that down thrust built in. For one thing, it is fighting the airfoil and making the plane inefficient. Anyway, that is my feeling on the matter and I'm sure a lot of people are going to argue the opposite.

BillS

rustypep,

My opinion is too little down thrust. Simply put a couple of washers under the rear of the engine and flight-test it. It should get better and if it does you have gone in the correct direction.

With a trainer and a flat bottom wing the down thrust will never be correct at all throttle settings. Don’t be afraid to experiment. The knowledge will make you a better pilot.

Bill

Blue_Moon_

trim the aircraft in level flight when it flies at for example at full throttle. when youre done, simply find out if the offset is correct by running your engine at idle.

blw

My Feedback: (3)

I wouldn't crank in more down thrust if a little trim helps out. Check the amount of elevator needed when you land.