How much do you pay for weight savings? To go faster?

[Space Cadet]

Curious if anyone else has a figure of merit in $ per pound of weight saved or $ per MPH gained they use for deciding when it's worth an upgrade to save weight?

I personally think if it costs under $100 per pound of weight saved or per mph gained it's a no-brainer, between $100-250 it takes some thought, and above that it's not likely to make the cut.

Thought about putting a poll out but first want to see if there's any interest...

Other thoughts/opinions?
Dwight

 

[klovning]

gut-be-gone

I bought some running shoes for 60$ and saved 15 pounds of pilot weight.

- most cost effective weight reduction I can find on the (future) plane.

 

[Ted RV8]

MPH gain??

Don't think weight savings will change your top speed mph. It will change take off roll and rate of climb.

Agree with Jason, best weight savings you can easily and cheaply do is your weight!

Ted

 

[Space Cadet]

Clarification

What I meant to ask was two separate questions, one for weight and one for speed. I understand the relationship between weight and speed is small. But for those building heavier aircraft and/or plan to fly close to gross, weight is a factor. At least it is for my mission.

And for those of use with worn running shoes and light-weight frames, the pilot (and co-pilot ) weight reduction options are not practical!

Dwight

 

[Bob Axsom]

Depends on your stage in building

Depends on your stage in building. If things were available then that are available now I would have paid whatever it took for the highest speed option - thousands of dollars just like I did then. Once the money is spent for an option I simply cannot replace it because something faster becomes available (like a Hartzell blended airfoil prop with 7496 blades for $11,000). I have spent time and money since the first flight in March of 2004 trying to make the airplane (RV-6A) faster and there is a lot of potential for speed gain in that type of work. Most of my changes require a lot of work but the raw material cost is usually less than a $300. The gains so far in five years over the baseline speed established by consistent testing has been 12 knots. I doubt that I have spent $1,000 on raw materials or around $83 per knot. Would I spend $300 for a 1 knot gain - well nothing is certain and I have spent time and money on modifications that lost speed - I have to say I probably would if if was a gradual expense. At that level of accepted risk I would have grudgingly spent $3,600 for the 12 knots. If I bought the 7496 bladed Hartzell I would almost certainly gain an immediate 3 knots but at $3,667 per knot I have to wait for a normal prop replacement to justify that kind of outlay.

Bob Axsom

 

[f1rocket]

I think the answer kind of depends on your mission. If you are building a RV-10, I'm guessing you aren't looking to go racing (although if you did, that would be okay and much fun). So if your motivation is to do it for fun and education, then you have to decide if the money detracts any from the fun. As Bob pointed out, if you pick "fast" components from the start, you will gain speed with less investment. Only you can decide if it is worth it.

I can provide one data point for you to consider. A couple of years ago, about 10 of us flew down to Sun-N-Fun with a stop-over in Cedar Key. We were all flying together. After our final fuel stop, we were about 200 nm away from Cedar Key. After departure, we all split up to find our own way. Now, in the 10 airplanes, we had a great mix of different RV models, engines, props, weights, etc. We all flew at different altitudes as well. The end result was this. The group who had the larger engines and CS props landed first. They were just calling their turn to base as I entered the pattern. Just as I landed, the last group was entering the pattern.

I know it's not scientific, but my point is that to spend a lot of money to arrive at your destination 5 minutes early is probably not worth it IF you are doing it for traveling purposes.

 

[David-aviator]

A light weight airplane handles better and can be more efficient in terms of fuel used to go somewhere, but how that is quantified in dollars and cents is difficult - as is all money spent on what we do here. Everyone has their own wishes to fulfill and they can be quite different.

I spent a ton of money getting rid of about 180 pounds with an engine switch and am beginning to really like it, the airplane gets off the ground quick even with a FP prop and climbs very nicely at somewhere between 1500-2000 fpm at 110 KIAS, lands slower and generally seems more nimble in flight. This is all stuff I like.

The next pilot may prefer spending that much money on a glass panel with big time EFIS displays, an auto pilot that does everything but talk in 5 languages and piped in favorite music.

We all dance to the same tune with different motions and when you start writing down the cost numbers, none of it makes any sense to anyone on the outside - but we know it's worth it.

 

[Alan Carroll]

Prop differences small?

If I bought the 7496 bladed Hartzell I would almost certainly gain an immediate 3 knots
Bob Axsom

Bob,

Just to stir the pot I'd like to question this statement. I too have struggled to justify a new prop, but have found it hard to do based on speed alone.

My impression is that the real world speed differences between the 7666 Hartzell, 7496 Hartzell, and WW 200RV are pretty small. As evidence I point to Randy Lervold's tests, which found the 200RV to be only 1-2 mph faster than the 7666, and Larry Vetterman's recent test, which found the 7496 and the 200RV to be identical.

These tests weren't done at race speeds, but in two races my RV-8 with 7666 blades was essentially identical in speed to RV-8s with blended airfoil props. In the 2007 AVC I was 10 seconds behind behind Jon Ross, who has a 7496 I believe (2008 I was farther behind, but we flew at different altitudes). In this year's Taylor 100 I was 8 seconds ahead of an RV-8 with a blended airfoil (not sure which model) and an IO-360. I took off behind him and had him in sight the whole race, so I know we flew the same course. My plane has a stock 8.5 compression O-360 with a carb , with a Lightspeed EI in place of one mag (I doubt the EI makes much difference down low). There's anything special about the plane other than careful detailing, and I can't claim any special piloting skills.

On the flip side, Vans reported the 7496 is 3-4 mph faster than the 7666, and your inside info from Hartzell seems to support this conclusion. John Huft has used 4 different props, and says the 7496 is fastest (I don't think he's tried the 200RV). If I were still building I would probably want the 7496 Hartzell, but I don't think Vans is selling it any more.

 

[David Shelton]

Speed

GENERAL PHILOSOPHY:
I personally don't stress a lot about the last knot. Some weight savings are cheap and easy. You can keep weight down with a lighter paint job, lighter pilot, less interior decoration and less equipment. You can easily reduce drag by properly sealing your flaps and canopy. Before spending too much time and money, remember that many speed mods won't have a measurable gain. Here is an example:

Should I pay $200 for a titanium widget that?s 1 pound lighter than the stock one? I typically fly my RV-10 around 2,300 pounds and climb at about 1,500 fpm. This widget would reduce weight by (1/2300 lb) x 100 = 0.043%. The rate of climb would be increased by the same percent, 1,500 fpm x 0.00043 = 0.645 fpm. I probably couldn't detect or measure the benefit of my widget... but if I use the money to upgrade my headset, I'll probably notice and enjoy it on every flight!

RACING PHILOSOPHY:
If you race competitively, you can probably justify a greater investment in performance.

A DIFFERENT KIND OF SPEED MOD:
I use my airplane for travel so I?m mostly concerned about my door-to-door trip speed. NASA found that trip speed is extremely vulnerable to ?inter-modal delays? and prefers strategies that address delays over the traditional approach of increasing cruise speed.

I carry a 250cc motorcycle beneath my RV-10 and save lot?s of time:

1)Reduced trip planning? no need to find or reserve ground transportation.

2)Reduced inter-modal delay? hit the roads in 3 minutes instead of 21.5 min (national avg.) to rent a car.

3)Shorter Drive? only 1/3 of airports have ground transportation services so I can often land closer to my final destination.

My motorcycle frequently allows me to reach my final destination 20-60 minutes sooner. On a 200 nm trip, I would need to fly 56 to 583 knots faster to save those 20-60 minutes through conventional speed mods! My neighbor flies a much faster SR-22 but I almost always beat him to the final destination

 

[Noah]

Baseline: $65.21/Lb & $416.67/knot

Interesting Question. One way to baseline this is to start with your basic flying airplane. If you spent $75K to build an RV-7 and it weighed 1150 Lbs empty, then your baseline cost per pound would be ($75,000 / 1150 Lb) or $65.21. If you can do 180 kts, then your baseline cost of speed would be ($75,000 / 180) or $416.67 per knot.

Obviously throw whatever numbers in there you want - maybe you only include your airframe & powerplant costs and neglect your interior, panel, and paint. Anyway, this might be helpful for determining whether future mods make economic sense or not.

 

[pierre smith]

More mud...

.....Two guys I know, one of them a good friend, spent $23,000 or so for an automotive engine setup and another $7000 for their props, $30,000 total and the speed was 162 MPH.

Later, they spent another $23,000 each with Van for a 180 Lyc and another engine mount, I guess $1000 and new props, Catto's for $1900, for a second total of $25,900. The speed gain with the Lyc's is 36 MPH and over 100 lbs. lighter.

So $25,900 ~ 36= $719.44 per mile gained

Regards,

 

[Space Cadet]

Thanks, good inputs!

Thanks everyone, there are certainly a wide range of opinions on the subject, and for me personally, a couple new ways to think about the subject.

Dwight

 

[breister]

I'll throw in some more for ya'.

Ask yourself, "what is my mission?" If you rarely or never expect to take trips more than, say, 500 miles than +/- a few knots is irrelevant. 50 lbs will be noticeable in climb performance (barely), but probably not noticeable in cruise.

Once you define your mission, and define your budget, then it is time to start weighting every single "feature" you want - including the value of speed and climb differences TO YOU. I would use at least a 5 point scale to provide enough difference in weighting between features. Then compare weighting to cost when deciding which features are "worth the extra money."

If, for example, speed and long distance are of the essence to you - start looking at the fiberglass birds. If grass strips are essential to you, skip the Laincairs - at least the earlier 235s and 360s. Time is money, too - so factor in your time to build. RVs can be completed more quickly than a fiberglass bird. The Rocket is the fastest "RV" - but fewer people build them so be prepared for more challenges.

Building an airplane (or restoring an old one) is a significant life undertaking and should be handled like a professional project. You man not feel it necessary to plan every detail in advance, but spending a few WEEKS of planning ahead of time can save you a great deal of heartache later on.

Good luck!

 

[Bill Palmer]

And Balance

Just a Thought/Reminder: It's not just about weight. It's about weight AND balance. Sometimes, depending on your aircraft's configuration and planned loads, it's wise to invest in the balance part of weight and balance.

Take Care!

Bill Palmer

 

[David Shelton]

Balance

Great point Bill! Trim drag is typically between 1-4% of the total aircraft drag during cruise. You can minimize it by keeping your CG near the aft limit. Many racing sailplanes have a ballast box built into the tail so the pilot may precisely adjust his CG.

I've been tempted to mold a streamlined hunk of lead to attach at the tail tie-down. When my rear seats are empty (most of the time!), I could mount it on the tail to move my CG aft.

Next week, I'll try to calculate the RV-10 trim drag at forward and aft CG limits. I should be able to figure out the possible speed gains. I'll let you guys know what I come up with.

Another simple performance gain: Don't haul around extra fuel. My buddy gets some peace of mind from full tanks but it doesn't help performance. We obviously need our legal reserves (FAR 91.151) but there is no need to top-off the tanks before a short flight. Many airlines have lowered their reserves to cope with rising fuel prices.

 

[Alan Carroll]

Effect of prop weight on speed?

Great point Bill! Trim drag is typically between 1-4% of the total aircraft drag during cruise. You can minimize it by keeping your CG near the aft limit. Many racing sailplanes have a ballast box built into the tail so the pilot may precisely adjust his CG.

I've been tempted to mold a streamlined hunk of lead to attach at the tail tie-down. When my rear seats are empty (most of the time!), I could mount it on the tail to move my CG aft.

Next week, I'll try to calculate the RV-10 trim drag at forward and aft CG limits. I should be able to figure out the possible speed gains. I'll let you guys know what I come up with.

I've wondered about this from the perspective of prop weight. The 200RV is said to be ~19 pounds lighter than the Hartzell CS, which I estimate would move CG aft about 0.8" on the RV-8. This would be a good thing on my plane, which is near the forward CG limit when flown solo (I usually add weight in the rear baggage area).

The same affect could be achieved with the Hartzell by adding about 8 lbs. of lead to the tail. I've never seen this mentioned in any of the prop tests; maybe the effect on speed is small?