downthrust on biplane pusher models

[dputt7]

Thanks George, much appreciated, just one question, what do you call downthrust on a pusher is it with the prop shaft pointing up at the rear. Thanks again
                 Dave

[g_kandylakis]

Hi Dave,

honestly? No idea.

I am not even sure how it will work, exactly, that is why I opted for both side adjustment, just in case.

Downthrust in "conventional" pull aircraft, blows the horizontal tail from the bottom, thus creating a lifting force and keeping the nose down... we know that.

In a pusher with the motor much higher than the C.G. we already have a rotational torgue caused by the distance between motor position and C.G., which we want to counteract, by blowing the horizontal tail on the top side. So, quite the opposite effect, and the angle.

So, I guess you could say, upthrust for a pusher.

Anyone more into aerodynamics, help please...

[dputt7]

Thanks George, I would like to hear from some experts as well

[OZPAF]

Not an expert but here is my 2c.

In a conventional front mounted propeller - tractor, aircraft down thrust is achieved as commonly seen, with the thrust line angled downwards. How far will depend on the trim and to a lesser extent on the centre of drag. This last factor is usually not an issue with most modern aircraft designs in a tractor configuration.

With a monoplane pusher - then in general to push the nose down - the thrust line will need to angle upwards, with the prop tilted backwards - passing above the CG, to provide the required nose down moment.

The situation becomes a bit more involved with a pusher biplane of similar design to the Bat.

First the extra wing weight, rigging , struts etc moves the CG up in a vertical direction.

Second - drag is much greater due to the biplane configuration and the centre of drag is also much higher than with a monoplane.

Third - the vertical position of the motor. A motor mounter between the wings may be close to acting through the centre of drag - thus possibly offsetting it. This is extremely helpful as the drag moment around the CG varies with the speed squared.

The position may also help by providing a built in nose down thrust if required(usually) under power. It may actually be more than required which would require some degree of upthrust - ie thrust line passing under the CG withe prop tilted forward

In short for any biplane layout - in particular with large stagger such as the Bat or the 1930's airliner - the vertical position of the motor is critical.

Hope this helps.

John

[dputt7]

   Thanks John, Nice to hear from you.  That all follows what I've read before, mostly, So I shall sit down and go through it bit by bit. Certainly more than 2 cent worth, just about a dollar I would say . Got some major repairs before I get to try it again.                          Thanks again
                                                       Dave

[Stunthenk]

Recently I built a Curtiss Wright Junior, using the Paul Matt three view enlarged to a span of approx. 20 inches. It is electrically powered using a motor and battery from a Chinese flying toy, coupled with an Atomic Workshop controller. The three view showed 4 degrees downthrust on the pusher engine, so that is what I used. I did make the mounting adjustable.

The motor is inserted in the dummy motor which is glued to a 1 mm plywood base plate. This is mounted on a beechwood post so that it can rotate for side thrust, the downthrust can  be altered by inserting  a spacer. The assembly is fixed by a woodscrew through a slot in the plywood base plate.

Battery an electronics are housed in the nose covered by a hatch on the underside.


The model is as yet unflown.... And detailing will be done after initial flights have proved the model - or not!

[ramses]

For my Savoia S.13 i first followed the plans for the downthrust, a few degrees down:



After the first test flights it was clear that was too much downthrust. I changed the downthrust to neutral:



That flew much better!



Cheers, Ramses

[dputt7]

  Sorry for my late reply.
             Stunthenk, loverly model, good luck with the test flights, I've built several models of a similar layout and they trim relatively easy but I think a biplane is a different story but thanks for your input.
Ramses, Thank you for your input and another loverly model. About 36 years ago   our group of modelers put on a Schnider Cup meeting at a local lake. It went for about 8 years and in that time my brother and I both managed to win the meeting I built a Savoia 51 and my brother a Savoia 13 built to his own design. Neither of us can remember what thrust angle our models had    Your testing will be a good place to start, the model has crashed twice in 2 attempts to fly it.  Itried to find a photo of the motor setup but I was unable'
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[THB]

Here's a pic of Dave and his brother Trev setting up for a bipe/pusher flight. I had a go at pushers a couple of times also - one was a Farman for the same Schneider events (KPO2 powered - only managed an extended ground effect / water effect glide) and a Vickers Gunbus rubber powered. From memory both were zero thrust lines. The fuselage is the wrong way round, the engine and prop is in the back seat, there's no room for a rear fuselage so they have to build a pergola out the back to hold the tail bits on...      I admire your your courage in taking these on! 

[dputt7]

   Wow Tim, that was along time ago. I had to scratch my head to remember it. A Hansa Brandenberg CC with a flexible wire leading from a pair of gears in the frount of the hull up to the motor.  It was no ceiling scraper but it did fly.
  Sorry George this is your thread on your model, maybe that's enouugh of a diversion.                                                       

[g_kandylakis]

I split the previous messages from the Sopwith Batboat thread, mainly because I think they deserve their own topic and should not get lost inside a long build thread.

So, continuing on this topic, back in 2019 Jiri Dolezel (CZ) came to the IIFI in the Netherlands with a beautiful Hansa Brandenburg W.20 for F4E electric.

Rather small in size, it put in some beautiful flights despite the very unusual configuration. I cannot find a video of them at the moment. Neither does Mike S have one in his report of that event.

https://www.ffscale.co.uk/page3aq.htm

A couple of months later I asked him about his setup and he sent me the attached pictures and the following message

"Hi George, if I have a fuselage axis and elevator in zero, the lower wing has +5°, upper wing +1,5°, motor  in zero, to side in zero, elevator +8° for slow flight with C.G. in 20% of upper wing"

So, zero downthrust, zero rightthrust, zero stabiliser, +8 degrees up elevator.

[dputt7]

   Well I think we are on to something now.  I recently watched a video of full size Sea Bee's taking off and landing and they had a tremendous amount of up elevator. so that with an amount of nose weight may be the answer.     Thanks for that George it was most helpful and a good Idea to remove the diversion from your thread.             
                                            Dave

[Squirrelnet]

Very interesting thread. I did a Telco powered Walrus in foam and used upthrust (ie downthrust on a pusher. It was foam wall foam and sadly didn't last more than one indoor Trinity. It did fly, I got it to fly on power but the glide was non existent I think due to the thrust line being wrong. It was set to what looks like close to 0 with a hint of up but felt like it needed some downthrust on the engine (ie more upthrust, due to the high mounted engine ) . I would like to try a stronger built version at some point, I even have a tissue skin to go over the foam but the thrust line to use is tricky. I have the Stan Cole CO2 plan as was thinking of copying that but just not sure

There's a video of it on it's one successful flight, about 20's into the video  Thanks 'iccjaw' for the catch :-) It looks great but as the power runs down it straightens  out , so not enough side thrust and more worryingly on other flights it goes into a parabolic dive without power. I thought this was because the glide was way out and trimmed it under power to fly... ok wait.. so maybe I need more downthrust if the glide is correctly trimmed. You can see why I haven't yet built another one

[Jiri Dolezel]

So, continuing on this topic, back in 2019 Jiri Dolezel (CZ) came to the IIFI in the Netherlands with a beautiful Hansa Brandenburg W.20 for F4E electric.

There is a W.20 in flight in 1:30 min what I found from Czech indoor session in Hluk, 2021.

https://m.youtube.com/watch?v=kpvccCXKLx4

[Squirrelnet]

Superb Jiri