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Need for Speed

Zazoos

Well Known Member
Did a speed test today and want some advice.
Full throttle and leaned. Max rpm was right at 2600.
Sterba Prop
Lycoming 0320 160 hp
Wheel pants and fairings installed.

If I had a prop that allowed the engine to hit 2700 rpm how much difference in speed should I expect.
Screen shots below:

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d

Thank you,
Trent
 
Different props (different pitch) will give slightly different results. I see about a 7 knot/100 rpm change at or around cruise settings ~ 2450 rpm/157 knots. More at lower airspeeds (and RPM), less at higher airspeed/rpm.
 
150Ktas at 9GPH and 7K' seems a bit slow. My 6A (320 with Catto FP) does 160 Ktas on 8.1 GPH at 8K' (maybe 8.2 or .3 at 7K) and I am sure far from the fastest or most efficient. A key difference is that I have FI with balanced injectors and EI, both of which drive close to 1 GPH efficiency improvement when LOP.

I would start by verifying your TAS speed with the NPTS GPS box. Mine was off by 7 knots at cruise and I don't think it is all that uncommon on the 6. If the TAS is accurate, there are several things you can probably do to improve.

I am not sure how much more speed you would get with another 100 RPM. Maybe 8-10HP, which is about 3 knots. I think that a more advanced prop would bear more fruit than re-pitching the Sterba. I have never heard anyone call the Sterba's a fast prop. I like my Catto. The difference you see between 2500 and 2600 on your prop is different than the difference you would see from re-pitching to reach 2700. By going to a finer pitch, you will be pulling less MAP (i.e. less load on the engine) at that higher RPM, so the scale slides a bit.

I like under pitched props, as I can still get good speeds at altitude. My prop will easily turn 2800+ at 8K if ROP. However, I can still get 2690 at 12K at peak. Given that I always cruise LOP, I routinely turn 2720-30 at 8K. They also improve climbs rate. They are a negative if you like to run full power ROP at cruise.

Larry
 
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The three legs in the OP's three posted EFIS pics works out to 151.7 KTAS, fairly close to the screens' reported TAS.
 

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Beside prop any other suggestions to increase speed?


Thank you,

Drag reduction. Are your gear fairings "properly" aligned? I see your slip/skid ball is consistently out... That's an out of trim condition and costs you speed.
 
Drag reduction. Are your gear fairings "properly" aligned? I see your slip/skid ball is consistently out...


+1

That is where I would start as it is the most likely to be off and it comes with a pretty big drag penalty. Also confirm the rigging of the ailerons and flaps. Seems to be another area where some builders don't spend as much time/attention as necessary. A rudder trim tab will keep the plane from slipping / skidding and provides some reduction.

Larry
 
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I note that you are from Texas. The headquarters for SARL, sport aircraft racing league, is based at Taylor tx, T74. Check out their website, the participants are welcoming and will share ideas and tricks of the trade. Over the years that I participated I increased my top speed by a significant amount. Much of that was through better flight techniques, but cleaning the airframe Added many knots. These extra knots are with each and every flight
 
If I had a prop that allowed the engine to hit 2700 rpm how much difference in speed should I expect. Thank you, Trent

About 2 knots.

I see about a 7 knot/100 rpm change at or around cruise settings ~ 2450 rpm/157 knots.

Kyle, I assume you are using the throttle to vary RPM. If so, you're changing torque as well as RPM, meaning a significant change in power.

That's not the same as Trent's proposal, in which the sole change is RPM. As a practical matter, we might consider his torque to be constant; the full throttle Lycoming torque curve is quite flat in that region.

A crude approach might be to assume Trent's 320 can make full rated power, 160HP, at 2700. If so, the usual HP equation says torque is 311 lbf-ft. If we then recalculate using 311 and 2600 RPM, current power is 154. All else equal, the RPM change alone is worth six HP.

So what will he get for six additional HP? Power required is proportional to velocity cubed, so..

New speed = 150*(160/154)^0.333333 = roughly 2 knots

Remember, this assumes the exact same prop, repitched for 100 more RPM. Changing to a more efficient prop might net better results, just because better prop efficiency means more of the available HP is applied to thrust.

BREAK

I wrote the above about 5:30 this AM. Then (given a second cup of coffee) it dawned on me I was up early to fly to TN and see a friend, and I have both props. Well, not quite, but same thing, a constant speed. Hmm...time for a test!

See photos below.

2600 and 2700 RPM, full throttle, mixture adjusted for essentially the same EGT. No other changes. We'll use the indicated power percentage:

210*0.68 = 142.8 HP @ 2600
210*0.71 = 149.1 HP @ 2700

New speed = Previous speed * (new power / old power)^0.3333333

so

New speed = 191 * (149.1 / 142.8)^0.3333333

New speed = 193.7.

I guess 194 indicated is close enough.
 

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About 2 knots.

Kyle, I assume you are using the throttle to vary RPM. If so, you're changing torque as well as RPM, meaning a significant change in power.

No doubt. But isn't that the OP's situation - his engine is RPM limited and another 100 RPM would give him more power and more speed? After all, his airplane has a FP prop, just like mine...

You picked a great day to go flying. It was gorgeous here this morning.
 
Using Dan's very reasonable assumption of a flat torque curve at that rpm, tweaking the prop to get 2700 rpm will increase the power by 2700/2600 or 3.85%. Assuming power required at hi speed with negligible induced drag reduction, power required is cubed. So, for small increments, 3.85/3=1.3%. 1.3% added to 150kt = 151.95kt. ~2kt increase. Dan = 100%

Less math, more filling.

On the practical side, best to find some drag reduction items 1st, then the same prop might even exceed 2700. Cooling drag is a biggy hidden item in many cases. Other tweaks that have been mentioned as well will likely get you there. Then you will have the whole thing, 150kt * 2700/2600 or ~156kt.
 
No doubt. But isn't that the OP's situation - his engine is RPM limited and another 100 RPM would give him more power and more speed? After all, his airplane has a FP prop, just like mine...

Nope. He would have the same torque at both 2600 and 2700. You are reducing MP to shift 100 RPM, i.e. changing both torque and RPM

Less math, more filling.

Indeed!
 
Accept that no matter how fast you’re going, you always want to go a little faster. Life does not revolve around a few MPH +/- 😉
 
you always want to go a little faster

Doesn’t apply to all I guess...

The part of that hobby I like is the flying. Now why would I fly WOT from A to B or A to A, thereby shortening the amount of pleasure?
Why for instance fly for only 50 minutes at hi FF and noise levels, if on the other hand I can make that same flight in quietness at say 19/2000 using Carson’s efficiency speed in maybe 10 or 15 minutes more, with an increased range and lower engine temps?

Beats me, and quite a few other RVers around here as well, just saying...
 
I guess some people just LOVE to fly in a 50 MPH headwind then... Because it increases the pleasure of flying.

So are some people actually participating in a thread dedicated to "speed", and trying to convince others it's NOT important? Doesn't that seem just a little counterproductive? That's like going on the Corvette forum and trying to sell the virtues of a Prius.

Some people want to go fast - I'm one of them. The Rocket is not nearly fast enough, but I can't afford to keep gas in my L-39. The Rocket will have to do, and every knot that I find hiding in rigging, cooling, poor gaps, or extra power makes me happier. That's why I'm in this hobby. If I want to just fly slow, I have my Taylorcraft.
 
BTW, if Trent re-props for 2700 RPM in level flight, he will pick up about 100 fpm additional climb rate.
 
Michael, surely not trying to convince anybody. I'll make that more intrusion and then let you play amongst yourselves ;)

If I had the need for speed, I probably would not own an RV, nor a Rocket for that matter. Well, not the kind of Rocket you are talking about. Even more so when living in the US where long runways are plenty.
There are quite a few homebuilts that were designed and are built for that ever so elusive speed. The RVs are not. What the RVs were designed for is versatility, everything, as always in aviation being a compromise.

For me (and other non-speed addicts), efficiency and the pure joy of flying is the name of the game.
 
Yeah, and some guys like fat girls...

Thanks, Dan. You made my morning. I really did laugh out loud at this comment as it was exactly what I was thinking.

On the topic of "need for speed," I recently purchased a 1980's vintage Glasair 1RG. It doesn't have the fantastic all-around flying qualities of my RV-3, but it sure is fast and efficient. My -3 is faster than it should be for various reasons, but the Glasair is still 20 knots faster with the same engine and an old fashioned constant speed prop.

The speed is addictive. I find myself flying the Glasair just because I want the extra 20 knots. I didn't think I'd care, but I do. It really reinforces my longterm plan to build an F1 Rocket!
 
Speed and efficiency are the same thing - energy preservation.

If you build an airplane for max speed, when pulled back to "normal", its just that much more efficient.
 
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From the website...
"“Total Performance”
In the early days, Van coined the term “Total Performance” to describe his design goals: highly-capable on both ends of the performance spectrum as well as in-between. In more than forty years, we haven’t come up with a better description. While the RVs are excellent cross-country airplanes, they are not simply “go-fast” machines. They have outstanding low-speed characteristics and short-field capabilities; a rare combination. Most are capable of delightful sport aerobatics."

High speed is one end of the spectrum. Yes anything that flies is a compromise. I was attracted to RVs because it was the best compromise I could find between speed, handling, cost, comfort, and supportability. Probably most of got here with similar thoughts. I'd always like more capability, what's wrong with a little more speed. Same can be said on the other end, I'd like to be able to land even slower. However, I wouldn't give up the top end for the bottom and vice versa. With the RV you can go faster without affecting to much at the bottom end (except adding heavier motors); but not sure how much slower I can go without affecting top end.
 
I bought my RV for a few reasons, speed wasn't the top reason, it was cause the machine can do a LOT quite well just not perfectly!
Bang for buck you can't beat an RV:)
 
Speed Vs Efficiency

Speed and efficiency are the same thing - energy preservation.

If you build an airplane for max speed, when pulled back to "normal", its just that much more efficient.

That’s often the case but not always. For example, a fixed pitch prop optimized for max full power speed probably won’t yield the best cruise efficiency....

Skylor
 
That’s often the case but not always. For example, a fixed pitch prop optimized for max full power speed probably won’t yield the best cruise efficiency....

Skylor

Same thing for CS prop.

The optimal twist of the blade would be different for cruise vs high speed.
 
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