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Contrasting Opinions: Throttle control in Loops

tom paul

Active Member
Hi All.
I have begun exploring the light aerobatic capabilities of my RV7A and of myself as a pilot. I have been instructed in loops and rolls by two different experienced RV pilots/Builders/Owners.
One of these teachers advised that I leave the throttle wide open during the loop, since I have a CS prop. The other advised the I cut the throttle to idle on the way down the back to avoid over-speeding the aircraft.

Both seem to work. In my perusing on youtube, this forum and anywhere else I have looked, I have seen very experienced pilots espousing both techniques as if they were no-brainers.

This is confusing. I'd like to open the discussion of this to help me do the "right" thing, as I begin to commit this maneuver to muscle memory.

Yes, I have done supervised stalls and spins, and I practice my aero at 5k AGL or above.
Thanks for reading. I'm looking forward to the debate.
 
I would do what is necessary to have the aircraft come out of the bottom of the loop at the same altitude and airspeed as you entered the maneuver. In my aircraft I normally start at 160 knots and setting 24”/2500 RPM and not touching it works fine. Every aircraft will be different.
 
If I am just doing plane old “lets have fun” loops (not competition stuff) in my RV’s with C/S props, I usually loop from normal cruise speed/power settings, so I don’t touch a thing on the power quadrant, and I end up at exactly the same conditions as I started with - altitude and speed. So leaving power alone works for me.

Paul
 
since I have a CS prop. The other advised the I cut the throttle to idle on the way down the back to avoid over-speeding the aircraft

and


well, all basic stuff, keep doing the second one... or take some aerobatic tuition...

Where is that stash of fresh popcorn gone?
 
I would do what is necessary to have the aircraft come out of the bottom of the loop at the same altitude and airspeed as you entered the maneuver. In my aircraft I normally start at 160 knots and setting 24”/2500 RPM and not touching it works fine. Every aircraft will be different.

I have extensive aerobatic experience in RVs and non-RVs, in competition and not in competition.

Do what Salvi767 and Pauls Dye say. Do whatever makes you feel comfortable. A typical RV at cruise power and a 3.5-4 g initial (and finishing) pull will produce a loop that exits at approximately the same entry airspeed and altitude without touching the power. The amount of G you pull will control your airspeed.

There is no right or wrong. Safe and enjoyable should be your primary goal. If you can nail the entry altitude at exit at approximately the same entry speed and hit your own wake even better. Bonus points if you can make a perfectly round loop in front of a panel of 5 IAC judges in wildly varying wind conditions during competition!
 
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You don't have a fixed pitch prop, but on the back side of a loop you might need to retard the throttle so as not to overspeed the engine. That may be where some folks were coming from.

It's been too long since I had the RV-4 to remember any specifics...

Ed
 
There are lots of threads on this already but even with a fixed pitch prop you don’t overspeed anything as long as you are not finishing lower then you started.

Oliver
 
I am in agreement with Paul dye on this.

I would strongly recommend if your exploring acrobatics for the first time get a couple hours on basic acrobatics with an good acro pilot instructor.

I went to St.Augustine,fl. A long time ago 1980’s and flew with them Al Moser who was a great acro pilot. There are many great acro schools.
Enjoy the acro even if you never do competitions. I have had a couple refreshers since.

Jack
 
Sorry but those who advocate pulling power to idle on the way down never had decent training and never learned good aerobatic fundamentals and I'd recommend passing on using them as a teacher. It's terrible technique, wastes tons of energy, produces a horribly ugly loop, and is totally unnecessary regardless of fixed or CS prop. You will not overspeed the engine or airframe anywhere within the ballpark of proper loop technique. RVs are subject to the same laws of conservation of energy as any other airplane. No need to change power setting.
 
From https://www.aopa.org/news-and-media/all-news/2013/may/01/technique-learning-to-loop

"A constant-speed prop is a real luxury in a loop as you can go to high power and rpm and simply “set it and forget it.” Fixed-pitch props usually require full power in the initial climb and throttling back during the descent to avoid exceeding rpm redlines."

The best source of aerobatic information is in person with a qualified aerobatic instructor. The above is just a (mostly) passable article from an AOPA editor who's not necessarily an aerobatic expert.

First, the OP asked about pulling power to IDLE on the way down. Nothing in this AOPA article advocates for that. The descriptions in this article don't apply to all airplanes. If you're flying loops in Cubs and Stearmans, yeah you'll for sure want to add FULL power on the way up and it's possible you may want to pull a little power off (NOT IDLE) on the way down to exit at your starting altitude, RPM, and airspeed...assuming you did not already enter at full power. RVs (even with FP props) easily loop from low cruise configuration without touching the power setting.

And it's totally wrong to say 3.5G's is the sweet spot for a loop. I've never seen an airplane do a nice actual round loop from 3.5G. Takes 4-4.5G in most airplanes, including RVs. You are for sure not scrubbing too much energy by pulling more than 3.5G. But most pilots fly severe oval shaped loops and aren't being judged so sure, 3.5G works fine, but don't hang on the words of an AOPA editor.
 
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I agree. Best source is a qualified instructor. Now unfortunately there are lots of acro instructors around I wouldn't consider qualified and stories I have heard go from bizarre to outright dangerous.

Contact your local IAC chapter and they can refer you to somebody who knows what s/he is doing. It's a small community and people know each other.

Alternatively check if your instructor ever competed. Not sure if you have to be an IAC member to get to that link but you can find everybody who ever competed here:

https://iaccdb.iac.org/pilots

Dave Hirschman is nowhere to be found.

If they haven't and they are not flying airshows I would be very careful. E.g. there is no way of knowing if you fly a round loop if you never received ground coaching and most people that did receive ground coaching did compete at least at one point in their flying career.

So if they haven't competed and are not flying airshows ask them if anybody ever observed them from the ground? Who? When? If nobody ever did run ... .

I know some of the more well known guys ask for top $$ but in my experience they are worth every cent of it compared to the local instructor who does some acro on the side ... .

Oliver
p.s. The best book on the topic I ever found is: Better Aerobatics from Alan Cassidy. He did compete and explains the physics well... :)
p.p.s. I can confirm that in particular in an RV-8 with a fixed pitch prop you will NOT overspeed the airplane or the engine if you fly a loop that doesn't end lower then it started. I have tried that literally hundreds of times sometimes round sometimes not. I can see a benefit for a constant speed prop in other maneuvers but a loop is not one of them.
 
Has anyone played with how slowly you can fly a loop a loop in an RV without stalling-out?

Assuming your spin recovery technique is competent: Try it! It's great fun, and it's pretty eye-opening to discover where the limit is. Was for me, anyway.

Don't worry about keeping it round. Just treat it as a 360º-pitched slow-flight exercise, feeling for the stall the whole way 'round.

If you're entering above 100 knots you aren't trying hard enough :)

- mark
 
I've looped as low as 120 kts but prefer 140 kts in my 8. 22"/2300, 3-3.5 gee's, I don't touch the gas pedal -).
 
Never stop moving it.

OR, if you really want to have fun and never STOP moving your throttle, do formation loops! Then it becomes about inside or outside the circle.
 
Has anyone played with how slowly you can fly a loop a loop in an RV without stalling-out?

Assuming your spin recovery technique is competent: Try it! It's great fun, and it's pretty eye-opening to discover where the limit is. Was for me, anyway.

Don't worry about keeping it round. Just treat it as a 360º-pitched slow-flight exercise, feeling for the stall the whole way 'round.

If you're entering above 100 knots you aren't trying hard enough :)

- mark

Yes I did. It is lots of fun reducing 10mph every time till you stall out. Just make sure you don't tail slide when you do so I wouldn't recommend that to somebody who still worries about if he should remove throttle or not... .

Oliver
 
I figured I add a little bit of actual data to the discussion. So I went out this am with my fixed pitch (Cato prop, IO-360) RV-8 and did some loops so I can share the data.

First I did a straight forward loop at about 4g.

r.jpg


Savy only shows the g load in 1 second interval so it looks less smooth than it actually is. This was a roughly round loop (not perfect you can actually see that in the trace, out of practice a bit...) starting at 131 knots ending at 124 knots with 2500 max rpm all around. I tried the same from 3 to 5 g with different entry speeds all the way up to 170 knots with the same result (did about 30 loops this morning). Each time speed is a bit lower at the end than it was at the beginning. RPM never goes above entry RPM (I am red line RPM on entry at 170 knots).


I also wanted to demonstrate what happens if you exit low. So here a trace on that:

l.jpg


I entered at 143 knots 2600 rpm and exited 400 feet low. That got me up to 167 knots and 2880 rpm.

So consistent with what was claimed in prior posts data shows that you can't over RPM my RV-8 with a fixed pitch prop IF you exit the loop at the same altitude or higher.

If you don't all bets are off. So maybe some instructors which also rent out their aircraft recommend pulling power because they don't trust their students (and therefore their own instruction capability) to finish above the starting altitude.... . If you routinely do that of course you have to pull power ... .

Oliver
 
Great post. ^^. There are only three situations where you should ever go to idle power while doing acro - spins, tailslides, and loss of control stall/spin/severely blown maneuver recovery. Tailslides definitely not recommended in RVs.
 
Wow. Great stuff Thanks everyone!
I went out to practice this morning and tried a couple of loops while not touching the throttle. I was set for 24/24 cruise. I did brush up on Alan Cassidy's chapter on loops before I went. The first one was pretty great. 3G max on the meter afterwords, floated over the top with what felt like .5 G, finished on the same heading, altitude and airspeed as I started, and felt the bump of my own wake as I came to the bottom.
On the second one, I think I relaxed the pull too early as I approached the top because I was floating in what felt like zero G for a bit, but it ended up in a good place afterall. At any rate, I was more comfortable keeping the power on all the way around.
I intended to do a bunch this morning, but I was spooked by a drop in my oil pressure. I have a 1200 hour IO360 which has typically gotten down to 58 lbs on the oil pressure during slow flight, but after a couple of loops, including this 0 G one, it was at 50, well below the green, so I ran for home. Five minutes later it had climbed back up to the very bottom of the green arc. I may start another thread about this. Is it normal for the oil pressure to drop like this during acro? I run 6 quarts normally. I have tried to keep it at 7, but when I do, it seems to spit it out and stay at 6.
Thanks again, everyone. I will be consulting my local IAC to get some first-hand training in the light aerobatics that I am interested in.
 
Oil pressure will drop going over the top if you are going 0g unless you have an inverted oil system. If you don't have an inverted oil system you will also be making a lot of work for yourself cleaning oil off the belly of your plane.
 
Oil pressure will drop going over the top if you are going 0g unless you have an inverted oil system. If you don't have an inverted oil system you will also be making a lot of work for yourself cleaning oil off the belly of your plane.

Is that dangerous? I wonder why it takes so long for it to come back up into the green after back to normal flight...
Makes me really nervous to see the OP in the yellow. I only have one engine and I fly over congested NJ to back home to KLDJ.
 
Is it normal for the oil pressure to drop like this during acro?

Depending on what you do yes. You get about 4-5 seconds inverted before it becomes an issue with my engine (I have a half Raven so my belly is reasonably clean). I am surprised though that it took that long for pressure to come back. For me it's pretty much instantaneous.

Below my oil pressure over a couple of loops.

Oliver

op.jpg
 
Is that dangerous? I wonder why it takes so long for it to come back up into the green after back to normal flight...
Makes me really nervous to see the OP in the yellow. I only have one engine and I fly over congested NJ to back home to KLDJ.

As said in my other post I am surprised too it didn't come back quick. What type of oil pressure gage do you have? Is it electronic and has a log you can look at off line? It might be an instrument issue. It shouldn't take 5 minutes if your engine is under load.

Even with a fully inverted oil system you have short moments of lower oil pressure (at 0 g neither the upright nor inverted configuration works) so short burst are normal and are not known to harm engines.

Oliver

p.s. Say high to the folks in NJ. Was flying out of Sky Manor for many years... .
 
What type of oil pressure gage do you have? Is it electronic and has a log you can look at off line? It might be an instrument issue. It shouldn't take 5 minutes if your engine is under load.

I have mechanical "steam" gauges. no EFIS. Perhaps there is an issue with the sender or the gauge. I am not well-versed in this setup. I am not the builder and I have not owned the plane for very long.
 
Oil Pressure

You probably do not have a purely mechanical gage. If you have an electrical sender check the ground. Clean the area where the ground connects to the sender. If that doesn't help replace the sender.
You may want to test with a purely mechanical automotive gage where the oil line connects directly to the gage.
When you have established a base line pressure reading adjust it to at least the middle of the green arc, higher is better. This should reduce the length of time at low oil pressure at zero or negative G and shorten the recovery time to normal pressure.
In your situation it is best to keep some positive G on top of the loop. Your engine will be much happier if you do this.
 
Oil pressure will drop going over the top if you are going 0g unless you have an inverted oil system. If you don't have an inverted oil system you will also be making a lot of work for yourself cleaning oil off the belly of your plane.

You shouldn't need to unload to 0g though. Even a small fraction of positive g keeps the oil system running, just need to make sure the oil pickup in the sump isn't unported.

It is astonishing to discover how slow and how gentle you can do a loop. Be patient over the top, the nose is going to come down eventually even if you need to hang at 0.2 or 0.3g for longer than you expect :)

- mark
 
... just need to make sure the oil pickup in the sump isn't unported.

- mark

How do I do that? I think it will just take some practice to float over the top while maintaining a positive g loading.
I'll have to research the type of oil pickup I have. I like the idea of putting in a mechanical sensor at least to calibrate this. A Lycoming tech suggested that I might adjust the oil pressure regulator at well. I haven't gotten into that yet. Adding oil was another idea, but my engine spits out anything over 6 on the first flight. 6 seems to be its happy place.
 
Below my oil pressure over a couple of loops.
op.jpg

FYI - Your images aren't showing up, for me at least. Chrome under Windows and Linux, same result. This is likely because your site is only http, not https. Browsers are now blocking embedded includes from non-https sites.
 
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