Big Downthrust angle.


Working on an Old Timer again. Says in instructions to tilt engine down anywhere from 8 degrees to 16 degrees. I think the old Ign. motor noted was a bit underpowered for the A/C.

Is there a way to offset this considerable downthrust angle? Say with change of main wing incidence angle or some other way.


Wm.

leave it alone - they were likely just concrned about possibly making the thing loop - remember these planes were not ARFS - they took time and work to build and so flying had to be carefully done
many times a poorly trimmed plane would go into a series of dives n climbs and hopefully this would not happen as a "stalled loop" attempt may be that last maneuver seen- before the model crashed.
No on line forums either -
you took it slow n easy
I started free flight in 1950 and had no one to help me either so I made some real boo boos and these always resulted in lots of work going into the trash.
Trim flights were short and with power settings carefully "guessed at "

Wm, this was a free flight design, right? And you're going to use RC? If so, you can reduce the downthrust and compensate with elevator trim. The old free flight designs had to have a floating glide and also be stable under power without swooping up into a loop. The wing typically had more incidence than the stab, which gave longitudinal stability, but if you trim it to glide well, it will swoop up under power, hence the downthrust. Downthrust works by directing prop wash under the stab, which lowers the nose. With downthrust, you can have high or low power without trim change. Nice for RC, but essential for free flight.

Lots of people reduce the incidence of the wing, but that actually reduces the effective downthrust (just visualize the flying surfaces and engine without the fuselage), so don't change it.

What you will need to do is carry some down elevator when you fly. If you haven't built it yet, you can increase the stab incidence so it is closer to the incidence of the wing. That, of course, is like down elevator. In either case, the prop wash will strike the stab/elevator underneath which is what downthrust does. If you reduced the wing incidence, you wouldn't have to carry down trim under power, but you would have to add more up elevator in the glide mode. It works, but you lose the helpful effect of the downthrust, so you have more work to do with elevator trim while flying.

I'm afraid I'm making this sound more complicated than it really is - a couple of diagrams would make it easier. Does this make sense? Bottom line...being able to control the elevator makes the downthrust much less critical. There is no reason to change the wing incidence.

Jim

Jim....

Right now, am working on a Modelcraft Spook circa 1941 and original plan says Zero degrees of downthrust when using an O&R 60. Fellow from Washington who has built several of them through out the 1950's says that does not work unless you want a lot of loops once engine gets up to full power. So he adds in 8 degrees of downthrust, and adds power, and on ocasion goes up to 16 degrees downthrust to control things, if he has a good running engine. He is a bit too old to try anything else, but knows the big downthrust stopped things getting away from him.

I would like to go a bit more modern power, and maybe less downthrust too.


Wm.

That's such a cool looking plane. A guy in our club flies one occasionally - looks fantastic cruising along slowly overhead. Downthrust does work, but down trim on the elevator does too, if you have enough available. As I mentioned, I increase the incidence of the stab on old designs when I build.

Some planes have downthrust even when they don't look like it. My DeBolt Champ doesn't look like it has any, but if you look at the wing angle, and the elevator/stab (after putting in enough down trim to make it fly straight) and ignore the fuselage, you can see that it actually does have downthrust. Jim

Basically, in freeflights, if you have a forward CG, you will need downthrust. Whether this is from having to have more incidence in the wing, or what, I don't know. The free flight spiral climb is actually a loop with rotation. I went out one day with a free flight and a roll of solder. By moving the CG back, I was able to flatten out the climb until the aiplane flew level under power. Remember the very successful Ramrod with 10 degrees downthrust and more forward than most CG?

Yes, if you move the CG back toward the point of neutral stability you reduce difference in trim between high speed and low speed. That in turn reduces or eliminates the need for down thrust.

Jim

and if you get all the trim out of it the thing won't fly-
yet another lesson free flighters larnt long ago - Like I said
leave it alone.

A couple of the old designs had some really crazy amounts of downthrust specified. The gull winged one by Ontario Modelcraft is one but the name of the model elludes me at the moment. A flying buddy has built a few of them and they all needed a silly amount of downthrust.

Also the closer the engine to the balance point the bigger the angle has to be to achieve the desired effect. The ones that need this the most are also the ones with the power axis located well below the wing, which forms the lion's share of the overall drag of course.

Am not catching you here. This was a 1941 design when Free Flight was popular. The article was written circa 1999, and used a circa 1990's (large) radio. But then too this was first report of the need for huge downthrust. Up until then, it was up to the builder. From 1990 on to today the A/C has been flying bertter and better, but requires the large downthrust.

If I re-engine, and have some higher winding glow motor, will I still have to do the large downthrust as the fellow mentioned or not? It would be nice to get rid of much of it.

The model was yanked from the Modelcraft catalogue circa 1961 after it was really-really obsolete.

Wm.

Dick....

I want this model to R.O.G. not be a hand launched effort as per the original.

Another worry here too is that the large downthrust may tend to KEEP it on the pavement, instead of turning nose high and climbing.


Wm.

The further the thrust POINT from the wing - the more the moment arm for thrust becomes
this is the REAL problem - just like a old rc trainer with a very high wing location and a very low thrust line
sam ting.
This is a bult in loop setup - th so called excessive thrust simply was an attempt to circumvent this problem
The fast fix is to move the thrust line up -
I know -that spoils the old timer setup
You can't have your Kate and Edith too.

Dick:


I don't mind moving the thrust line up an amount. Basically it would help the design some by positioning the new glow motor closer on to the apparent centerline. This also increases the possibility of using a big propeller, but not by all that much.

If I go ahead and raise the glow engine some, will this eliminate some of the necessary downthrust, and will it enable the model to R.O.G. then?


Wm.

Even with the downthrust you will have no problem ROGing, but if the wheels are well forward keeping it straight could be problematical. Raising the motor thrustline might help, but a glow motor swings a much smaller prop than an old ignition engine, so prop size won't be an issue. As Dick suggests, part of the problem is the couple from a high drag centre and the excessive angular difference 'tween wing and tail. If you fit a rudder and elevator you change everything. You no longer have a vintage F/F but a sport R/C. If you want it to fly like a sport R/C then you can do a couple of things to help, reduce the wing tailplane difference to a degree or so, get the tailplane section as close to symmetric as you can, move the mainwheels back to just forward of the wing leading edge and the you can reduce the downthrust and move the CG to about 30% MAC. It'll still look like a vintage, but be much easier to fly in the normal manner. If you want it to fly like a vintage F/F then a moveable rudder trim tab is all you need, just to keep the thing in the same county, and fly it like a F/F.
Evan, WB #12.