Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

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Old 04-20-2017, 06:13 PM
  #1
vermontbadboy
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Angry engine thust

I have a question. Many years ago I made a model Piper TriPacer from a Sterling kit, about 56" span. Maybe I over powered it ,but the first time I got it to fly,it went into a steep climb ,got up to maybe 100 feet ,stalled ,came down and crashed. I don't think that I held it in the climb mode,maybe I just didn't know how to handle a model[I had flown full scale],so maybe it was just my mistake,. But I have been thinking of getting the model in shape again[I rebuilt it] And see if it will fly now that I have more experience in flying models. My question is would engine down thrust have anything to do with why it went into a steep climb,I other words should it have more down thrust than it had?
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Old 04-20-2017, 07:09 PM
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All Day Dan
 
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I built this 40cc Tripacer with everything set at 0-0-0. The CG is set at 30% of the chord. No trim was necessary. Dan.
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Old 04-21-2017, 10:00 AM
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Welcome to RCU BadBoy. There are much more knowledgeable experts on these forums than I, but I believe there are many things that could have caused your terminal climb. A general inspection by a trusted flyer would help. AD Dan hits on some of the issues, wing/stab incidence and CG. Certainly, a far rearward CG would do it.

Good luck and keep us posted.

Bedford
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Old 04-22-2017, 05:07 AM
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ibuild
 
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I would say so too, that there is many things that individually or perhaps a combination of many things that could cause such behavior, but in my experience - in most cases - a center of gravity that is mis-calculated or perhaps even more often simply not properly checked and being too far aft for the design or configuration would be the cause of an seemingly in-controllable plane and end in a crash.
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Old 04-23-2017, 12:13 PM
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When the model first lifted off it got enough speed to raise the nose that high but apparently didn't have enough power to maintain speed at that high a climb angle.. And being fairly new to models you didn't recognize that and it stalled and then it was too late.

There are full size examples of this you could easily find I'm sure. New pilot allows the plane to assume too high a climb angle and doesn't watch the airspeed until too late. Stalls and doesn't have enough room to re-establish flying speed and still have the room to pull out.

The idea of checking the trim as mentioned is important. As a full size pilot you know how important it is to consider your loadout. Same for models. So do check that balance point.

The other thing is that after liftoff you should be ready to control the climb as required with some slight elevator stick pressure. Don't let the model assume a high angle of climb on your first few flights. Hold the nose down to a moderate climb and keep the airspeed up while climbing up to the safe altitude. THEN start playing with elevator trim and learning how the model behaves.

If it shows a strong trim change between high power and low power then you can do a few things. My own personal choice is to sneak clost to Dan's 0-0-0 and set the CG to get a nice level flying trim at something short of the full on 0-0-0 settings for a model of something like the Tripacer. My hot zip around aerobatic models I'll go for 0-0-0 and set the CG so they fly like an arrow at any attitude and power setting. But for sailplanes, old timers and if I built scale models I'd go for more like 0, +1 to +1.5, 0. That 1 to 1.5 on the wing gives me a little positive stability pitch up from speed changes and at the same time puts the thrust line at -1.5 to the wing. And it's the wing that is the key player. So from one wing angle I get both downthrust and stabilizer decalage.

Also do not assume that the elevator will end up level with the stabilizer when the model is correctly trimmed to your tastes. The angle between wing and stabilizer was set by one person's opinion. And is likely a compromise based on the presumed buying audience. It's OK if the elevator ends up a few degrees up or down from the stabilizer.

In your case from reading how it flew SOME added downthrust would not be a bad idea. But I'd still want to fly it and just use some forward stick to hold the climb in check. It'll only take a whisper so don't jam a lot in or you'll dork it into the deck.
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