thrust line on pushers?

[gouch]

I am building a pusher plane based on the "frog" by Dan Schwartz, but have modified it for AP use with more wing area longer fuse and it will be using a much higher output power plant and AUW.
I am at the stage of fitting the motor mount, I began to wonder about the "correct" thrust line. I have drawn a simple image to help explain it.

The prop centre line will sit higher than the wing and obv. behind it.

Which line should I follow? the blue one, or the yellow one, assuming the prop centre line sits where the point where the green fuse line meets the yellow line.

I'm guessing the yellow line, but would like confirmation and some assistance on exactly the reason. I have built one previously and noticed a big nose down under full power, and after moving the angle, I still noticed it was pushing down.

Can I get rid of the nose down tendency with motor alignment 100% given the prop sits higher than the the wing? I can't get it in line with the wing due to prop size needed, without making the fuse excessivly "high". I now have it about 2" above the top most section of the wing.

It is an undercambered wing. Does this also have an effect on this?

Also whilst I have your attention, how much decalage should I aim for ?
The original I built had the same as the plan, which was quite a lot.I found it did pitch up a lot at speed. The second one I built had a bit less,and did seem better in this regard, but I deviated from the plans a bit in other areas so I wasn't sure of the change.

I am used to slope planes that I generally set at 0*, but this slowflyer stuff is still new to me, esp with undercambered wings.

Any advice is greatly appreciated :D

Cheers
Paul

[globemaster3c17]

What exactly is decalage anyway?

[HELModels]

I am building a pusher plane based on the "frog" by Dan Schwartz, but have modified it for AP use with more wing area longer fuse and it will be using a much higher output power plant and AUW.
I am at the stage of fitting the motor mount, I began to wonder about the "correct" thrust line. I have drawn a simple image to help explain it.

The prop centre line will sit higher than the wing and obv. behind it.

Which line should I follow? the blue one, or the yellow one, assuming the prop centre line sits where the point where the green fuse line meets the yellow line.

I'm guessing the yellow line, but would like confirmation and some assistance on exactly the reason. I have built one previously and noticed a big nose down under full power, and after moving the angle, I still noticed it was pushing down.

Can I get rid of the nose down tendency with motor alignment 100% given the prop sits higher than the the wing? I can't get it in line with the wing due to prop size needed, without making the fuse excessivly "high". I now have it about 2" above the top most section of the wing.

It is an undercambered wing. Does this also have an effect on this?

Also whilst I have your attention, how much decalage should I aim for ?
The original I built had the same as the plan, which was quite a lot.I found it did pitch up a lot at speed. The second one I built had a bit less,and did seem better in this regard, but I deviated from the plans a bit in other areas so I wasn't sure of the change.

I am used to slope planes that I generally set at 0*, but this slowflyer stuff is still new to me, esp with undercambered wings.

Any advice is greatly appreciated :D

Cheers
Paul
I'd go with the yellow line, based on my own experience trying to build a pusher using salvaged components. I have no formula to offer for thrust line, except that it relates to where the CG is when holding the model by her nose. I tested this initially on mine and noticed when held at the vertical CG, a nose down tendency, so angle was increased and now no pitching tendencies. Since your motor sits real high on the wing, I'm betting that the angle will need to be steeper than if sitting lower on the wing. One thing I'm finding quickly is that there is no replacement for actual testing to isolate a problem with models and those tests can have frustrating results.

The yardbird I'm trying to debug is Nasty! I think prop wash spiral slipstream is striking the vertical at an angle which creates lifting force on the vertical stab, i.e. turns right. One suggestion has been to replace centerlined vertical with 2 outboard stabs, so as to get the vertical stab out of the propwash. It is possible that with yours, that power-on turning tendency might be much tamer with the motor up real high. Again, no substitute for actual testing. prop slipstream effects can decrease with airspeed.

As for your decalage angle, it is what works for your tail moment, CG, and wing. Cant offer any formula here, but I know there is at least one.


Maybe an actual engineer will chime in.

Good Luck

[gouch]

Thanks electrostorch, some interesting info there. The twin verticals sound interesting, although I am using rudder, elevator so that would increase the build a bit. mmmm how do ailerons go on an undercambered wing?


The yellow line was drawn on purpose through the point where the plane balances on the wing (the Cof G). Now I don't know if I'm blowing hot air here (excuse the pun :p ) , but that theory was to have the thrust line pass through that point??? Am I dribbling or is there something in that?

I forgot to mention that when built as per plans with the "normal" power plant, (GWS IPS A) it is an extremely easy to fly plane, which is exactly what Dan had in mind when he designed it. I even had my missus fly a std one without problem (except for orientation) i don't want anyone to think it's a bad flyer, I have modified mine far from the original.

But me loading it up with a ton of weight, (but with only slightly more loading with bigger wing) and firing near on 100 watts into it has certainly changed it's characteristics, but I am enjoying modifying an existing design to learn from and hopefully end up with something that is totally suited to my needs.


globemaster:

Decalage should be explained by anyone else but me! :D
I fix motorcyles, I won't even attempt to be an expert on this whole flying stuff!

I understand what it is, I think another name is "incidence" but can offer no more knowledge than that. If I even try to explain it, it will make no sense! HELP OLLIE !!!!!!

Cheers
Paul

[Sparky Paul]

"decelage" when used properly refers to the rigging angles between the wings of a biplane.
It is commonly used to describe the difference between the wing zero-chord line and the horizontal zero-chord line, which is more properly termed "longitudinal dihedral".
The apparent upthrust of the rear of motors on high-mounted pushers alleviates the nose-down component of the thrust. As the c.g. of a plane is usually below the wing, pointing the motor so its thrust line passes thru that c.g. would result in a large angle on the motor mount, but there's other things that sum their forces around the c.g.. fuselage, tail, landing gear, so the actual motor thrust angle will usually not have to go thru the c.g., just near it.

[gouch]

Thanks for both explanations sparky, I have included ability to allow adjustment in motor angle so I can experiment.

[Salto]

Hi Paul,

In non accellerated flight, the distance from the thrust line to the C/G is not important at all. What matters is the distance from the thrust line to the vertical centre of drag of the model. It's the couple produced by the thrust at some height acting against the drag at some other height that will produce a pitching moment. BTW think of a couple as being two equal but opposite forces acting on a body and giving it a "twist" - in this case, a pitch up or down.

If your model has some dihedral, and the outer portions of the wing are therefore well above the motor, you may be able to keep the motor thrust line pretty close to the centre of drag.

If the model is accellerating or decellerating then the relative position of the thrust line and the vertical C/G will have some influence, but for the type of model you are designing the accellerations will be low, so this dynamic effect will be low.

And anyway, the dynamic effect will only matter when you are actually increasing or decreasing speed, and won't have any effect in steady flight. What matters in steady flight is the distance to the vertical centre of drag.

Also, a major effect with high (or low) mounted motors, or for that matter with high or low mounted stabs, is how the prop wash reacts with the stab. In steady flight this can influence the pitch trim considerably.

You asked about the undercambered wing. As drawn, this wing section will have a strong pitch down moment, which will increase with increasing airspeed. And, if you have little or no dihedral, the thrust from your high mounted motor will also act to pitch the model down, So you may have a problem.

But by having the horizontal stab below the prop centre, and angling the thrust line down to direct the prop wash onto the top of the stab, you should be able to create a nice pitch up moment, which will also increase with airspeed. If correctly "tuned" this could be made to cancel out the two pitch down moments mentioned previously.

So my advice is to use enough dihedral so that the motor is roughly half way up the vertical extent of the wing. This will reduce the first pitch down moment.

Then I'd leave some adjustment in thrust line so that you can aim the prop wash onto the horizontal stab to tune out the remaining pitch down moment caused by the high camber airfoil and the fuselage drag (which is all below the thrust line).

Hope this helps.

Graham.

[gouch]

Hi graham, Wow, some excellent info in there! And with your suggestions, I think I can nail it out. I will ensure I leave some adjustment in the motor mount so I can try different settings and see what differences in flight i can acheive.
How would be a good way to also create a stab setup that could be adjusted at the field? The wing slides into the fuse so adjusting it is not an option. I would like to experiment with the "longitudinal dihedral" (thanks sparky ;) ) as I still haven't seen what differences it makes to the characteristics of my planes, as they are all permanently mounted, and all my slope models have zero angles anyway, and they most certainly don't have undercambered wings! My slope and powered flying wings aren't much help either in this regard :rolleyes:

My biggest problem with fine tuning my planes is deciding whether it is an actual problem or just me, and then trying to decypher what it is the plane is doing, then finding out how to try and correct it!
Like "non-mechanics" trying to diagnose a problem with their car when they don't know how the car works in the first place :p
Except in this situation, I'm the "non aero guy" :D

Cheers
And thanks
Paul

P.S Do you have any 6mm depron in stock at the moment?

[Salto]

Paul,

One of the neatest ways to make the stab incidence adjustable is to use an all flying stab. Then you can adjust the servo centre or the linkage for different incidences without any messy cutting or packing on the fuse.

Some further information: With your proposed design using a fixed wing and an adjustable stab, the stab incidence will obviously determine the longtitudinal dihedral. The longtitudinal dihedral is always intimately connected with the C/G position and with the pitch trim and therefore flying speed. For a given C/G position, each different stab incidence will result in a different pitch trim speed. So it's not like you can vary stab incidence alone without changing your pitch trim speed.

A good way to look at all this is to adjust the C/G position to give the required pitch stability - rearwards for good response, forwards for stability. Then adjust the stab incidence to give the required pitch trim speed - more for slower, less for faster.

In practice, with your kind of plane, you test fly the model, see if you are happy with the C/G position (pitch stability), then adjust the elevator trim to give you the required speed at a variety of throttle settings. Now land, and look at the elevator position. If it's in lne with the stab then you've got the stab incidence right, if it's slightly "up" then you need less incidence, and vice-versa.

If you want a more stable set-up you must move the C/G forward AND re-adjust the longtitudinal dihedral to get back to your required pitch trim speed. Of course, if you've got an all flying stab then you won't need to cut of glue anything to adjust the stab incidence,

Graham.

P.S. Yes I've got stock of 2,3 and 6mm Depron in stock at present. Also got EPP blocks and sheet now. Strathfield indoor flying is on the 9th September.

[gouch]

Once again, great info Graham, and is easily understood with your explanations. I am in Sydney on the 9th and 10th, so I will see you there.

Cheers
Paul

[globemaster3c17]

Thanks for the definition, Sparky. I knew I've heard of it before, but couldn't remember what it was at all.

[HELModels]

Apparently Reynolds Number is not enough to distinguish man rated from models. alot of times set A will intersect set B, but even in the intersection, elements of set B remain elements of set B.

I'd read Model Aviation, monthly by AMA. It will change a modeler's perspective for the better.

To those down under,

That is good info and shows a thorough understanding of what modelers face. As usual when I see good info I will try to use it Where I can.
Good tuning makes the difference between a Lawn Dart and a flier.